- Email: [email protected]
Time-to-contact demonstrates modulation of postural control during a dynamic lower extremity task
Gait & Posture 38 (2013) 658–662
Contents lists available at SciVerse ScienceDirect
Gait & Posture journal homepage: www.elsevier.com/locate/gaitpost
Time-to-contact demonstrates modulation of postural control during a dynamic lower extremity task§ Sarah Schloemer a,1, Joshua Cotter b,1, Steve Jamison c,1, Ajit Chaudhari a,1,* a
The Ohio State University, Columbus, OH, USA University of California, Irvine, CA, USA c University of Delaware, Newark, DE, USA b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 June 2012 Received in revised form 7 December 2012 Accepted 20 February 2013
Postural control deficits are associated with increased risk of loss of balance and potential injury. To assess balance deficits and estimate injury risk, there is a need to evaluate postural control during dynamic activities. Analysis during dynamic activities could assess if an individual’s ability to control their posture is a fixed condition or if it is dependent on the demands of a task. The purpose of this study was to evaluate changes in postural control during a dynamic lower extremity task using time-tocontact (TtC) analysis. 3D motion capture with a force plate was used to evaluate 46 healthy recreational athletes performing an anterior reach with the right foot while standing on their left leg. TtC was calculated for nine valid trials. For each trial, the time from the toe leaving the force plate to the toe touching the floor at the maximum reach distance was divided into five epochs of equal duration. TtC was averaged over each epoch. Differences in TtC were evaluated with an unbalanced mixed effects ANOVA and post hoc Tukey’s HSD comparisons. Epoch was a significant main effect (p < 0.001), with both Epoch 4 and Epoch 5 having significantly greater TtC from all other epochs (p = 0.05). Increasing TtC in later epochs suggests a higher demand for postural control when the task becomes more challenging. As an individual’s reaching foot extends further from the body, postural control is adjusted to match the changing demands of the dynamic task. ß 2013 Elsevier B.V. All rights reserved.
Keywords: Center of pressure Balance Functional movement
1. Introduction Human beings carry out a wide variety of dynamic activities during their daily lives. Throughout a movement and even while standing, humans are in dynamic, rather than static, equilibrium [1]. Dynamic equilibrium is the maintenance of control over the body segments during movement to prevent falling [2]. Balance is essential to successfully completing a task, and thus, skilled movements are dependent upon an individual’s ability to maintain equilibrium and a stable posture in a variety of positions and under many conditions [3]. Individuals who are incapable of controlling their posture throughout an activity are likely to experience a fall and are at risk of incurring injuries associated with falls, as well as
§
Partial funding for this study was provided by Makkar Athletics, Inc. * Corresponding author at: OSU Sports Medicine, The Ohio State University, 2050 Kenny Rd Suite 3100, Columbus, OH 43221, USA. Tel.: +1 614 293 2409. E-mail address: [email protected] (A. Chaudhari). 1 A all authors were fully involved in the study and preparation of the manuscript. The material within has not been and will not be submitted for publication elsewhere. 0966-6362/$ – see front matter ß 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gaitpost.2013.02.012
other injuries such as ankle sprains [4] and both primary [5] and secondary [6] anterior cruciate ligament tears. To better evaluate and treat individuals with musculoskeletal pathologies, biomechanical abnormalities, a loss of proprioception, and impaired neuromuscular control systems, it is necessary to understand the mechanisms and strategies by which humans control their posture during dynamic tasks. To successfully complete the dynamic activities of daily life, it may be necessary to modify the degree of postural control exerted during the course of the task. Analysis of postural control during dynamic tasks could provide insight into whether an individual is capable of adjusting his/her postural control as the demands of a task change or if the level of control is held constant throughout the task. Postural time to contact (TtC) is a quantitative measure of postural control which represents the time it would take the center of pressure (CoP) to move outside the body’s base of support, based on a trajectory calculated from the CoP’s instantaneous position, velocity, and acceleration [7]. The time required for the CoP to travel along the trajectory and contact the base of support is the TtC. A shorter TtC indicates that the postural control system has less time to make a correction in posture before potentially experiencing a loss of balance. Therefore, shorter TtC will be
S. Schloemer et al. / Gait & Posture 38 (2013) 658–662
659
associated with instances of less postural control because CoP movement variability is less restricted. By evaluating TtC at each frame in a trial, it is possible to track temporal changes in postural control through the course of a trial. Because TtC quantifies postural control at each frame, it allows for detection of instantaneous modifications in postural control during dynamic tasks [8]. Thus, TtC can distinguish between periods of relative postural stability and instability within a task [9]. To assess the utility of TtC to quantify postural control, Haddad et al. used TtC to investigate changes in postural control during a dynamic upper extremity task. Participants stood with both feet on a force plate, picked up a block off a table, and then pushed it through either a large or small hole positioned at shoulder height and arm’s length [8]. Changes in TtC within each trial were determined by normalizing each time series to 100 data points and averaging TtC data over epochs of 10 data points. Results showed that TtC was longer in the later epochs of the trial as the demand for precision increased while the subjects fit the block through the hole. Furthermore, subjects had a longer TtC when fitting the block through a smaller opening, suggesting that they maintained stricter postural control when the task required greater precision. While postural control has been quantified during quiet standing and dynamic upper extremity tasks, no study has evaluated postural control during dynamic lower extremity activities. Therefore, the purpose of this study was to test the hypothesis that postural control varies during the course of a dynamic balance test, using TtC as the measure of postural control.
