- Email: [email protected]
Preventing non-contact ACL injuries in female athletes: What can we learn from dancers?
Accepted Manuscript Preventing non-contact ACL injuries in female athletes: What can we learn from dancers? Catherine Turner, Sarah Crow, Thomas Crowther, Brittany Keating, Trenton Saupan, Jason Pyfer, Kimberly Vialpando, Szu-Ping Lee PII:
S1466-853X(17)30675-2
DOI:
10.1016/j.ptsp.2017.12.002
Reference:
YPTSP 855
To appear in:
Physical Therapy in Sport
Received Date: 25 August 2016 Revised Date:
14 November 2017
Accepted Date: 19 December 2017
Please cite this article as: Turner, C., Crow, S., Crowther, T., Keating, B., Saupan, T., Pyfer, J., Vialpando, K., Lee, S.-P., Preventing non-contact ACL injuries in female athletes: What can we learn from dancers?, Physical Therapy in Sports (2018), doi: 10.1016/j.ptsp.2017.12.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Preventing Non-Contact ACL Injuries in Female Athletes:
Catherine Turner, PT, DPT, OCS
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Sarah Crow, PT, DPT
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What Can We Learn from Dancers?
Thomas Crowther, PT, DPT
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Brittany Keating, PT, DPT Trenton Saupan, PT, DPT Jason Pyfer, PT, DPT
Kimberly Vialpando, PT, DPT
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Szu-Ping Lee, PT, PhD
Department of Physical Therapy, University of Nevada, Las Vegas, Nevada, USA
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This research was supported by a UNLVPT Student Opportunity Research Grant. This study was
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approved by the Institutional Review Board at University of Nevada, Las Vegas, NV. Address for Correspondence: Szu-Ping Lee, PT, PhD
Department of Physical Therapy, University of Nevada, Las Vegas 4505 S. Maryland Parkway, Box 453029, Las Vegas, NV 89154-3029, USA Phone: (702)895-3086 Email: [email protected]
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Word Count: Paper (4455)
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Preventing Non-Contact ACL Injuries in Female Athletes:
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What Can We Learn from Dancers?
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ABSTRACT Objectives: To investigate the effects of dance experience and movement instruction on lower
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extremity kinematics and muscle activation during a landing task. Design: Cross-sectional case control. Setting: Laboratory setting.
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Participants: 27 female subjects (age 18-25) in 2 groups: dancers (n = 12) and non-dancers (n =
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15).
Main Outcome Measures: Lower extremity biomechanics during drop landing were analyzed. Subjects performed drop landings after watching an instructional video without verbal instructions (NI), followed by repeat assessment after watching the same videos with specific verbal instructions (VI). Surface electromyography (EMG) was used to measure the activation of
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gluteus maximus and medius during the deceleration phase of landings. Peak knee and hip frontal plane angles during landing were acquired using a 3-D motion capture system.
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Results: Compared to non-dancers, dancers demonstrated generally greater gluteus maximus activation and a decreased knee abduction (i.e. valgus) angle during drop landing. A significant
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interaction showed that instruction led to increased knee abduction angle in non-dancers but not dancers (p=0.014).
Conclusions: Our findings suggest that experienced dancers demonstrate safer landing strategies compared to recreational athletes. Providing acute movement instruction was shown to disrupt the landing mechanics in those with no dance training experience. KEYWORDS: ACL injury, dancers, female athletes, landing, instruction
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Highlights: •
Dancers demonstrate generally greater gluteus maximus activation and a decreased knee
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abduction (i.e. valgus) angle during drop landing compared to non-dancers Non-dancers respond to acute movement instructions differently from dancers showing that additional instruction may lead to less optimal landing mechanics in this group.
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Extensive dance training may yield movement patterns that are protective against non-
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contact ACL injury.
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1. INTRODUCTION
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Female athletic participation has increased over the past decade, along with a disproportionate prevalence of knee injuries in comparison to male athletes.(Buller, Best,
Baraga, & Kaplan, 2015) The anterior cruciate ligament (ACL) accounts for 20% of all athletic
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knee injuries.(Majewski, Susanne, & Klaus, 2006) When comparing male and female athletes participating in soccer and basketball, female athletes experience ACL injuries 3 to 4 times more
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frequently, with reportedly 63-80% of injuries defined as non-contact.(Arendt, Agel, & Dick, 1999) Several theories have been proposed to explain the prevalence of non-contact ACL injuries,(Maetzel, Krahn, & Naglie, 2003) and the higher prevalence of ACL injuries in female athletes.(Chappell, Creighton, Giuliani, Yu, & Garrett, 2007; Hashemi, Chandrashekar, Mansouri, Slauterbeck, & Hardy, 2008; Uhorchak et al., 2003; Wojtys, Huston, Lindenfeld,
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Hewett, & Greenfield, 1998) Currently, a growing body of evidence suggests neuromuscular reeducation and movement training may be effective for preventing ACL injury during athletic
Straub, 2004)
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participation.(Alentorn-Geli et al., 2009b; Chappell et al., 2007; Chimera, Swanik, Swanik, &
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When specifically examining sex, female soccer players injure their ACLs at an incidence rate of 0.006 to 3.7 per 1000 hours of participation(Alentorn-Geli et al., 2009a) and female basketball players sustain ACL injuries at an incidence of 0.29 per 1000 hours.(Arendt et al., 1999) In comparison, female dancers reportedly have a very low ACL injury rate of 0.0009 per 1000 exposures (defined as participation in a class, rehearsal or performance).(Liederbach, Dilgen, & Rose, 2008) This data suggests a significantly lower risk of ACL injury, and only a limited number of studies have addressed why such a disparity between dancers and athletes
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might exist.(Ambegaonkar et al., 2011; Liederbach et al., 2008; Orishimo, Kremenic, Pappas, Hagins, & Liederbach, 2009; Orishimo, Liederbach, Kremenic, Hagins, & Pappas, 2014). Formal dance training focuses on the alignment of the body and lower extremities in
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aesthetically pleasing positions requiring a high level of neuromuscular control and balance. A typical 1.5 hour ballet class will often incorporate more than 200 jumps, with greater than 50% involving single-leg landings.(Liederbach et al., 2008) Within ballet training, external rotation of
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the lower extremities is a fundamental component of technique. Ideal external rotation, termed “turnout,” is essentially opposite to knee valgus, and results in a position with the feet forming an
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angle of 180° with the motion primarily occurring from the hip joints. (Negus, Hopper, & Briffa, 2005) As a result of this training, one could theorize that dancers may activate their gluteal muscles more than other athletes during jumping and landing. Gluteus maximus is a primary hip extensor also involved in generating hip abduction, external rotation and works eccentrically
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with gluteus medius to control hip adduction and internal rotation during weight bearing activities. (Delp, Hess, Hungerford, & Jones, 1999; Souza & Powers, 2009) The use of external rotation throughout ballet training including jumping and landing activities may allow for
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consistent activation of this important and powerful gluteal muscle by dancers, and subsequently be ACL protective for this population.
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Through the extensive feedback and movement instruction during training, dancers may
also develop heightened body awareness and controlled movement patterns when compared to other athletes. These effects were theorized to contribute to the clinically observed reduction of non-contact ACL injury risk in dancers.(Liederbach et al., 2008) However, the underlying protective biomechanical mechanism in this population is currently unknown. Most non-contact ACL injuries in athletes occur during activities that induce high knee loading (i.e. combined knee
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extensor, adductor, and rotational torques), most commonly during rapid deceleration, change of direction, and landing tasks.(Alentorn-Geli et al., 2009a) Potentially, a lack of eccentric control of gluteus maximus and gluteus medius may lead to reduced multi-directional control and
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increased injury risk in the lower extremity.(Souza & Powers, 2009) It is possible that dance training may enhance the control of these key muscles. But this theory has not been investigated. The purpose of this study was to compare lower extremity kinematics and activation of
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the gluteal muscles during drop landings between college-aged dancers and non-dancers, and investigate the effects of movement instruction on landing strategy. We hypothesized that
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dancers will land with smaller knee abduction and hip adduction angles, and greater activation of the gluteus medius and gluteus maximus muscles in comparison to non-dancers. We further hypothesized that specific movement instructions can help reduce knee abduction and hip
2. METHODS 2.1 Subjects
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adduction angles during landing.
All participants were female between the ages of 18 and 25 (12 dancers and 15 non-
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dancers). Only young active females were investigated due to this population’s higher reported
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risk of ACL injury.(Griffin et al., 2000; Liederbach et al., 2008). The inclusion criteria for the dancer group was that the participants must have a minimum of 7 years of dance experience and currently train for a minimum of 12 hours per week.(Steinberg, Aujla, Zeev, & Redding, 2014) Participants in the non-dancer group were required to be physically active and exercise at least 23 times per week for a minimum of 30 minutes per session. The exercise intensity needed to be in the range from moderate (3-6 METs) to vigorous (>6 METs), which is an intensity equivalent to various forms of dancing.(Ainsworth et al., 1993) Both dancers and non-dancers were asked to
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fill out a survey and provide the number of days per week and number of hours per session they were engaged in their relevant activity. Dancers were asked which style of dance they had the most experience in, while non-dancers were asked what type of exercise they participated in.
