Acute experimental hip muscle pain alters single-leg squat balance in healthy young adults

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Acute experimental hip muscle pain alters single-leg squat balance in healthy young adults Anna L. Hatton a,*, Kay M. Crossley a, Franc¸ois Hug a,b, James Bouma a, Bonnie Ha a, Kara L. Spaulding a, Kylie Tucker c a b c

School of Health and Rehabilitation Sciences, The University of Queensland, Brisbane, Queensland, Australia Laboratory EA 4334, University of Nantes, Nantes, France School of Biomedical Sciences, The University of Queensland, Brisbane, Queensland, Australia

A R T I C L E I N F O

A B S T R A C T

Article history: Received 11 July 2014 Received in revised form 12 February 2015 Accepted 22 February 2015

Background: Clinical musculoskeletal pain commonly accompanies hip pathology and can impact balance performance. Due to the cross-sectional designs of previous studies, and the multifactorial nature of musculoskeletal pain conditions, it is difficult to determine whether pain is a driver of balance impairments in this population. This study explored the effects of experimentally induced hip muscle pain on static and dynamic balance. Methods: Twelve healthy adults (4 women, mean[SD]: 27.1[3] years) performed three balance tasks on each leg, separately: single-leg standing (eyes closed), single-leg squat (eyes open), forward step (eyes open); before and after hypertonic saline injection (1 ml, 5% NaCl) into the right gluteus medius. Range, standard deviation (SD), and velocity of the centre of pressure (CoP) in medio-lateral (ML) and anterior– posterior (AP) directions were considered. Results: During the single-leg squat task, experimental hip pain was associated with significantly reduced ML range ( 4[13]%, P = 0.028), AP range ( 14[21]%, P = 0.005), APSD ( 15[28]%, P = 0.009), and AP velocity ( 6[13]%, P = 0.032), relative to the control condition, in both legs. No effect of pain was observed during single-leg standing and forward stepping. Significant between-leg differences in ML velocity were observed during the forward stepping task (P = 0.034). Discussion: Pain is a potentially modifiable patient-reported outcome in individuals with hip problems. This study demonstrates that acute hip muscle pain alone, without interference of musculoskeletal pathology, does not lead to the same impairments in balance as exhibited in clinical populations with hip pathologies. This is the first step in understanding how and why balance is altered in painful hip pathologies. ß 2015 Elsevier B.V. All rights reserved.

Keywords: Hip muscle pain Hypertonic saline Balance performance

1. Introduction Musculoskeletal pain commonly accompanies hip disease and injury, and can have a major impact on functional ability [1], leading to a poor quality of life [2]. Hip osteoarthritis, a common source of pain in the elderly, has been shown to alter anticipatory postural adjustments prior to sideways [3] and forward stepping

* Corresponding author at: School of Health and Rehabilitation Sciences, The University of Queensland, Brisbane, Queensland 4072, Australia. Tel.: +61 7 3365 4590; fax: +61 7 3365 1622. E-mail addresses: [email protected] (A.L. Hatton), [email protected] (K.M. Crossley), [email protected] (F. Hug), [email protected] (J. Bouma), [email protected] (B. Ha), [email protected] (K.L. Spaulding), [email protected] (K. Tucker).

[4], and impair recovery of balance after a sudden change in direction [5]. Consequently, hip pathology can have devastating effects on the ability to perform daily activities [2]. Few studies have examined similar balance deficits in younger adults, who present with hip pain resulting from disease or injury such as rupture of the ligamentum teres [6], or femoroacetabular impingement [7]. Significantly impaired dynamic single-leg balance performance has recently been reported in individuals with hip chondropathy compared to healthy adults [8]. However, due to the cross-sectional design of the aforementioned studies, it is difficult to determine whether balance impairments precede or result from the development of hip pain. Experimental pain has been used to simulate the nociceptive component of pain conditions and explore the effects of acute lower limb pain on balance performance in healthy adults, in

http://dx.doi.org/10.1016/j.gaitpost.2015.02.013 0966-6362/ß 2015 Elsevier B.V. All rights reserved.