2. Methods Forty-five healthy recreational athletes (37M/8F, 22.9 5.1y, 75.0 13.8 kg, 1.75 0.10 m) participated in this study. Prior to testing, all subjects provided IRB-approved informed consent. Subjects were required to be between 18 and 59 years of age with greater than two years of current involvement in a sport in which they competed at least once per year. Subjects regularly trained more than three times a week with a weekly training time greater than three hours for three months prior to testing. Individuals experiencing any pain that limited movement, wearing a cardiac pacemaker, or known to be pregnant were excluded from this study. Postural control was assessed during an anterior reach of the right foot performed while balancing on the left leg (Fig. 1). The task required subjects to stand on their left foot while reaching their right foot as far as possible without elevating their left heel or taking their hands off their hips [10]. Subjects lightly tapped their right foot at maximum reach and returned to the start position in a controlled manner. This task was chosen because it is more challenging than traditional balance tests and therefore may be more effective at discriminating between healthy, athletic individuals. Moreover, anterior reach distance asymmetry between limbs has been shown to be a reliable predictor of lower extremity injury in high school basketball players [11] as well as a metric for differentiating between healthy controls and individuals with chronic ankle instability [12,13], indicating it provides a sufficient challenge to discriminate within athletic populations. Subjects performed 4 practice trials because previous work has shown that normalized maximum reach distance stabilizes after approximately 4 trials [14]. Following the practice trials, 3 sets of 3 valid trials were recorded, for a total of 9 trials for each subject, or 405 total trials (45 subjects 9 trials per subject). Sets were performed several minutes apart. A trial was considered invalid and repeated if hands came off hips, the stance heel became elevated, the reach toe was not tapped lightly, or if any portion of the movement was performed in an uncontrolled manner. Due to missing markers or problems with the data collection software a
Fig. 1. Subject performing the anterior reaching task while balancing on a force plate.
small number of trials could not be processed, leaving a total of 397 trials used in the analysis. Kinematic data of the feet were collected for each trial at 120 Hz with eight Vicon MX-F40 motion capture cameras (Vicon, Oxford, UK). Markers placed on the 1st toe, lateral malleolus, medial malleolus, and heel of the left foot were used to identify the base of support (Fig. 2). The foot was aligned with the global y-axis and the perimeter of the base of support was formed by connecting the 1st toe to the medial malleolus, the medial malleolus to the heel, and the heel to the lateral malleolus with straight lines. The anterior and lateral borders of the base of support were drawn parallel to the global x- and y-axes, respectively (Fig. 2). A marker on the right 1st toe was used to identify the moment the subject tapped the ground at their maximum reach distance. CoP data were also collected at 600 Hz during each trial using one 4060-10 Bertec force plate (Bertec Corp., Columbus, OH). The time it would take the center of pressure (CoP), given its instantaneous position, velocity, and acceleration, to contact the perimeter of the base of support (TtC) was calculated for each frame between toe off and toe touch at maximum reach using
S. Schloemer et al. / Gait & Posture 38 (2013) 658–662
660
Fig. 2. (A) Base of support for the left foot showing a sample trajectory (dashed line) from the instantaneous center of pressure (black diamond). COP excursion (dotted line) shown for the entire reach time. (B) Left foot with the 4 markers used to define the base of support.