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Participants were excluded from this study if they had sustained any lower extremity injury within the past six months.(Arendt et al., 1999; Fernandez Regueiro, Garcia Fernandez, Nuno Mateo, & Gonzalvo Rodriguez, 2014) Lower extremity injuries that occurred more than 6
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months prior to data collection and were currently asymptomatic did not constitute
exclusion.(Scialom, Goncalves, & Padovani, 2006; Steinberg et al., 2014) Non-dancers were also
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excluded from the study if they had received any formal dance training within the past 10 years. All subjects were asked to identify which leg they would choose to kick a ball for maximum distance with, a method that has been used in previous drop landing studies to identify the dominant leg (Liederbach, Kremenic, Orishimo, Pappas, & Hagins, 2014; Orishimo et al., 2009;
2.2 Instrumentation
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Orishimo et al., 2014)
Hip muscle strength was measured using a handheld dynamometer (MicroFET2, Hoggan
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Scientific, Salt Lake City, UT, USA). Hip external rotation range of motion was measured using
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a standard goniometer, using the method described by Grossman et al. (2008) with the subject laying supine on the examination table simulating a standing first position in ballet terminology. (Figure 1) Gluteus maximus and gluteus medius activation were assessed using a wireless surface electromyography (EMG) system (Delsys Inc., Natick, MA, USA) with a sampling rate of 2000 Hz. Lower extremity 3D kinematic data were assessed using a 10-camera 3D motion capturing system (T-40S cameras, Vicon Motion Systems Ltd., Oxford, UK) with a sampling rate of 200 Hz. A 40 by 60 cm force platform (Type 9281CA, Kistler Instrument Corp., Amherst,
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NY, USA) was used to detect the instant of ground contact during the landing trials. The
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sampling rate was 2000 Hz. All systems were time synchronized.
2.3 Procedures
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FIGURE 1: Measurement of hip external rotation
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Prior to data collection, an informed consent approved by the University of Nevada, Las Vegas Institutional Review Board for Biomedical Research, was signed and collected from each participant. After enrollment, the participants were asked to complete a demographic and physical activity survey. The data collection procedures were outlined in Figure 2.
FIGURE 2: Experimental Procedures Overview
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2.3.1 Preparation of Muscle Activation Assessment
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Neuromuscular activation levels of the right gluteus maximus and gluteus medius were
assessed. Skin surfaces of the muscles were cleaned and lightly abraded. Self-adhesive wireless surface EMG electrodes were attached to the skin surface. For gluteus maximus, the electrode was placed lateral to the sacrum on the prominence of the muscle belly, in parallel to the muscle fibers. EMG signal quality was confirmed by asking the participant to contract the hip extensor muscle in a prone posture with the knee flexed. For gluteus medius, the greater trochanter was palpated and then EMG electrode was placed along the line formed by the iliac crest and greater 7
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trochanter on the prominence of the muscle belly. EMG signal quality was confirmed by asking the participant to contract the hip abductor muscles by lifting their leg up in a side-lying position. The EMG electrodes were further secured with additional adhesive tape to decrease excess
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motion and improve surface contact of the electrodes during activity. This method has been described and validated to quantify activation level of these specific muscles during weight-
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bearing activities in a previous study.(Lee & Powers, 2013)
Once electrodes were secured, maximal voluntary isometric contraction (MVIC) testing
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was performed to measure the strengths of hip abduction and hip extension and establish the peak activation level. Hip abduction strength and hip extension strength was assessed with a handheld dynamometer simultaneously during the performance of MVIC testing. Hip abduction strength was assessed as described previously (Figure 3; Leetun et al., 2004). Hip extension strength was assessed in the test position described by Souza and Powers (Figure 4), but the
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handheld dynamometer was utilized based on equipment availability, in contrast to the use of an isokinetic dynamometer. The use of a strap to secure the handheld dynamometer has been recommended to avoid the effect of examiner effort in influencing test results (Bohannon, 1999).
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In the current study, the investigator found it necessary to use light hand placement during the
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hip abduction test to prevent the dynamometer sliding anteriorly or posteriorly during testing. Every effort was made to any avoid compression of the device by the examiner during this process. The testing protocol was previously established and proven reliable (Mentiplay et al., 2015; Romero-Franco, Jimenez-Reyes, & Montano-Munuera, 2016).
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FIGURE 3: Isometric Hip Abduction Strength Testing
FIGURE 4: Isometric Hip Extension Strength Testing
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In a 5-second duration, participants were asked to generate the maximal contraction isometrically. Three trials were collected for each assessment. After the strength assessments, reflective markers were placed on the participant based on the Vicon Plug-in Gait full-body
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marker set.(Kadaba, Ramakrishnan, & Wootten, 1990) A static calibration trial that acquires the body geometry and marker placements was collected followed by a five-minute warm up on an elliptical machine to familiarize the participant with performing physical activity with the
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attached instruments.
2.3.2 Biomechanical Evaluation of Drop Landing
Participants performed bilateral drop landings from the top of a 30.5 cm box. (Fernandez Regueiro et al., 2014; Orishimo et al., 2009; Walsh, Waters, & Kersting, 2007) (Figure 5) All participants performed the test conditions with bare feet. Previous investigators have highlighted
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that dancers may not consistently wear shoes or wear the same type of shoes when dancing.(Ambegaonkar et al., 2011; Shultz & Schmitz, 2009) To control this potential confounder, we asked all study participants to perform drop landings barefoot. Participants were
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expected to perform drop landings with legs in a neutral position, with and without specific
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movement instructions (verbal instruction [VI]) vs. no instruction [NI]). The term neutral was used to describe the standard landing position for drop landings with the lower extremities in the sagittal plane, as neutral or parallel is the term dancers utilize when describing the natural, nonturned-out positions of the limbs, more commonly utilized in modern dance.(Liederbach et al., 2008)
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FIGURE 5: Drop landing Test
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In the NI conditions, participants were shown videos of the drop landing movement demonstrated by one of the investigators.(Ambegaonkar et al., 2011) The videos showed both a frontal and sagittal view. Participants were not given any verbal instruction or feedback, besides
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placing their hands on their hips to avoid obscuring the Vicon markers, but were allowed to practice at most 3 times before performing 3 captured landing trials.(Walsh et al., 2007) After
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completing the NI landings, participants watched the same demonstration videos but now with specific verbal instructions on how to perform the landings. The instructions focused on the positions of the body segments (i.e. trunk, hips, knees, and feet) during landing. Complete instructions can be found in Table 1. In this condition, the participants were allowed unlimited practice jumps and were given feedback by investigators on their movements during the practice, before 3 landing trials were captured. For the purposes of the practice trials and providing
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feedback, one investigator observed the frontal plane while a second investigator observed the sagittal plane. Verbal feedback was only provided if deemed necessary to achieve an optimal drop landing. Any instructions used were succinct, and did not deviate from the instructions on
remain as consistent as possible.
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TABLE 1: Verbal instructions given to the participants
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the video, e.g., “place your hands on your hips,” or “don’t let your knees fall in,” in order to
Instructions given: “Place your feet hip width apart and toes pointing forward. Be sure to place the balls of your feet at the front edge of the box so your toes are just hanging off. Stand with trunk erect. When you are ready, please step Drop landing off the box and land on the ground in the same position you started in. Focus on having your toes touch down first, and then your heels. Remember to maintain an upright trunk and do not let your knees fall in towards each other when landing.” *Participants were instructed to keep their hands on their hips at all times and to keep their eyes looking forward.
2.4 Data Analysis
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Test conditions:
Data from the right leg was processed and analyzed in our analysis. Collected EMG
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signals were bandpass filtered (35-500Hz, 4th order Butterworth filter), then full-wave rectified. The marker data were low-pass filtered with a cutoff frequency of 12 Hz. The 3D joint
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kinematics data were computed from the filtered marker data using a software (Visual 3D, Cmotion Inc., Rockville, MD, USA). Muscle activation and joint kinematic variables were extracted from the deceleration phase of each drop landing trial. The deceleration phase was defined from the instant of the foot contacting the force plate (defined as when the vertical ground reaction force exceeds 50 newtons), to when the knee flexion angle was greatest during a landing. Activations of the gluteus medius and maximus muscles during this period were timeaveraged and normalized as a percentage to the corresponding peak activation levels obtained
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during the MVIC trials. Peak hip adduction and knee abduction angles were also obtained during this period.
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2.5 Statistical Analysis Demographic information of the participants was compared using independent t-tests, specifically comparing age, height, weight, BMI, range of motion for hip external rotation,
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activity level, and hip abduction and extension strength between groups. Two-way mixed model ANOVAs were used to investigate the effects of group (dancers vs. non-dancers) and instruction
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(NI vs. VI, repeat-measure factor) on the lower extremity biomechanics and hip muscle activation levels during landings. When significant main effects or interactions were found, posthoc comparisons with Bonferroni correction were performed. All analyses were done using a statistical software (SPSS version 22, IBM Corp. Armonk, NY, USA). Significance level was set
3.0 RESULTS
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at 0.05.