custom MATLAB (MathWorks, Natick, MA) scripts and previously developed equations [7]. Toe off was defined as the sample during which the subject’s right foot was completely off the force plate and movement was initiated. Toe touch was defined as the frame during which the subject’s right 1st toe marker reached a vertical minimum. To determine how postural control changed during the trial, TtC was then averaged over 5 epochs, each equal to a fifth of the reach time, such that Epoch 1 began with toe off and Epoch 5 ended with toe touch at maximum reach. To evaluate epoch as a fixed effect and subject as a random effect on TtC, an unbalanced mixed effects ANOVA was used. Post hoc Tukey’s HSD comparisons (a = 0.05) were subsequently used to determine differences between individual epochs. Reaching velocity was not controlled between trials or between subjects. TtC is a temporal measure and therefore may be impacted by how quickly a subject completes a reach. To assess if reaching velocity significantly affected TtC, linear regressions were performed between mean reaching velocity and mean TtC at each epoch across all subjects. All statistical analyses were performed using custom MATLAB scripts. 3. Results The results of the ANOVA yielded epoch as a significant main effect (p < 0.001). The post hoc Tukey’s HSD showed that the TtC of Epochs 1, 2, and 3 were not significantly different from one another (Table 1). However, Epoch 4 had a significantly longer TtC than Epochs 1, 2, and 3 (p < 0.05), while Epoch 5 had a significantly longer TtC than all other epochs (p < 0.05; Table 1; Fig. 3). Mean reach velocity was not associated with mean TtC for any epoch (R2 = 0.001–0.068). Average statistics and ranges for variables of interest can be found in Table 2. 4. Discussion This study is the first to our knowledge to demonstrate that healthy individuals modulate their postural control during Table 1 Marginal means for the TtC differences between epochs. Epoch comparison
Mean difference (ms)
95% Confidence interval
1!2 2!3 3 ! 4a 4 ! 5a
3.6 7.5 13.4 22.4
(4.6, 11.8) (0.7, 15.7) (5.2, 21.6) (14.2, 30.6)
a
Indicates significance (p < 0.01).
dynamic lower extremity tasks. A period of less control occurs during Epochs 1, 2 and 3, while the reaching foot is held relatively close to the body, suggesting that healthy individuals may not perceive a need to maintain strict postural stability during ‘‘easier’’ periods of a task. These results are consistent with previous studies comparing athletes who are balance trained versus those who are not. Studies of ballet dancers compared to non-dancers during quiet standing show that dancers exhibit greater variability in postural sway as exemplified by lower mathematical stability and increased noisiness in the CoP time series [15] and shorter time-toboundary (calculated as instantaneous distance to the base of support divided by instantaneous velocity) [16]. The authors of these studies conclude that due to their superior balance training, ballet dancers can allow more flexibility in their posture during less demanding tasks. Fidler et al. theorized that the amount of variability in postural control an individual exhibits is related to the difficulty of the task, such that when the neuromuscular system can adeptly meet the demands of the task, the individual does not need to strictly maintain their posture [16]. The progressive increase in TtC from Epoch 3 to Epoch 5 represents a corresponding increase in postural stability as the subject’s right foot nears the point of toe touch. The challenge to the postural control system increases during this period of the test, both due to the requirement to touch the toe at a single point with minimal force and because the reaching foot is being extended further from the base of support. Therefore, the longer TtC observed suggests individuals may be exhibiting greater postural control to maintain stability. Haddad et al. observed a similar TtC pattern for the precision fitting task, noting that TtC was longer in the later epochs of the task [8]. Furthermore, longer TtC was observed throughout the trials that required the subjects to fit the block through the smaller hole. These results suggest that healthy individuals incrementally increase their control as the difficulty and precision demands of the task increase. Due to its ability to quantify modifications of control throughout a task TtC may be an effective measure for assessing postural control during a functional performance activity, which may be beneficial for evaluating risk of injury or rehabilitation progress. Our theory that longer TtC during the later stages of the reach indicate individuals exercise increased postural control during the more difficult portion of the task is also supported by the mean CoP velocities during each epoch (Table 2). Both medial–lateral (ML) and anterior–posterior (AP) CoP velocity are greater during the earlier epochs, suggesting subjects are allowing greater CoP movement variability at the beginning of the reach. During Epoch
S. Schloemer et al. / Gait & Posture 38 (2013) 658–662
661
Fig. 3. Average time to contact (TtC) during each epoch. Error bars indicate the 99% confidence intervals of the means.
Table 2 Mean CoP measures by epoch. Epoch
1
2
3
4
5
TtC (ms) Reach velocity (mm/s) R2 a
274.49 34.91 1030.3 301.3 0.017
270.50 38.56 851.4 368.1 0.068
277.40 36.16 386.5 229.1 0.022
290.74 31.65 126.2 97.4 0.007
313.12 36.78 18.43 44.02 0.001
ML COP velocity (mm/s) AP COP velocity (mm/s)
53.85 13.70 98.04 29.09
54.71 13.89 105.63 28.64
51.85 10.33 108.79 33.44
43.39 10.46 92.09 28.78
34.24 10.48 64.40 16.91
a
R2 values for subject mean epoch reaching velocity as a predictor of subject mean epoch TtC.