Independent t-tests revealed no significant differences in height, weight, BMI, range of
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motion for hip external rotation, hip abduction and extension strength between the dancer and
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non-dancer groups, but did reveal a significant difference in activity level per week between the two groups (p<0.001). This is likely due to the fact that the dancers were engaged in structured practices of long duration (Table 2). Seventy-eight percent of participants identified as right leg dominant. Subjects in the dancer group reported participating in a single or combined form of dance including ballet, jazz, and modern, while one subject participated in hip-hop in addition to the three prior forms of dance, and two subjects also listed participation in contemporary dance. The non-dancer group primarily participated in running and weight-training, with two
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participants reporting participation in soccer or volleyball, respectively, and three subjects reported additional participation in yoga or Pilates.
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TABLE 2. Demographic and Anthropometric Information of the Participants.
Dancer N=(12)
Non-Dancer N=(15)
Age (yrs)
20.58±1.13
22.07±1.68
Height (m)
1.65±0.06
1.62±0.07
Weight (kg)
60.88±6.01
58.77±6.51
0.395
BMI (kg/m2)
22.4±2.3
22.5±2.3
0.981
1255.0±434.5
415.3±336.6
<0.001
56.4±7.3
56.6±11.3
0.960
53.8±8.7
55.5±9.6
0.646
Hip Abduction Strength (N)
57.7±11.6
53.9±11.3
0.397
Hip Extension Strength (N)
101.4±26.9
101.3±26.5
0.995
Right Hip ER ROM(deg.)
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0.039 0.201
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Left Hip ER ROM(deg.)
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Activity level (min/wk)
P-value
3.1Landing Biomechanics
The 2-way ANOVAs revealed that there was a strong trend of a group main effect.
Specifically dancers demonstrated trends of having greater gluteus maximus contraction and smaller knee abduction angle (gluteus maximus activation: dancers=53.8±43.3 % vs. nondancers=28.0±21.2 %, p=0.050, Cohen’s d=-0.76; peak knee abduction angle: dancers=5.3±3.2˚,
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non-dancers=8.1±4.1˚, p=0.055, Cohen’s d=0.76). No significant difference between groups were found in gluteus medius activation and hip abduction angle (p=0.309 and 0.431,
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respectively; Figures 6 and 7).
FIGURE 6: Comparison of Frontal Plane Hip and Knee Angles between Non-Dancers and Dancers
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FIGURE 7: Comparison of Gluteal Muscle Activations between Non-Dancers and Dancers
A significant main effect of instruction was detected for peak knee and hip angles across
groups (peak knee abduction angle: before instruction=6.3±3.8˚, after instruction=7.4±4.1˚, p=0.003, Cohen’s d=0.28; hip abduction angle: before instruction=3.1±3.8˚, after instruction=4.6±4.0˚, p=0.001, Cohen’s d=-0.38). There was a significant interaction between instruction and group for knee abduction angle (p=0.014; Figure 8). Based on our primary variable of knee abduction angle, we calculated the achieved power levels to be 0.91.
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FIGURE 8: Knee Abduction Angle Group (Dancer vs. Non-dancer) by Condition (No Instruction vs. Verbal Instruction) Interaction
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The post-hoc analyses showed that within the non-dancer group, there was a significant difference in knee abduction angle before and after instruction (NI vs. VI, 7.3±3.9 ˚ vs. 9.0±4.3˚,
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p<0.001, Cohen’s d=0.41), but not in the dancers (NI vs. VI, 5.2±3.6˚ vs. 5.4±3.0˚, p=0.728, Cohen’s d=0.06).
4.0 DISCUSSION
Our primary finding was that dancers exhibited a smaller knee abduction angle and greater gluteus maximus activation during drop landing tasks when compared to non-dancers. Additionally, there was a difference in how dancers and non-dancers reacted to movement
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instructions during the drop landing task. After instructions, we observed an increase in the nondancer’s knee abduction angle while dancer’s knee angle did not change significantly.
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4.1 Gluteus Maximus Activation Preventing ACL injuries is a top priority in female youth sports. The effectiveness of specific movement training programs designed to address this concern has been demonstrated,
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and commonly focuses on tasks requiring gluteus maximus activation during landing and
deceleration. (Alentorn-Geli et al., 2009b; Pollard, Sigward, & Powers, 2010). ACL injury
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prevention programs have been developed that include activities known to target the hip extensors and hip abductors with specific lower extremity strengthening exercises and plyometric drills.(Alentorn-Geli et al., 2009b; Mandelbaum et al., 2005) The extensive training dancers receive in a turned-out position may increase their ability to engage the gluteus maximus
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muscles during the deceleration phase of landing tasks. Increased gluteus maximus activation may also lead to a decreased knee frontal angle on landing as a result of greater femoral rotation control.(Hollman, Hohl, Kraft, Strauss, & Traver, 2013) In our study, the dancers exhibited on
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average 20-25% greater gluteus maximus activation and a smaller knee abduction angle during the deceleration phase of landing when compared to non-dancers. While these two findings
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might be coincidental, one might theorize that the increased gluteus maximus activation plays a role in the smaller knee abduction angle. With excessive knee valgus (i.e. knee abduction) angle being a factor for knee ligamentous injuries (Alentorn-Geli et al., 2009a; Ford, Myer, & Hewett, 2003), the dancers appeared to be employing a protective landing strategy, potentially explaining the lower rate of ACL injuries within this population. From a clinical perspective, there was not a statistically significant difference in hip abduction or hip extension strength between the two groups on assessment. This may indicate that an athlete’s ability to activate the gluteus muscles
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during landings is a key component to ACL injury prevention and a more informative measurement to assess when screening athletes than isometric hip strength in isolation.
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In agreement with previous studies, we observed peak knee abduction angle occurring within 100 ms after ground contact. Previous video analysis of ACL injuries suggests that
injurious incidences typically occur 17-50 ms after ground contact.(Krosshaug et al., 2007) The
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knee is most vulnerable to ligamentous injuries quickly after contact, hence a person is unlikely to use feedback response to correct knee motions. A feed-forward motor planning strategy that
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aligns the lower extremity prior to landing would be far more efficient and protective during high velocity loading and impact activities. For the purposes of this study, we opted to use a bilateral drop landing technique as described in studies examining athletes, dancers and recreationally active individuals who were college aged in similarity to our population.(Carcia, Eggen, & Shultz, 2005; Didier, 2001; Walsh et al., 2007). Krosshaug et al. have questioned the use of
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bilateral drop landing tests in detecting ACL injury risks factors, although the authors themselves noted their study population was an elite group of athletes, compared to the previous studies. (Carcia et al., 2005; Hewett et al., 2005; Krosshaug et al., 2016; Walsh et al., 2007) The bilateral
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drop landing task we utilized does not challenge femoral adduction as much as a single-leg
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landing and may explain our findings of less than 4° peak knee abduction angle between the dancers and non-dancers. We might have observed greater knee abduction due to insufficient gluteal muscle activation, if a more challenging single leg task was used.(Powers, 2003) When examining the performance of a single-legged drop landing task, Orishimo et al.
found no difference between professional male and female dancers, and subsequently demonstrated female sports athletes landed with greater peak knee valgus in comparison to both male and female dancers during the same task.(Orishimo et al., 2009; Orishimo et al., 2014) It
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was revealing that the dancers in our study landed with the knee in a more neutral alignment when compared to the non-dancers. This confirms the findings from Orishomo et al., and raises the question of what can clinicians learn from dancers in the development of training strategies
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to prevent ACL injuries? In a clinical or athletic setting, it might be relevant to incorporate
specific interventions to target gluteus maximus muscle activation not just strengthening, along with repetitive jump training using single and bilateral landing techniques, to potentially reduce
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the risk of ACL injury in other athletic populations. Future studies may focus on examining the amount of practice that is required to lead to the level of difference between dancers and non-
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dancers observed in this study. Prospective studies are also needed to examine if gluteal muscle activation training is effective for reducing ACL injury rate.
4.2 Movement Instruction
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The addition of verbal instructions in this study resulted in a small but significant increase (~2°) in the knee abduction angle in the non-dancers. This was unexpected as explicit instruction: “…do not let your knees fall in toward each other when landing” was included in the
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cuing. The significant interaction between group and instruction demonstrated that dancers and non-dancers responded differently to the introduction of movement instruction. This may be due
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to different interpretation, processing, and execution of the verbal movement instruction by the two groups.
Our specific verbal movement instructions may have negatively influenced the landing
mechanics as the cues were primarily focused internally on body alignment. While such instructions have been commonly used in ACL injury prevention programs,(Myer, Ford, & Hewett, 2004) the internal attentional focus may interfere with movement performance. A
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number of previous studies have demonstrated the negative influence of internal attentional focus and excessive instruction on motor learning and performance.(Abdollahipour, Psotta, & Land, 2016; Halperin, Chapman, Martin, & Abbiss, 2017; Wulf, 2013; Wulf & Weigelt, 1997) This
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may explain why some ACL injury prevention programs were not effective. (Noyes & Barber Westin, 2012) However, dancers are used to receiving a combination of internally and externally focused instruction which may account for the lack of performance degradation. Research has
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suggested, that the enhanced proprioception documented in dancers provides them with the
ability to shift their attentional focus away from basic tasks in order to focus on the execution of
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more challenging tasks.(Manrique Arija et al., 2011) Therefore, one might argue that the basic landing activity required during this study was a task the dancers were already skilled in performing and consequently uninfluenced by the verbal instructions. Dancers possess enhanced proprioception, motor control and coordination considered to
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be a result of their training, although some authors have argued that individuals with enhanced abilities are more likely to become dancers. (Jola, Davis, & Haggard, 2011; Manrique Arija et al., 2011) There is likely an argument for a combination of both. Marmeleira et al. demonstrated
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improved knee joint position sense and knee kinesthesia following a 12 week creative dancing program for older adults suggesting training programs may be beneficial in enhancing lower
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limb proprioception regardless of age and experience.(Marmeleira et al., 2009) Based on our study, we suggest future research should look at multiple factors in the design of ACL injury prevention programs including focusing on enhancing proprioception of the entire lower kinetic chain, the effect of internal versus external attentional focus and prior training experience on the biomechanics of landing. Currently published research suggests external attentional focus is beneficial in improving movement patterns to reduce second ACL injury risk following ACL
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reconstruction (Gokeler et al., 2015). It could be argued there might be a role for a combination of both internal and external cuing based on the motor learning stage of the participant, with greater internal cuing required in the earlier stages of motor learning, although this has yet to be
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clearly established in the literature regarding ACL prevention programs. (Kiani et al., 2010; Myer, Sugimoto, Thomas, & Hewett, 2013) Screening for the existence of enhanced
proprioception prior to participation in an at-risk sport might also be relevant in decreasing the
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frequency of ACL injury. One might also question if the inclusion of specific jump training focusing on lower extremity alignment, technique and gluteus maximus activation, as dancers
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typically do from an early age, would have a role in diminishing ACL injury rates if integrated into injury prevention programs and the athletic settings at an earlier age for at-risk athletes? This study has a number of limitations. The dancers were recruited from the Dance Department of a university, and demonstrated significantly higher activity levels compared to
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non-dancers who were mostly recreationally active. The dancers had mixed training backgrounds including both modern and ballet, although the emphasis at this particular university is in modern dance. This may have explained the lack of statistically significant difference in hip external
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range of motion range of motion between the two groups, with modern dancers self-selecting for that style of performance, versus primarily ballet training with an expectation of greater hip
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turnout. Having the subjects perform the study barefoot may have provided the dancers with an advantage, particularly modern dancers who frequently perform barefooted. Furthermore, we only observed the acute effects of instruction on the kinematics of landing; it was unknown if the same effects would be observed if the participants had more time to practice the landing movements as in a typical ACL injury prevention program, or if the changes in kinematics would translate to kinetic risk factors such as increased knee abduction moment. Biomechanical
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comparison of dancers versus other high level athletes with similar levels of activity participation in their respective sports would help elucidate the influence of training experience on landing
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mechanics and injury.
5.0 CONCLUSION
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This study demonstrates that during landing dancers employed movement and
neuromuscular control strategies that were different from non-dancers. Specifically dancers
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exhibited increased activation of the gluteus maximus and a reduction of knee abduction angle; these differences are potentially protective attributes against non-contact ACL injuries. In addition, dancers and non-dancers appeared to respond differently to movement instructions. Our data showed that landing mechanics of non-dancers were disrupted after receiving movement cues, indicating that the lack of experience in processing explicit movement instructions can
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impact acute motor learning. Findings from this study may provide argument for incorporating motor learning concepts associated with dance education into ACL injury prevention programs. However, future research should investigate the most beneficial forms of cuing, and the optimal
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developmental motor learning period to initiate preventative training strategies for ACL injuries.
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Maetzel, A., Krahn, M., & Naglie, G. (2003). The cost effectiveness of rofecoxib and celecoxib in patients with osteoarthritis or rheumatoid arthritis. Arthritis Rheum, 49(3), 283-292. doi:10.1002/art.11121 Majewski, M., Susanne, H., & Klaus, S. (2006). Epidemiology of athletic knee injuries: A 10year study. Knee, 13(3), 184-188. doi:10.1016/j.knee.2006.01.005 Mandelbaum, B. R., Silvers, H. J., Watanabe, D. S., Knarr, J. F., Thomas, S. D., Griffin, L. Y., . . . Garrett, W., Jr. (2005). Effectiveness of a neuromuscular and proprioceptive training program in preventing anterior cruciate ligament injuries in female athletes: 2-year follow-up. Am J Sports Med, 33(7), 1003-1010. doi:10.1177/0363546504272261 Manrique Arija, S., Lopez Lasanta, M., Jimenez Nunez, F. G., Urena, I., Espino-Lorenzo, P., Romero Barco, C. M., . . . Fernandez Nebro, A. (2011). [Annual trends in knee and hip arthroplasty in rheumatoid arthritis 1998-2007]. Reumatol Clin, 7(6), 380-384. doi:10.1016/j.reuma.2011.05.012 Marmeleira, J. F., Pereira, C., Cruz-Ferreira, A., Fretes, V., Pisco, R., & Fernandes, O. M. (2009). Creative dance can enhance proprioception in older adults. J Sports Med Phys Fitness, 49(4), 480-485. Mentiplay, B. F., Perraton, L. G., Bower, K. J., Adair, B., Pua, Y. H., Williams, G. P., . . . Clark, R. A. (2015). Assessment of Lower Limb Muscle Strength and Power Using Hand-Held and Fixed Dynamometry: A Reliability and Validity Study. PLoS One, 10(10), e0140822. doi:10.1371/journal.pone.0140822 Myer, G. D., Ford, K. R., & Hewett, T. E. (2004). Rationale and Clinical Techniques for Anterior Cruciate Ligament Injury Prevention Among Female Athletes. J Athl Train, 39(4), 352-364. Myer, G. D., Sugimoto, D., Thomas, S., & Hewett, T. E. (2013). The influence of age on the effectiveness of neuromuscular training to reduce anterior cruciate ligament injury in female athletes: a meta-analysis. Am J Sports Med, 41(1), 203-215. doi:10.1177/0363546512460637 Negus, V., Hopper, D., & Briffa, N. K. (2005). Associations between turnout and lower extremity injuries in classical ballet dancers. Journal of Orthopaedic & Sports Physical Therapy, 35, 307-318. Noyes, F. R., & Barber Westin, S. D. (2012). Anterior cruciate ligament injury prevention training in female athletes: a systematic review of injury reduction and results of athletic performance tests. Sports Health, 4(1), 36-46. doi:10.1177/1941738111430203 Orishimo, K. F., Kremenic, I. J., Pappas, E., Hagins, M., & Liederbach, M. (2009). Comparison of landing biomechanics between male and female professional dancers. Am J Sports Med, 37(11), 2187-2193. doi:10.1177/0363546509339365 Orishimo, K. F., Liederbach, M., Kremenic, I. J., Hagins, M., & Pappas, E. (2014). Comparison of landing biomechanics between male and female dancers and athletes, part 1: Influence of sex on risk of anterior cruciate ligament injury. Am J Sports Med, 42(5), 1082-1088. doi:10.1177/0363546514523928 Pollard, C. D., Sigward, S. M., & Powers, C. M. (2010). Limited hip and knee flexion during landing is associated with increased frontal plane knee motion and moments. Clin Biomech (Bristol, Avon), 25(2), 142-146. doi:10.1016/j.clinbiomech.2009.10.005 Powers, C. M. (2003). The influence of altered lower-extremity kinematics on patellofemoral joint dysfunction: a theoretical perspective. J Orthop Sports Phys Ther, 33(11), 639-646. doi:10.2519/jospt.2003.33.11.639
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Romero-Franco, N., Jimenez-Reyes, P., & Montano-Munuera, J. A. (2016). Validity and reliability of a low-cost digital dynamometer for measuring isometric strength of lower limb. J Sports Sci, 1-6. doi:10.1080/02640414.2016.1260152 Scialom, M., Goncalves, A., & Padovani, C. R. (2006). Work and injuries in dancers: Survey of a professional dance company in Brazil. Med Probl Perform Art, 21, 29-33. Shultz, S. J., & Schmitz, R. J. (2009). Effects of transverse and frontal plane knee laxity on hip and knee neuromechanics during drop landings. Am J Sports Med, 37(9), 1821-1830. doi:10.1177/0363546509334225 Souza, R. B., & Powers, C. M. (2009). Predictors of hip internal rotation during running: an evaluation of hip strength and femoral structure in women with and without patellofemoral pain. Am J Sports Med, 37(3), 579-587. doi:10.1177/0363546508326711 Steinberg, N., Aujla, I., Zeev, A., & Redding, E. (2014). Injuries among talented young dancers: findings from the U.K. Centres for Advanced Training. Int J Sports Med, 35(3), 238-244. doi:10.1055/s-0033-1349843 Uhorchak, J. M., Scoville, C. R., Williams, G. N., Arciero, R. A., St Pierre, P., & Taylor, D. C. (2003). Risk factors associated with noncontact injury of the anterior cruciate ligament: a prospective four-year evaluation of 859 West Point cadets. Am J Sports Med, 31(6), 831842. Walsh, M. S., Waters, J., & Kersting, U. G. (2007). Gender bias on the effects of instruction on kinematic and kinetic jump parameters of high-level athletes. Res Sports Med, 15(4), 283-295. doi:10.1080/15438620701693306 Wojtys, E. M., Huston, L. J., Lindenfeld, T. N., Hewett, T. E., & Greenfield, M. L. V. H. (1998). Association between the menstrual cycle and anterior cruciate ligament injuries in female athletes. Am J Sports Med, 26(5), 614-619. Wulf, G. (2013). Attentional focus and motor learning: a review of 15 years. International Review of Sport and Exercise Psychology, 6(1), 77-104. doi:10.1080/1750984x.2012.723728 Wulf, G., & Weigelt, C. (1997). Instructions about physical principles in learning a complex motor skill: to tell or not to tell. Res Q Exerc Sport, 68(4), 362-367. doi:10.1080/02701367.1997.10608018
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ACKNOWLEDGEMENTS We would like to acknowledge the UNLV Department of Dance for their help and cooperation
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with our participant recruitment, UNLV Department of Kinesiology for allowing us to use the Sports Injury Research Center for data collection, and UNLV Department of Physical Therapy and the Clinical Locomotion Neuromechanics Laboratory for research support. This study
received funding from the UNLV Physical Therapy Department in the form of a UNLVPT
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Student Opportunity Research Grant. We would also like to thank Josh Bailey for his help with
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data collection, and Ron Garcia for his help with data processing. This study was approved by
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the Institutional Review Board at University of Nevada, Las Vegas, NV.