5, AP and ML CoP velocity are at their minimum, providing evidence that individuals are restricting the movement of their CoP considerably more as they execute the toe tap at maximum reach. There are limitations to this study that may provide direction for future investigations. The subject population examined in this study consisted of healthy, recreational athletes. While this population can provide insights into how healthy individuals modulate postural control, we cannot establish what thresholds of TtC are appropriate or allowable during a dynamic task to avoid injury. To gain a fuller understanding of how TtC relates to postural stability and instability, comparisons of TtC between healthy controls, individuals with postural control deficiencies and individuals with elite levels of balance (such as dancers, ice skaters, martial artists or gymnasts) should be evaluated during functional dynamic activities. Future studies should evaluate the efficacy of using TtC analysis during the anterior reach or other dynamic balance tasks in individuals following acute lower extremity injury to monitor rehabilitation and evaluate side to side asymmetries. Other single-leg lower extremity tasks evaluated using the TtC measure may prove useful to better understand balance deficits in individuals with neuropathologies such as Parkinson’s disease or stroke. In addition, this study did not compare TtC to traditional CoP measures such as CoP excursion or velocity. It may be possible to average these measures over epochs within a trial to reveal changes in postural control as well. Further research is necessary to determine which measures may be most sensitive and meaningful to postural control changes within a task. Another limitation of this study is reaching speed was not controlled between trials or subjects. Epoch length across all subjects averaged 286.7 ms with a standard deviation of 142.5 ms. Due to the relationship between the movements of the center of mass (CoM) and CoP, to stay balanced while reaching, the movement of the CoM must be controlled such that the CoP remains within the base of support. Assuming subjects reached at a
speed they felt would not compromise their balance, it is reasonable that reaching speed is not strongly associated with TtC because subjects are maintaining their balance, and thus limiting the movement of their CoM regardless of reaching speed. This study has shown how an individual modulates postural control in accordance with the demands of a dynamic lower extremity task. Our results support the theory that an increase in the difficulty of a task elicits a corresponding increase in postural control in healthy individuals. This ability of TtC to quantify instantaneous modifications in postural control during functional movement tasks may increase the capacity to discriminate between subtle differences in neurological or orthopedic pathologies in both the research and clinical setting.
Conflict of interest statement The authors have no financial or personal relationships to disclose.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.gaitpost.2013. 02.012. References [1] Green J. An introduction to human physiology. 4th ed. Oxford: Oxford University Press; 1978. [2] Howe T, Oldham J. Posture and balance. In: Trew M, Everett T, editors. Human movement: an introductory text. 4th ed., Edinburgh: Churchill Livingstone; 2001. p. 225–39. [3] Davies P. Steps to follow. Berlin: Springer-Verlag; 1985.
662
S. Schloemer et al. / Gait & Posture 38 (2013) 658–662
[4] McGuine TA, Greene JJ, Best T, Leverson G. Balance as a predictor of ankle injuries in high school basketball players. Clinical Journal of Sport Medicine 2000;10:239–44. [5] Paterno MV, Schmitt LC, Ford KR, et al. Biomechanical measures during landing and postural stability predict second anterior cruciate ligament injury after anterior cruciate ligament reconstruction and return to sport. The American Journal of Sports Medicine 2010;38:1968–78. [6] Vrbanic´ TS, Ravlic´-Gulan J, Gulan G, Matovinovic´ D. Balance index score as a predictive factor for lower sports results or anterior cruciate ligament knee injuries in Croatian female athletes—preliminary study. Collegium Antropologicum 2007;31:253–8. [7] Slobounov SM, Slobounova ES, Newell KM. Virtual time-to-collision and human postural control. Journal of Motor Behavior 1997;29: 263–81. [8] Haddad JM, Ryu JH, Seaman JM, Ponto KC. Time-to-contact measures capture modulations in posture based on the precision demands of a manual task. Gait Posture 2010;32:592–6. [9] Hertel J, Olmsted-Kramer LC, Challis JH. Time-to-boundary measures of postural control during single leg quiet standing. Journal of Applied Biomechanics 2006;22:67–73.
[10] Gribble PA, Hertel J, Denegar CR, Buckley WE. The effects of fatigue and chronic ankle instability on dynamic postural control. Journal of Athletic Training 2004;39:321–9. [11] Plisky PJ, Rauh MJ, Kaminski TW, Underwood FB. Star excursion balance test as a predictor of lower extremity injury in high school basketball players. The Journal of Orthopaedic and Sports Physical Therapy 2006;36:911–9. [12] Olmsted LC, Carcia CR, Hertel J, Shultz SJ. Efficacy of the star excursion balance tests in detecting reach deficits in subjects with chronic ankle instability. Journal of Athletic Training 2002;37:501–6. [13] Hertel J, Braham RA, Hale SA, Olmsted-Kramer LC. Simplifying the star excursion balance test: analyses of subjects with and without chronic ankle instability. The Journal of Orthopaedic and Sports Physical Therapy 2006;36:131–7. [14] Robinson RH, Gribble PA. Support for a reduction in the number of trials needed for the star excursion balance test. Archives of Physical Medicine and Rehabilitation 2008;89:364–70. [15] Schmit JM, Regis DI, Riley MA. Dynamic patterns of postural sway in ballet dancers and track athletes. Experimental Brain Research 2005;163:370–8. [16] Fidler AH, Gagnon JM, Van Emmerik JL, Hammil REAJ. In: Wang IQ, editor. Postural control strategies in dancers and non dancers. Beijing, China: ISBS; 2005. p. 642–5.