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Highlights: Dancers showed higher hip muscle activation compared to non-dancers during landing.
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Dancers land with more neutral knee kinematics (less valgus) than non-dancers.
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Acute movement instruction deteriorated landing mechanics in non-dancers only.
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Dance training experience may provide protection against ACL injury.
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Movement-based ACL injury prevention program may be modelled after dance training.
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Funding Statement:
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This study received funding from the UNLV Physical Therapy Department in the form of a UNLVPT Student Opportunity Research Grant.
S1466-853X(17)30675-2
DOI:
10.1016/j.ptsp.2017.12.002
Reference:
YPTSP 855
To appear in:
Physical Therapy in Sport
Received Date: 25 August 2016 Revised Date:
14 November 2017
Accepted Date: 19 December 2017
Please cite this article as: Turner, C., Crow, S., Crowther, T., Keating, B., Saupan, T., Pyfer, J., Vialpando, K., Lee, S.-P., Preventing non-contact ACL injuries in female athletes: What can we learn from dancers?, Physical Therapy in Sports (2018), doi: 10.1016/j.ptsp.2017.12.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Preventing Non-Contact ACL Injuries in Female Athletes:
Catherine Turner, PT, DPT, OCS
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Sarah Crow, PT, DPT
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What Can We Learn from Dancers?
Thomas Crowther, PT, DPT
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Brittany Keating, PT, DPT Trenton Saupan, PT, DPT Jason Pyfer, PT, DPT
Kimberly Vialpando, PT, DPT
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Szu-Ping Lee, PT, PhD
Department of Physical Therapy, University of Nevada, Las Vegas, Nevada, USA
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This research was supported by a UNLVPT Student Opportunity Research Grant. This study was
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approved by the Institutional Review Board at University of Nevada, Las Vegas, NV. Address for Correspondence: Szu-Ping Lee, PT, PhD
Department of Physical Therapy, University of Nevada, Las Vegas 4505 S. Maryland Parkway, Box 453029, Las Vegas, NV 89154-3029, USA Phone: (702)895-3086 Email: [email protected]
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Word Count: Paper (4455)
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Preventing Non-Contact ACL Injuries in Female Athletes:
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What Can We Learn from Dancers?
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ABSTRACT Objectives: To investigate the effects of dance experience and movement instruction on lower
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extremity kinematics and muscle activation during a landing task. Design: Cross-sectional case control. Setting: Laboratory setting.
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Participants: 27 female subjects (age 18-25) in 2 groups: dancers (n = 12) and non-dancers (n =
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15).
Main Outcome Measures: Lower extremity biomechanics during drop landing were analyzed. Subjects performed drop landings after watching an instructional video without verbal instructions (NI), followed by repeat assessment after watching the same videos with specific verbal instructions (VI). Surface electromyography (EMG) was used to measure the activation of
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gluteus maximus and medius during the deceleration phase of landings. Peak knee and hip frontal plane angles during landing were acquired using a 3-D motion capture system.
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Results: Compared to non-dancers, dancers demonstrated generally greater gluteus maximus activation and a decreased knee abduction (i.e. valgus) angle during drop landing. A significant
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interaction showed that instruction led to increased knee abduction angle in non-dancers but not dancers (p=0.014).
Conclusions: Our findings suggest that experienced dancers demonstrate safer landing strategies compared to recreational athletes. Providing acute movement instruction was shown to disrupt the landing mechanics in those with no dance training experience. KEYWORDS: ACL injury, dancers, female athletes, landing, instruction
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Highlights: •
Dancers demonstrate generally greater gluteus maximus activation and a decreased knee
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abduction (i.e. valgus) angle during drop landing compared to non-dancers Non-dancers respond to acute movement instructions differently from dancers showing that additional instruction may lead to less optimal landing mechanics in this group.
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Extensive dance training may yield movement patterns that are protective against non-
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contact ACL injury.
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1. INTRODUCTION
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Female athletic participation has increased over the past decade, along with a disproportionate prevalence of knee injuries in comparison to male athletes.(Buller, Best,
Baraga, & Kaplan, 2015) The anterior cruciate ligament (ACL) accounts for 20% of all athletic
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knee injuries.(Majewski, Susanne, & Klaus, 2006) When comparing male and female athletes participating in soccer and basketball, female athletes experience ACL injuries 3 to 4 times more
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frequently, with reportedly 63-80% of injuries defined as non-contact.(Arendt, Agel, & Dick, 1999) Several theories have been proposed to explain the prevalence of non-contact ACL injuries,(Maetzel, Krahn, & Naglie, 2003) and the higher prevalence of ACL injuries in female athletes.(Chappell, Creighton, Giuliani, Yu, & Garrett, 2007; Hashemi, Chandrashekar, Mansouri, Slauterbeck, & Hardy, 2008; Uhorchak et al., 2003; Wojtys, Huston, Lindenfeld,
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Hewett, & Greenfield, 1998) Currently, a growing body of evidence suggests neuromuscular reeducation and movement training may be effective for preventing ACL injury during athletic
Straub, 2004)
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participation.(Alentorn-Geli et al., 2009b; Chappell et al., 2007; Chimera, Swanik, Swanik, &
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When specifically examining sex, female soccer players injure their ACLs at an incidence rate of 0.006 to 3.7 per 1000 hours of participation(Alentorn-Geli et al., 2009a) and female basketball players sustain ACL injuries at an incidence of 0.29 per 1000 hours.(Arendt et al., 1999) In comparison, female dancers reportedly have a very low ACL injury rate of 0.0009 per 1000 exposures (defined as participation in a class, rehearsal or performance).(Liederbach, Dilgen, & Rose, 2008) This data suggests a significantly lower risk of ACL injury, and only a limited number of studies have addressed why such a disparity between dancers and athletes
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might exist.(Ambegaonkar et al., 2011; Liederbach et al., 2008; Orishimo, Kremenic, Pappas, Hagins, & Liederbach, 2009; Orishimo, Liederbach, Kremenic, Hagins, & Pappas, 2014). Formal dance training focuses on the alignment of the body and lower extremities in
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aesthetically pleasing positions requiring a high level of neuromuscular control and balance. A typical 1.5 hour ballet class will often incorporate more than 200 jumps, with greater than 50% involving single-leg landings.(Liederbach et al., 2008) Within ballet training, external rotation of
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the lower extremities is a fundamental component of technique. Ideal external rotation, termed “turnout,” is essentially opposite to knee valgus, and results in a position with the feet forming an
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angle of 180° with the motion primarily occurring from the hip joints. (Negus, Hopper, & Briffa, 2005) As a result of this training, one could theorize that dancers may activate their gluteal muscles more than other athletes during jumping and landing. Gluteus maximus is a primary hip extensor also involved in generating hip abduction, external rotation and works eccentrically
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with gluteus medius to control hip adduction and internal rotation during weight bearing activities. (Delp, Hess, Hungerford, & Jones, 1999; Souza & Powers, 2009) The use of external rotation throughout ballet training including jumping and landing activities may allow for
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consistent activation of this important and powerful gluteal muscle by dancers, and subsequently be ACL protective for this population.
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Through the extensive feedback and movement instruction during training, dancers may
also develop heightened body awareness and controlled movement patterns when compared to other athletes. These effects were theorized to contribute to the clinically observed reduction of non-contact ACL injury risk in dancers.(Liederbach et al., 2008) However, the underlying protective biomechanical mechanism in this population is currently unknown. Most non-contact ACL injuries in athletes occur during activities that induce high knee loading (i.e. combined knee
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extensor, adductor, and rotational torques), most commonly during rapid deceleration, change of direction, and landing tasks.(Alentorn-Geli et al., 2009a) Potentially, a lack of eccentric control of gluteus maximus and gluteus medius may lead to reduced multi-directional control and
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increased injury risk in the lower extremity.(Souza & Powers, 2009) It is possible that dance training may enhance the control of these key muscles. But this theory has not been investigated. The purpose of this study was to compare lower extremity kinematics and activation of
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the gluteal muscles during drop landings between college-aged dancers and non-dancers, and investigate the effects of movement instruction on landing strategy. We hypothesized that
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dancers will land with smaller knee abduction and hip adduction angles, and greater activation of the gluteus medius and gluteus maximus muscles in comparison to non-dancers. We further hypothesized that specific movement instructions can help reduce knee abduction and hip
2. METHODS 2.1 Subjects
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adduction angles during landing.