Contents lists available at SciVerse ScienceDirect
Gait & Posture journal homepage: www.elsevier.com/locate/gaitpost
Time-to-contact demonstrates modulation of postural control during a dynamic lower extremity task§ Sarah Schloemer a,1, Joshua Cotter b,1, Steve Jamison c,1, Ajit Chaudhari a,1,* a
The Ohio State University, Columbus, OH, USA University of California, Irvine, CA, USA c University of Delaware, Newark, DE, USA b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 June 2012 Received in revised form 7 December 2012 Accepted 20 February 2013
Postural control deficits are associated with increased risk of loss of balance and potential injury. To assess balance deficits and estimate injury risk, there is a need to evaluate postural control during dynamic activities. Analysis during dynamic activities could assess if an individual’s ability to control their posture is a fixed condition or if it is dependent on the demands of a task. The purpose of this study was to evaluate changes in postural control during a dynamic lower extremity task using time-tocontact (TtC) analysis. 3D motion capture with a force plate was used to evaluate 46 healthy recreational athletes performing an anterior reach with the right foot while standing on their left leg. TtC was calculated for nine valid trials. For each trial, the time from the toe leaving the force plate to the toe touching the floor at the maximum reach distance was divided into five epochs of equal duration. TtC was averaged over each epoch. Differences in TtC were evaluated with an unbalanced mixed effects ANOVA and post hoc Tukey’s HSD comparisons. Epoch was a significant main effect (p < 0.001), with both Epoch 4 and Epoch 5 having significantly greater TtC from all other epochs (p = 0.05). Increasing TtC in later epochs suggests a higher demand for postural control when the task becomes more challenging. As an individual’s reaching foot extends further from the body, postural control is adjusted to match the changing demands of the dynamic task. ß 2013 Elsevier B.V. All rights reserved.
Keywords: Center of pressure Balance Functional movement
1. Introduction Human beings carry out a wide variety of dynamic activities during their daily lives. Throughout a movement and even while standing, humans are in dynamic, rather than static, equilibrium [1]. Dynamic equilibrium is the maintenance of control over the body segments during movement to prevent falling [2]. Balance is essential to successfully completing a task, and thus, skilled movements are dependent upon an individual’s ability to maintain equilibrium and a stable posture in a variety of positions and under many conditions [3]. Individuals who are incapable of controlling their posture throughout an activity are likely to experience a fall and are at risk of incurring injuries associated with falls, as well as
§
Partial funding for this study was provided by Makkar Athletics, Inc. * Corresponding author at: OSU Sports Medicine, The Ohio State University, 2050 Kenny Rd Suite 3100, Columbus, OH 43221, USA. Tel.: +1 614 293 2409. E-mail address: [email protected] (A. Chaudhari). 1 A all authors were fully involved in the study and preparation of the manuscript. The material within has not been and will not be submitted for publication elsewhere. 0966-6362/$ – see front matter ß 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gaitpost.2013.02.012
other injuries such as ankle sprains [4] and both primary [5] and secondary [6] anterior cruciate ligament tears. To better evaluate and treat individuals with musculoskeletal pathologies, biomechanical abnormalities, a loss of proprioception, and impaired neuromuscular control systems, it is necessary to understand the mechanisms and strategies by which humans control their posture during dynamic tasks. To successfully complete the dynamic activities of daily life, it may be necessary to modify the degree of postural control exerted during the course of the task. Analysis of postural control during dynamic tasks could provide insight into whether an individual is capable of adjusting his/her postural control as the demands of a task change or if the level of control is held constant throughout the task. Postural time to contact (TtC) is a quantitative measure of postural control which represents the time it would take the center of pressure (CoP) to move outside the body’s base of support, based on a trajectory calculated from the CoP’s instantaneous position, velocity, and acceleration [7]. The time required for the CoP to travel along the trajectory and contact the base of support is the TtC. A shorter TtC indicates that the postural control system has less time to make a correction in posture before potentially experiencing a loss of balance. Therefore, shorter TtC will be
S. Schloemer et al. / Gait & Posture 38 (2013) 658–662
659
associated with instances of less postural control because CoP movement variability is less restricted. By evaluating TtC at each frame in a trial, it is possible to track temporal changes in postural control through the course of a trial. Because TtC quantifies postural control at each frame, it allows for detection of instantaneous modifications in postural control during dynamic tasks [8]. Thus, TtC can distinguish between periods of relative postural stability and instability within a task [9]. To assess the utility of TtC to quantify postural control, Haddad et al. used TtC to investigate changes in postural control during a dynamic upper extremity task. Participants stood with both feet on a force plate, picked up a block off a table, and then pushed it through either a large or small hole positioned at shoulder height and arm’s length [8]. Changes in TtC within each trial were determined by normalizing each time series to 100 data points and averaging TtC data over epochs of 10 data points. Results showed that TtC was longer in the later epochs of the trial as the demand for precision increased while the subjects fit the block through the hole. Furthermore, subjects had a longer TtC when fitting the block through a smaller opening, suggesting that they maintained stricter postural control when the task required greater precision. While postural control has been quantified during quiet standing and dynamic upper extremity tasks, no study has evaluated postural control during dynamic lower extremity activities. Therefore, the purpose of this study was to test the hypothesis that postural control varies during the course of a dynamic balance test, using TtC as the measure of postural control.
2. Methods Forty-five healthy recreational athletes (37M/8F, 22.9 5.1y, 75.0 13.8 kg, 1.75 0.10 m) participated in this study. Prior to testing, all subjects provided IRB-approved informed consent. Subjects were required to be between 18 and 59 years of age with greater than two years of current involvement in a sport in which they competed at least once per year. Subjects regularly trained more than three times a week with a weekly training time greater than three hours for three months prior to testing. Individuals experiencing any pain that limited movement, wearing a cardiac pacemaker, or known to be pregnant were excluded from this study. Postural control was assessed during an anterior reach of the right foot performed while balancing on the left leg (Fig. 1). The task required subjects to stand on their left foot while reaching their right foot as far as possible without elevating their left heel or taking their hands off their hips [10]. Subjects lightly tapped their right foot at maximum reach and returned to the start position in a controlled manner. This task was chosen because it is more challenging than traditional balance tests and therefore may be more effective at discriminating between healthy, athletic individuals. Moreover, anterior reach distance asymmetry between limbs has been shown to be a reliable predictor of lower extremity injury in high school basketball players [11] as well as a metric for differentiating between healthy controls and individuals with chronic ankle instability [12,13], indicating it provides a sufficient challenge to discriminate within athletic populations. Subjects performed 4 practice trials because previous work has shown that normalized maximum reach distance stabilizes after approximately 4 trials [14]. Following the practice trials, 3 sets of 3 valid trials were recorded, for a total of 9 trials for each subject, or 405 total trials (45 subjects 9 trials per subject). Sets were performed several minutes apart. A trial was considered invalid and repeated if hands came off hips, the stance heel became elevated, the reach toe was not tapped lightly, or if any portion of the movement was performed in an uncontrolled manner. Due to missing markers or problems with the data collection software a
Fig. 1. Subject performing the anterior reaching task while balancing on a force plate.
small number of trials could not be processed, leaving a total of 397 trials used in the analysis. Kinematic data of the feet were collected for each trial at 120 Hz with eight Vicon MX-F40 motion capture cameras (Vicon, Oxford, UK). Markers placed on the 1st toe, lateral malleolus, medial malleolus, and heel of the left foot were used to identify the base of support (Fig. 2). The foot was aligned with the global y-axis and the perimeter of the base of support was formed by connecting the 1st toe to the medial malleolus, the medial malleolus to the heel, and the heel to the lateral malleolus with straight lines. The anterior and lateral borders of the base of support were drawn parallel to the global x- and y-axes, respectively (Fig. 2). A marker on the right 1st toe was used to identify the moment the subject tapped the ground at their maximum reach distance. CoP data were also collected at 600 Hz during each trial using one 4060-10 Bertec force plate (Bertec Corp., Columbus, OH). The time it would take the center of pressure (CoP), given its instantaneous position, velocity, and acceleration, to contact the perimeter of the base of support (TtC) was calculated for each frame between toe off and toe touch at maximum reach using
S. Schloemer et al. / Gait & Posture 38 (2013) 658–662
660
Fig. 2. (A) Base of support for the left foot showing a sample trajectory (dashed line) from the instantaneous center of pressure (black diamond). COP excursion (dotted line) shown for the entire reach time. (B) Left foot with the 4 markers used to define the base of support.
custom MATLAB (MathWorks, Natick, MA) scripts and previously developed equations [7]. Toe off was defined as the sample during which the subject’s right foot was completely off the force plate and movement was initiated. Toe touch was defined as the frame during which the subject’s right 1st toe marker reached a vertical minimum. To determine how postural control changed during the trial, TtC was then averaged over 5 epochs, each equal to a fifth of the reach time, such that Epoch 1 began with toe off and Epoch 5 ended with toe touch at maximum reach. To evaluate epoch as a fixed effect and subject as a random effect on TtC, an unbalanced mixed effects ANOVA was used. Post hoc Tukey’s HSD comparisons (a = 0.05) were subsequently used to determine differences between individual epochs. Reaching velocity was not controlled between trials or between subjects. TtC is a temporal measure and therefore may be impacted by how quickly a subject completes a reach. To assess if reaching velocity significantly affected TtC, linear regressions were performed between mean reaching velocity and mean TtC at each epoch across all subjects. All statistical analyses were performed using custom MATLAB scripts. 3. Results The results of the ANOVA yielded epoch as a significant main effect (p < 0.001). The post hoc Tukey’s HSD showed that the TtC of Epochs 1, 2, and 3 were not significantly different from one another (Table 1). However, Epoch 4 had a significantly longer TtC than Epochs 1, 2, and 3 (p < 0.05), while Epoch 5 had a significantly longer TtC than all other epochs (p < 0.05; Table 1; Fig. 3). Mean reach velocity was not associated with mean TtC for any epoch (R2 = 0.001–0.068). Average statistics and ranges for variables of interest can be found in Table 2. 4. Discussion This study is the first to our knowledge to demonstrate that healthy individuals modulate their postural control during Table 1 Marginal means for the TtC differences between epochs. Epoch comparison
Mean difference (ms)
95% Confidence interval
1!2 2!3 3 ! 4a 4 ! 5a
3.6 7.5 13.4 22.4
(4.6, 11.8) (0.7, 15.7) (5.2, 21.6) (14.2, 30.6)
a
Indicates significance (p < 0.01).