All participants were female between the ages of 18 and 25 (12 dancers and 15 non-
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dancers). Only young active females were investigated due to this population’s higher reported
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risk of ACL injury.(Griffin et al., 2000; Liederbach et al., 2008). The inclusion criteria for the dancer group was that the participants must have a minimum of 7 years of dance experience and currently train for a minimum of 12 hours per week.(Steinberg, Aujla, Zeev, & Redding, 2014) Participants in the non-dancer group were required to be physically active and exercise at least 23 times per week for a minimum of 30 minutes per session. The exercise intensity needed to be in the range from moderate (3-6 METs) to vigorous (>6 METs), which is an intensity equivalent to various forms of dancing.(Ainsworth et al., 1993) Both dancers and non-dancers were asked to
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fill out a survey and provide the number of days per week and number of hours per session they were engaged in their relevant activity. Dancers were asked which style of dance they had the most experience in, while non-dancers were asked what type of exercise they participated in.
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Participants were excluded from this study if they had sustained any lower extremity injury within the past six months.(Arendt et al., 1999; Fernandez Regueiro, Garcia Fernandez, Nuno Mateo, & Gonzalvo Rodriguez, 2014) Lower extremity injuries that occurred more than 6
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months prior to data collection and were currently asymptomatic did not constitute
exclusion.(Scialom, Goncalves, & Padovani, 2006; Steinberg et al., 2014) Non-dancers were also
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excluded from the study if they had received any formal dance training within the past 10 years. All subjects were asked to identify which leg they would choose to kick a ball for maximum distance with, a method that has been used in previous drop landing studies to identify the dominant leg (Liederbach, Kremenic, Orishimo, Pappas, & Hagins, 2014; Orishimo et al., 2009;
2.2 Instrumentation
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Orishimo et al., 2014)
Hip muscle strength was measured using a handheld dynamometer (MicroFET2, Hoggan
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Scientific, Salt Lake City, UT, USA). Hip external rotation range of motion was measured using
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a standard goniometer, using the method described by Grossman et al. (2008) with the subject laying supine on the examination table simulating a standing first position in ballet terminology. (Figure 1) Gluteus maximus and gluteus medius activation were assessed using a wireless surface electromyography (EMG) system (Delsys Inc., Natick, MA, USA) with a sampling rate of 2000 Hz. Lower extremity 3D kinematic data were assessed using a 10-camera 3D motion capturing system (T-40S cameras, Vicon Motion Systems Ltd., Oxford, UK) with a sampling rate of 200 Hz. A 40 by 60 cm force platform (Type 9281CA, Kistler Instrument Corp., Amherst,
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NY, USA) was used to detect the instant of ground contact during the landing trials. The
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sampling rate was 2000 Hz. All systems were time synchronized.
2.3 Procedures
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FIGURE 1: Measurement of hip external rotation
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Prior to data collection, an informed consent approved by the University of Nevada, Las Vegas Institutional Review Board for Biomedical Research, was signed and collected from each participant. After enrollment, the participants were asked to complete a demographic and physical activity survey. The data collection procedures were outlined in Figure 2.
FIGURE 2: Experimental Procedures Overview
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2.3.1 Preparation of Muscle Activation Assessment
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Neuromuscular activation levels of the right gluteus maximus and gluteus medius were
assessed. Skin surfaces of the muscles were cleaned and lightly abraded. Self-adhesive wireless surface EMG electrodes were attached to the skin surface. For gluteus maximus, the electrode was placed lateral to the sacrum on the prominence of the muscle belly, in parallel to the muscle fibers. EMG signal quality was confirmed by asking the participant to contract the hip extensor muscle in a prone posture with the knee flexed. For gluteus medius, the greater trochanter was palpated and then EMG electrode was placed along the line formed by the iliac crest and greater 7
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trochanter on the prominence of the muscle belly. EMG signal quality was confirmed by asking the participant to contract the hip abductor muscles by lifting their leg up in a side-lying position. The EMG electrodes were further secured with additional adhesive tape to decrease excess
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motion and improve surface contact of the electrodes during activity. This method has been described and validated to quantify activation level of these specific muscles during weight-
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bearing activities in a previous study.(Lee & Powers, 2013)
Once electrodes were secured, maximal voluntary isometric contraction (MVIC) testing
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was performed to measure the strengths of hip abduction and hip extension and establish the peak activation level. Hip abduction strength and hip extension strength was assessed with a handheld dynamometer simultaneously during the performance of MVIC testing. Hip abduction strength was assessed as described previously (Figure 3; Leetun et al., 2004). Hip extension strength was assessed in the test position described by Souza and Powers (Figure 4), but the
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handheld dynamometer was utilized based on equipment availability, in contrast to the use of an isokinetic dynamometer. The use of a strap to secure the handheld dynamometer has been recommended to avoid the effect of examiner effort in influencing test results (Bohannon, 1999).
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In the current study, the investigator found it necessary to use light hand placement during the
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hip abduction test to prevent the dynamometer sliding anteriorly or posteriorly during testing. Every effort was made to any avoid compression of the device by the examiner during this process. The testing protocol was previously established and proven reliable (Mentiplay et al., 2015; Romero-Franco, Jimenez-Reyes, & Montano-Munuera, 2016).
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FIGURE 3: Isometric Hip Abduction Strength Testing
FIGURE 4: Isometric Hip Extension Strength Testing
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In a 5-second duration, participants were asked to generate the maximal contraction isometrically. Three trials were collected for each assessment. After the strength assessments, reflective markers were placed on the participant based on the Vicon Plug-in Gait full-body
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marker set.(Kadaba, Ramakrishnan, & Wootten, 1990) A static calibration trial that acquires the body geometry and marker placements was collected followed by a five-minute warm up on an elliptical machine to familiarize the participant with performing physical activity with the
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attached instruments.
2.3.2 Biomechanical Evaluation of Drop Landing
Participants performed bilateral drop landings from the top of a 30.5 cm box. (Fernandez Regueiro et al., 2014; Orishimo et al., 2009; Walsh, Waters, & Kersting, 2007) (Figure 5) All participants performed the test conditions with bare feet. Previous investigators have highlighted
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that dancers may not consistently wear shoes or wear the same type of shoes when dancing.(Ambegaonkar et al., 2011; Shultz & Schmitz, 2009) To control this potential confounder, we asked all study participants to perform drop landings barefoot. Participants were
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expected to perform drop landings with legs in a neutral position, with and without specific
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movement instructions (verbal instruction [VI]) vs. no instruction [NI]). The term neutral was used to describe the standard landing position for drop landings with the lower extremities in the sagittal plane, as neutral or parallel is the term dancers utilize when describing the natural, nonturned-out positions of the limbs, more commonly utilized in modern dance.(Liederbach et al., 2008)
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FIGURE 5: Drop landing Test
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In the NI conditions, participants were shown videos of the drop landing movement demonstrated by one of the investigators.(Ambegaonkar et al., 2011) The videos showed both a frontal and sagittal view. Participants were not given any verbal instruction or feedback, besides
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placing their hands on their hips to avoid obscuring the Vicon markers, but were allowed to practice at most 3 times before performing 3 captured landing trials.(Walsh et al., 2007) After
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completing the NI landings, participants watched the same demonstration videos but now with specific verbal instructions on how to perform the landings. The instructions focused on the positions of the body segments (i.e. trunk, hips, knees, and feet) during landing. Complete instructions can be found in Table 1. In this condition, the participants were allowed unlimited practice jumps and were given feedback by investigators on their movements during the practice, before 3 landing trials were captured. For the purposes of the practice trials and providing
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feedback, one investigator observed the frontal plane while a second investigator observed the sagittal plane. Verbal feedback was only provided if deemed necessary to achieve an optimal drop landing. Any instructions used were succinct, and did not deviate from the instructions on
remain as consistent as possible.
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TABLE 1: Verbal instructions given to the participants
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the video, e.g., “place your hands on your hips,” or “don’t let your knees fall in,” in order to
Instructions given: “Place your feet hip width apart and toes pointing forward. Be sure to place the balls of your feet at the front edge of the box so your toes are just hanging off. Stand with trunk erect. When you are ready, please step Drop landing off the box and land on the ground in the same position you started in. Focus on having your toes touch down first, and then your heels. Remember to maintain an upright trunk and do not let your knees fall in towards each other when landing.” *Participants were instructed to keep their hands on their hips at all times and to keep their eyes looking forward.
2.4 Data Analysis
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Test conditions:
Data from the right leg was processed and analyzed in our analysis. Collected EMG
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signals were bandpass filtered (35-500Hz, 4th order Butterworth filter), then full-wave rectified. The marker data were low-pass filtered with a cutoff frequency of 12 Hz. The 3D joint
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kinematics data were computed from the filtered marker data using a software (Visual 3D, Cmotion Inc., Rockville, MD, USA). Muscle activation and joint kinematic variables were extracted from the deceleration phase of each drop landing trial. The deceleration phase was defined from the instant of the foot contacting the force plate (defined as when the vertical ground reaction force exceeds 50 newtons), to when the knee flexion angle was greatest during a landing. Activations of the gluteus medius and maximus muscles during this period were timeaveraged and normalized as a percentage to the corresponding peak activation levels obtained
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during the MVIC trials. Peak hip adduction and knee abduction angles were also obtained during this period.