dynamic lower extremity tasks. A period of less control occurs during Epochs 1, 2 and 3, while the reaching foot is held relatively close to the body, suggesting that healthy individuals may not perceive a need to maintain strict postural stability during ‘‘easier’’ periods of a task. These results are consistent with previous studies comparing athletes who are balance trained versus those who are not. Studies of ballet dancers compared to non-dancers during quiet standing show that dancers exhibit greater variability in postural sway as exemplified by lower mathematical stability and increased noisiness in the CoP time series [15] and shorter time-toboundary (calculated as instantaneous distance to the base of support divided by instantaneous velocity) [16]. The authors of these studies conclude that due to their superior balance training, ballet dancers can allow more flexibility in their posture during less demanding tasks. Fidler et al. theorized that the amount of variability in postural control an individual exhibits is related to the difficulty of the task, such that when the neuromuscular system can adeptly meet the demands of the task, the individual does not need to strictly maintain their posture [16]. The progressive increase in TtC from Epoch 3 to Epoch 5 represents a corresponding increase in postural stability as the subject’s right foot nears the point of toe touch. The challenge to the postural control system increases during this period of the test, both due to the requirement to touch the toe at a single point with minimal force and because the reaching foot is being extended further from the base of support. Therefore, the longer TtC observed suggests individuals may be exhibiting greater postural control to maintain stability. Haddad et al. observed a similar TtC pattern for the precision fitting task, noting that TtC was longer in the later epochs of the task [8]. Furthermore, longer TtC was observed throughout the trials that required the subjects to fit the block through the smaller hole. These results suggest that healthy individuals incrementally increase their control as the difficulty and precision demands of the task increase. Due to its ability to quantify modifications of control throughout a task TtC may be an effective measure for assessing postural control during a functional performance activity, which may be beneficial for evaluating risk of injury or rehabilitation progress. Our theory that longer TtC during the later stages of the reach indicate individuals exercise increased postural control during the more difficult portion of the task is also supported by the mean CoP velocities during each epoch (Table 2). Both medial–lateral (ML) and anterior–posterior (AP) CoP velocity are greater during the earlier epochs, suggesting subjects are allowing greater CoP movement variability at the beginning of the reach. During Epoch
S. Schloemer et al. / Gait & Posture 38 (2013) 658–662
661
Fig. 3. Average time to contact (TtC) during each epoch. Error bars indicate the 99% confidence intervals of the means.
Table 2 Mean CoP measures by epoch. Epoch
1
2
3
4
5
TtC (ms) Reach velocity (mm/s) R2 a
274.49 34.91 1030.3 301.3 0.017
270.50 38.56 851.4 368.1 0.068
277.40 36.16 386.5 229.1 0.022
290.74 31.65 126.2 97.4 0.007
313.12 36.78 18.43 44.02 0.001
ML COP velocity (mm/s) AP COP velocity (mm/s)
53.85 13.70 98.04 29.09
54.71 13.89 105.63 28.64
51.85 10.33 108.79 33.44
43.39 10.46 92.09 28.78
34.24 10.48 64.40 16.91
a
R2 values for subject mean epoch reaching velocity as a predictor of subject mean epoch TtC.