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2.5 Statistical Analysis Demographic information of the participants was compared using independent t-tests, specifically comparing age, height, weight, BMI, range of motion for hip external rotation,
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activity level, and hip abduction and extension strength between groups. Two-way mixed model ANOVAs were used to investigate the effects of group (dancers vs. non-dancers) and instruction
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(NI vs. VI, repeat-measure factor) on the lower extremity biomechanics and hip muscle activation levels during landings. When significant main effects or interactions were found, posthoc comparisons with Bonferroni correction were performed. All analyses were done using a statistical software (SPSS version 22, IBM Corp. Armonk, NY, USA). Significance level was set
3.0 RESULTS
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at 0.05.
Independent t-tests revealed no significant differences in height, weight, BMI, range of
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motion for hip external rotation, hip abduction and extension strength between the dancer and
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non-dancer groups, but did reveal a significant difference in activity level per week between the two groups (p<0.001). This is likely due to the fact that the dancers were engaged in structured practices of long duration (Table 2). Seventy-eight percent of participants identified as right leg dominant. Subjects in the dancer group reported participating in a single or combined form of dance including ballet, jazz, and modern, while one subject participated in hip-hop in addition to the three prior forms of dance, and two subjects also listed participation in contemporary dance. The non-dancer group primarily participated in running and weight-training, with two
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participants reporting participation in soccer or volleyball, respectively, and three subjects reported additional participation in yoga or Pilates.
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TABLE 2. Demographic and Anthropometric Information of the Participants.
Dancer N=(12)
Non-Dancer N=(15)
Age (yrs)
20.58±1.13
22.07±1.68
Height (m)
1.65±0.06
1.62±0.07
Weight (kg)
60.88±6.01
58.77±6.51
0.395
BMI (kg/m2)
22.4±2.3
22.5±2.3
0.981
1255.0±434.5
415.3±336.6
<0.001
56.4±7.3
56.6±11.3
0.960
53.8±8.7
55.5±9.6
0.646
Hip Abduction Strength (N)
57.7±11.6
53.9±11.3
0.397
Hip Extension Strength (N)
101.4±26.9
101.3±26.5
0.995
Right Hip ER ROM(deg.)
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0.039 0.201
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Left Hip ER ROM(deg.)
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Activity level (min/wk)
P-value
3.1Landing Biomechanics
The 2-way ANOVAs revealed that there was a strong trend of a group main effect.
Specifically dancers demonstrated trends of having greater gluteus maximus contraction and smaller knee abduction angle (gluteus maximus activation: dancers=53.8±43.3 % vs. nondancers=28.0±21.2 %, p=0.050, Cohen’s d=-0.76; peak knee abduction angle: dancers=5.3±3.2˚,
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non-dancers=8.1±4.1˚, p=0.055, Cohen’s d=0.76). No significant difference between groups were found in gluteus medius activation and hip abduction angle (p=0.309 and 0.431,
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respectively; Figures 6 and 7).
FIGURE 6: Comparison of Frontal Plane Hip and Knee Angles between Non-Dancers and Dancers
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FIGURE 7: Comparison of Gluteal Muscle Activations between Non-Dancers and Dancers
A significant main effect of instruction was detected for peak knee and hip angles across
groups (peak knee abduction angle: before instruction=6.3±3.8˚, after instruction=7.4±4.1˚, p=0.003, Cohen’s d=0.28; hip abduction angle: before instruction=3.1±3.8˚, after instruction=4.6±4.0˚, p=0.001, Cohen’s d=-0.38). There was a significant interaction between instruction and group for knee abduction angle (p=0.014; Figure 8). Based on our primary variable of knee abduction angle, we calculated the achieved power levels to be 0.91.
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FIGURE 8: Knee Abduction Angle Group (Dancer vs. Non-dancer) by Condition (No Instruction vs. Verbal Instruction) Interaction
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The post-hoc analyses showed that within the non-dancer group, there was a significant difference in knee abduction angle before and after instruction (NI vs. VI, 7.3±3.9 ˚ vs. 9.0±4.3˚,
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p<0.001, Cohen’s d=0.41), but not in the dancers (NI vs. VI, 5.2±3.6˚ vs. 5.4±3.0˚, p=0.728, Cohen’s d=0.06).
4.0 DISCUSSION
Our primary finding was that dancers exhibited a smaller knee abduction angle and greater gluteus maximus activation during drop landing tasks when compared to non-dancers. Additionally, there was a difference in how dancers and non-dancers reacted to movement
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instructions during the drop landing task. After instructions, we observed an increase in the nondancer’s knee abduction angle while dancer’s knee angle did not change significantly.
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4.1 Gluteus Maximus Activation Preventing ACL injuries is a top priority in female youth sports. The effectiveness of specific movement training programs designed to address this concern has been demonstrated,
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and commonly focuses on tasks requiring gluteus maximus activation during landing and
deceleration. (Alentorn-Geli et al., 2009b; Pollard, Sigward, & Powers, 2010). ACL injury
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prevention programs have been developed that include activities known to target the hip extensors and hip abductors with specific lower extremity strengthening exercises and plyometric drills.(Alentorn-Geli et al., 2009b; Mandelbaum et al., 2005) The extensive training dancers receive in a turned-out position may increase their ability to engage the gluteus maximus
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muscles during the deceleration phase of landing tasks. Increased gluteus maximus activation may also lead to a decreased knee frontal angle on landing as a result of greater femoral rotation control.(Hollman, Hohl, Kraft, Strauss, & Traver, 2013) In our study, the dancers exhibited on
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average 20-25% greater gluteus maximus activation and a smaller knee abduction angle during the deceleration phase of landing when compared to non-dancers. While these two findings
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might be coincidental, one might theorize that the increased gluteus maximus activation plays a role in the smaller knee abduction angle. With excessive knee valgus (i.e. knee abduction) angle being a factor for knee ligamentous injuries (Alentorn-Geli et al., 2009a; Ford, Myer, & Hewett, 2003), the dancers appeared to be employing a protective landing strategy, potentially explaining the lower rate of ACL injuries within this population. From a clinical perspective, there was not a statistically significant difference in hip abduction or hip extension strength between the two groups on assessment. This may indicate that an athlete’s ability to activate the gluteus muscles
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during landings is a key component to ACL injury prevention and a more informative measurement to assess when screening athletes than isometric hip strength in isolation.
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In agreement with previous studies, we observed peak knee abduction angle occurring within 100 ms after ground contact. Previous video analysis of ACL injuries suggests that
injurious incidences typically occur 17-50 ms after ground contact.(Krosshaug et al., 2007) The
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knee is most vulnerable to ligamentous injuries quickly after contact, hence a person is unlikely to use feedback response to correct knee motions. A feed-forward motor planning strategy that
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aligns the lower extremity prior to landing would be far more efficient and protective during high velocity loading and impact activities. For the purposes of this study, we opted to use a bilateral drop landing technique as described in studies examining athletes, dancers and recreationally active individuals who were college aged in similarity to our population.(Carcia, Eggen, & Shultz, 2005; Didier, 2001; Walsh et al., 2007). Krosshaug et al. have questioned the use of
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bilateral drop landing tests in detecting ACL injury risks factors, although the authors themselves noted their study population was an elite group of athletes, compared to the previous studies. (Carcia et al., 2005; Hewett et al., 2005; Krosshaug et al., 2016; Walsh et al., 2007) The bilateral
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drop landing task we utilized does not challenge femoral adduction as much as a single-leg
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landing and may explain our findings of less than 4° peak knee abduction angle between the dancers and non-dancers. We might have observed greater knee abduction due to insufficient gluteal muscle activation, if a more challenging single leg task was used.(Powers, 2003) When examining the performance of a single-legged drop landing task, Orishimo et al.
found no difference between professional male and female dancers, and subsequently demonstrated female sports athletes landed with greater peak knee valgus in comparison to both male and female dancers during the same task.(Orishimo et al., 2009; Orishimo et al., 2014) It
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was revealing that the dancers in our study landed with the knee in a more neutral alignment when compared to the non-dancers. This confirms the findings from Orishomo et al., and raises the question of what can clinicians learn from dancers in the development of training strategies
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to prevent ACL injuries? In a clinical or athletic setting, it might be relevant to incorporate
specific interventions to target gluteus maximus muscle activation not just strengthening, along with repetitive jump training using single and bilateral landing techniques, to potentially reduce
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the risk of ACL injury in other athletic populations. Future studies may focus on examining the amount of practice that is required to lead to the level of difference between dancers and non-
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dancers observed in this study. Prospective studies are also needed to examine if gluteal muscle activation training is effective for reducing ACL injury rate.
4.2 Movement Instruction
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The addition of verbal instructions in this study resulted in a small but significant increase (~2°) in the knee abduction angle in the non-dancers. This was unexpected as explicit instruction: “…do not let your knees fall in toward each other when landing” was included in the
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cuing. The significant interaction between group and instruction demonstrated that dancers and non-dancers responded differently to the introduction of movement instruction. This may be due
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to different interpretation, processing, and execution of the verbal movement instruction by the two groups.
Our specific verbal movement instructions may have negatively influenced the landing
mechanics as the cues were primarily focused internally on body alignment. While such instructions have been commonly used in ACL injury prevention programs,(Myer, Ford, & Hewett, 2004) the internal attentional focus may interfere with movement performance. A
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number of previous studies have demonstrated the negative influence of internal attentional focus and excessive instruction on motor learning and performance.(Abdollahipour, Psotta, & Land, 2016; Halperin, Chapman, Martin, & Abbiss, 2017; Wulf, 2013; Wulf & Weigelt, 1997) This
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may explain why some ACL injury prevention programs were not effective. (Noyes & Barber Westin, 2012) However, dancers are used to receiving a combination of internally and externally focused instruction which may account for the lack of performance degradation. Research has
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suggested, that the enhanced proprioception documented in dancers provides them with the
ability to shift their attentional focus away from basic tasks in order to focus on the execution of
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more challenging tasks.(Manrique Arija et al., 2011) Therefore, one might argue that the basic landing activity required during this study was a task the dancers were already skilled in performing and consequently uninfluenced by the verbal instructions. Dancers possess enhanced proprioception, motor control and coordination considered to
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be a result of their training, although some authors have argued that individuals with enhanced abilities are more likely to become dancers. (Jola, Davis, & Haggard, 2011; Manrique Arija et al., 2011) There is likely an argument for a combination of both. Marmeleira et al. demonstrated
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improved knee joint position sense and knee kinesthesia following a 12 week creative dancing program for older adults suggesting training programs may be beneficial in enhancing lower
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limb proprioception regardless of age and experience.(Marmeleira et al., 2009) Based on our study, we suggest future research should look at multiple factors in the design of ACL injury prevention programs including focusing on enhancing proprioception of the entire lower kinetic chain, the effect of internal versus external attentional focus and prior training experience on the biomechanics of landing. Currently published research suggests external attentional focus is beneficial in improving movement patterns to reduce second ACL injury risk following ACL
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reconstruction (Gokeler et al., 2015). It could be argued there might be a role for a combination of both internal and external cuing based on the motor learning stage of the participant, with greater internal cuing required in the earlier stages of motor learning, although this has yet to be
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clearly established in the literature regarding ACL prevention programs. (Kiani et al., 2010; Myer, Sugimoto, Thomas, & Hewett, 2013) Screening for the existence of enhanced
proprioception prior to participation in an at-risk sport might also be relevant in decreasing the
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frequency of ACL injury. One might also question if the inclusion of specific jump training focusing on lower extremity alignment, technique and gluteus maximus activation, as dancers
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typically do from an early age, would have a role in diminishing ACL injury rates if integrated into injury prevention programs and the athletic settings at an earlier age for at-risk athletes? This study has a number of limitations. The dancers were recruited from the Dance Department of a university, and demonstrated significantly higher activity levels compared to
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non-dancers who were mostly recreationally active. The dancers had mixed training backgrounds including both modern and ballet, although the emphasis at this particular university is in modern dance. This may have explained the lack of statistically significant difference in hip external
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range of motion range of motion between the two groups, with modern dancers self-selecting for that style of performance, versus primarily ballet training with an expectation of greater hip
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turnout. Having the subjects perform the study barefoot may have provided the dancers with an advantage, particularly modern dancers who frequently perform barefooted. Furthermore, we only observed the acute effects of instruction on the kinematics of landing; it was unknown if the same effects would be observed if the participants had more time to practice the landing movements as in a typical ACL injury prevention program, or if the changes in kinematics would translate to kinetic risk factors such as increased knee abduction moment. Biomechanical
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comparison of dancers versus other high level athletes with similar levels of activity participation in their respective sports would help elucidate the influence of training experience on landing
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mechanics and injury.
5.0 CONCLUSION
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This study demonstrates that during landing dancers employed movement and
neuromuscular control strategies that were different from non-dancers. Specifically dancers
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exhibited increased activation of the gluteus maximus and a reduction of knee abduction angle; these differences are potentially protective attributes against non-contact ACL injuries. In addition, dancers and non-dancers appeared to respond differently to movement instructions. Our data showed that landing mechanics of non-dancers were disrupted after receiving movement cues, indicating that the lack of experience in processing explicit movement instructions can
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impact acute motor learning. Findings from this study may provide argument for incorporating motor learning concepts associated with dance education into ACL injury prevention programs. However, future research should investigate the most beneficial forms of cuing, and the optimal
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developmental motor learning period to initiate preventative training strategies for ACL injuries.
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players. Part 1: Mechanisms of injury and underlying risk factors. Knee Surg Sports Traumatol Arthrosc, 17(7), 705-729. doi:10.1007/s00167-009-0813-1 Alentorn-Geli, E., Myer, G. D., Silvers, H. J., Samitier, G., Romero, D., Lazaro-Haro, C., & Cugat, R. (2009b). Prevention of non-contact anterior cruciate ligament injuries in soccer players. Part 2: a review of prevention programs aimed to modify risk factors and to reduce injury rates. Knee Surg Sports Traumatol Arthrosc, 17(8), 859-879. doi:10.1007/s00167-009-0823-z Ambegaonkar, J. P., Shultz, S. J., Perrin, D. H., Schmitz, R. J., Ackerman, T. A., & Schulz, M. R. (2011). Lower body stiffness and muscle activity differences between female dancers and basketball players during drop jumps. Sports Health, 3(1), 89-96. doi:10.1177/1941738110385998 Arendt, E. A., Agel, J., & Dick, R. (1999). Anterior cruciate ligament injury patterns among collegiate men and women. J Athl Train, 34(2), 86-92. Bohannon, R. W. (1999). Intertester reliability of hand-held dynamometry: a concise summary of published research. Percept Mot Skills, 88(3 Pt 1), 899-902. Buller, L. T., Best, M. J., Baraga, M. G., & Kaplan, L. D. (2015). Trends in Anterior Cruciate Ligament Reconstruction in the United States. Orthop J Sports Med, 3(1), 2325967114563664. doi:10.1177/2325967114563664 Carcia, C., Eggen, J., & Shultz, S. (2005). Hip-abductor fatigue, frontal-plane landing angle, and excursion during a drop jump. J Sports Rehabil, 14, 321-331. Chappell, J. D., Creighton, R. A., Giuliani, C., Yu, B., & Garrett, W. E. (2007). Kinematics and electromyography of landing preparation in vertical stop-jump: risks for noncontact anterior cruciate ligament injury. Am J Sports Med, 35(2), 235-241. doi:10.1177/0363546506294077 Chimera, N. J., Swanik, K. A., Swanik, C. B., & Straub, S. J. (2004). Effects of Plyometric Training on Muscle-Activation Strategies and Performance in Female Athletes. J Athl Train, 39(1), 24-31. Delp, S. L., Hess, W. E., Hungerford, D. S., & Jones, L. C. (1999). Variation of rotation moment arms with hip flexion. J Biomech, 32(5), 493-501. Didier, J. J. (2001). Vertical jumping and landing mechanics: Female athletes and nonathletes. International Journal of Athletic Therapy & Training(November), 17-20. Fernandez Regueiro, R., Garcia Fernandez, M. E., Nuno Mateo, F. J., & Gonzalvo Rodriguez, P. (2014). Patient with arthritis knee and palmoplantar pustulosis. Rev Clin Esp, 214(6), e73-e74. doi:10.1016/j.rce.2014.02.005 Ford, K. R., Myer, G. D., & Hewett, T. E. (2003). Valgus knee motion during landing in high school female and male basketball players. Med Sci Sports Exerc, 35(10), 1745-1750. doi:10.1249/01.MSS.0000089346.85744.D9 Gokeler, A., Benjaminse, A., Welling, W., Alferink, M., Eppinga, P., & Otten, B. (2015). The effects of attentional focus on jump performance and knee joint kinematics in patients after ACL reconstruction. Phys Ther Sport, 16(2), 114-120. doi:10.1016/j.ptsp.2014.06.002 Griffin, L. Y., Agel, J., Albohm, M. J., Arendt, E. A., Dick, R. W., Garrett, W. E., . . . Wojtys, E. M. (2000). Noncontact anterior cruciate ligament injuries: risk factors and prevention strategies. J Am Acad Orthop Surg, 8(3), 141-150.
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ACKNOWLEDGEMENTS We would like to acknowledge the UNLV Department of Dance for their help and cooperation
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with our participant recruitment, UNLV Department of Kinesiology for allowing us to use the Sports Injury Research Center for data collection, and UNLV Department of Physical Therapy and the Clinical Locomotion Neuromechanics Laboratory for research support. This study
received funding from the UNLV Physical Therapy Department in the form of a UNLVPT
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Student Opportunity Research Grant. We would also like to thank Josh Bailey for his help with
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data collection, and Ron Garcia for his help with data processing. This study was approved by
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the Institutional Review Board at University of Nevada, Las Vegas, NV.
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Highlights: Dancers showed higher hip muscle activation compared to non-dancers during landing.
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Dancers land with more neutral knee kinematics (less valgus) than non-dancers.
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Acute movement instruction deteriorated landing mechanics in non-dancers only.
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Dance training experience may provide protection against ACL injury.
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Movement-based ACL injury prevention program may be modelled after dance training.
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Funding Statement:
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This study received funding from the UNLV Physical Therapy Department in the form of a UNLVPT Student Opportunity Research Grant.