5, AP and ML CoP velocity are at their minimum, providing evidence that individuals are restricting the movement of their CoP considerably more as they execute the toe tap at maximum reach. There are limitations to this study that may provide direction for future investigations. The subject population examined in this study consisted of healthy, recreational athletes. While this population can provide insights into how healthy individuals modulate postural control, we cannot establish what thresholds of TtC are appropriate or allowable during a dynamic task to avoid injury. To gain a fuller understanding of how TtC relates to postural stability and instability, comparisons of TtC between healthy controls, individuals with postural control deficiencies and individuals with elite levels of balance (such as dancers, ice skaters, martial artists or gymnasts) should be evaluated during functional dynamic activities. Future studies should evaluate the efficacy of using TtC analysis during the anterior reach or other dynamic balance tasks in individuals following acute lower extremity injury to monitor rehabilitation and evaluate side to side asymmetries. Other single-leg lower extremity tasks evaluated using the TtC measure may prove useful to better understand balance deficits in individuals with neuropathologies such as Parkinson’s disease or stroke. In addition, this study did not compare TtC to traditional CoP measures such as CoP excursion or velocity. It may be possible to average these measures over epochs within a trial to reveal changes in postural control as well. Further research is necessary to determine which measures may be most sensitive and meaningful to postural control changes within a task. Another limitation of this study is reaching speed was not controlled between trials or subjects. Epoch length across all subjects averaged 286.7 ms with a standard deviation of 142.5 ms. Due to the relationship between the movements of the center of mass (CoM) and CoP, to stay balanced while reaching, the movement of the CoM must be controlled such that the CoP remains within the base of support. Assuming subjects reached at a
speed they felt would not compromise their balance, it is reasonable that reaching speed is not strongly associated with TtC because subjects are maintaining their balance, and thus limiting the movement of their CoM regardless of reaching speed. This study has shown how an individual modulates postural control in accordance with the demands of a dynamic lower extremity task. Our results support the theory that an increase in the difficulty of a task elicits a corresponding increase in postural control in healthy individuals. This ability of TtC to quantify instantaneous modifications in postural control during functional movement tasks may increase the capacity to discriminate between subtle differences in neurological or orthopedic pathologies in both the research and clinical setting.
Conflict of interest statement The authors have no financial or personal relationships to disclose.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.gaitpost.2013. 02.012. References [1] Green J. An introduction to human physiology. 4th ed. Oxford: Oxford University Press; 1978. [2] Howe T, Oldham J. Posture and balance. In: Trew M, Everett T, editors. Human movement: an introductory text. 4th ed., Edinburgh: Churchill Livingstone; 2001. p. 225–39. [3] Davies P. Steps to follow. Berlin: Springer-Verlag; 1985.
662
S. Schloemer et al. / Gait & Posture 38 (2013) 658–662
[4] McGuine TA, Greene JJ, Best T, Leverson G. Balance as a predictor of ankle injuries in high school basketball players. Clinical Journal of Sport Medicine 2000;10:239–44. [5] Paterno MV, Schmitt LC, Ford KR, et al. Biomechanical measures during landing and postural stability predict second anterior cruciate ligament injury after anterior cruciate ligament reconstruction and return to sport. The American Journal of Sports Medicine 2010;38:1968–78. [6] Vrbanic´ TS, Ravlic´-Gulan J, Gulan G, Matovinovic´ D. Balance index score as a predictive factor for lower sports results or anterior cruciate ligament knee injuries in Croatian female athletes—preliminary study. Collegium Antropologicum 2007;31:253–8. [7] Slobounov SM, Slobounova ES, Newell KM. Virtual time-to-collision and human postural control. Journal of Motor Behavior 1997;29: 263–81. [8] Haddad JM, Ryu JH, Seaman JM, Ponto KC. Time-to-contact measures capture modulations in posture based on the precision demands of a manual task. Gait Posture 2010;32:592–6. [9] Hertel J, Olmsted-Kramer LC, Challis JH. Time-to-boundary measures of postural control during single leg quiet standing. Journal of Applied Biomechanics 2006;22:67–73.
[10] Gribble PA, Hertel J, Denegar CR, Buckley WE. The effects of fatigue and chronic ankle instability on dynamic postural control. Journal of Athletic Training 2004;39:321–9. [11] Plisky PJ, Rauh MJ, Kaminski TW, Underwood FB. Star excursion balance test as a predictor of lower extremity injury in high school basketball players. The Journal of Orthopaedic and Sports Physical Therapy 2006;36:911–9. [12] Olmsted LC, Carcia CR, Hertel J, Shultz SJ. Efficacy of the star excursion balance tests in detecting reach deficits in subjects with chronic ankle instability. Journal of Athletic Training 2002;37:501–6. [13] Hertel J, Braham RA, Hale SA, Olmsted-Kramer LC. Simplifying the star excursion balance test: analyses of subjects with and without chronic ankle instability. The Journal of Orthopaedic and Sports Physical Therapy 2006;36:131–7. [14] Robinson RH, Gribble PA. Support for a reduction in the number of trials needed for the star excursion balance test. Archives of Physical Medicine and Rehabilitation 2008;89:364–70. [15] Schmit JM, Regis DI, Riley MA. Dynamic patterns of postural sway in ballet dancers and track athletes. Experimental Brain Research 2005;163:370–8. [16] Fidler AH, Gagnon JM, Van Emmerik JL, Hammil REAJ. In: Wang IQ, editor. Postural control strategies in dancers and non dancers. Beijing, China: ISBS; 2005. p. 642–5.