Please cite this article in press as: Hatton AL, et al. Acute experimental hip muscle pain alters single-leg squat balance in healthy young adults. Gait Posture (2015), http://dx.doi.org/10.1016/j.gaitpost.2015.02.013

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isolation from pathological changes [9–12]. Pain induced by hypertonic saline injections into gluteus medius muscle and tendon, is associated with similar patterns of referred pain, regional deep tissue hyperalgesia, and pain provocation test responses, as observed in clinical populations with hip pathology [13]. Unilateral injections of hypertonic saline into the quadriceps, hamstrings, dorsiflexors and plantarflexors are associated with increased anterior–posterior (AP) and medio-lateral (ML) displacement of the centre of pressure (CoP) during bilateral standing and following platform perturbations [10,12]: interpreted to indicate a decline in balance performance. In comparison, the effects of experimentally induced knee pain, from injection into the infrapatellar fat pad, are less conclusive. Hirata et al. [11] reported that acute knee pain increased the displacement and velocity of CoP movement in AP and ML directions during quiet standing but not after an external perturbation. Bennell and Hinman [9] observed no significant effects of induced knee pain on static or dynamic balance control. Taken together, we can conclude that the motor adaptations are likely to be specific to the body region with pain, and the motor task being performed. At the hip, the effects of experimental pain on static or dynamic balance control are unknown. It is particularly important to address this question as hip muscle control is essential to maintain trunk and pelvic stability, and to control balance in the ML direction, especially during single-limb tasks [14]. It is possible that if hip pain (alone) is associated with deterioration in balance performance, then strategies to reduce pain may be one of the first considerations in the management of functional changes in people with hip pain pathologies. The aim of this study was to investigate the effects of experimentally induced hip muscle pain on static and dynamic balance performance in healthy young adults. We hypothesised that hip pain would result in increased CoP displacement and velocity during single-leg standing, single-leg squat, and forward stepping tasks, compared to a control condition. 2. Methods 2.1. Participants Twelve healthy adults (4 women), with mean (SD) age 27.1 (3) years; height 1.76 (9) m; weight 74.0 (17.3) kg, completed the study. Participants were recruited from The University of Queensland in response to advertising. Exclusion criteria included current or previous hip or low back pain; hip surgery; musculoskeletal, neurological, or sensory impairments that could affect balance control; current use of pain medication; or needle phobia. Eleven participants reported right leg dominance. The study was approved by the Institutional Medical Research Ethics Committee (#2004000654) and conformed to the Declaration of Helsinki. Participants provided written informed consent and were permitted to withdraw from the study at any point. 2.2. Design In a within-subject experimental design, all participants were tested in one session under two conditions (no pain [control] and pain). Balance was assessed during three tasks: single-leg standing with eyes closed (StandEC), single-leg squat with eyes open (SquatEO), and forward stepping with eyes open (FwdStep). 2.3. Equipment Force data were obtained from two Kistler force platforms (Model 9296AA, Kistler, Alton, UK). Data were sampled at 100 Hz (Power1401 Data Acquisition System, Cambridge Electronic Design, UK) and low-pass filtered (20 Hz, 4th order Butterworth filter) off-line.

2.4. Procedures 2.4.1. Balance tasks All balance tasks were performed barefoot. Prior to data collection, participants practiced 1–2 trials of the three balance tasks to allow familiarity with the testing procedures. The balance tasks without pain were performed first, followed by the pain condition. This is important as acute pain is associated with changes in motor control, which do not necessarily recover immediately after pain has ceased [15]. The order of balance tasks, and starting leg (left or right) were randomised within-conditions. 2.4.2. Single-leg standing with eyes closed (StandEC) Participants stood on their right or left leg in the middle of a force platform, with their arms folded across their chest, and eyes closed, for 12 s. Participants were given the standardised instruction to ‘lift your [right/left] leg up and keep balanced for 10 s, keeping your arms crossed, I will instruct you when to lower your leg’. The foot was raised above the ground by flexing the knee to 908. Three trials were completed for each leg. If necessary participants were permitted to touch down with their uplifted foot to regain balance and return to the test position as quickly as possible. A 10 s rest period was given between trials. The StandEC task was chosen to assess static balance as removing visual information increases the challenge of an otherwise simple standing task. 2.4.3. Single-leg squat with eyes open (SquatEO) Participants stood on their right or left leg in the middle of a force platform, with their arms folded across their chest, and their eyes open. A plinth was positioned behind the participant, with the height adjusted so that an angle of 608 knee flexion was achieved when the participant’s buttock touched the top surface of the plinth. From a single-leg standing position, participants were instructed to ‘squat down until your buttocks lightly touch the bed behind, then return to the starting position and repeat 3 times in time with the count’. Three sets of three repetitions were performed for each leg (the first leg was randomly presented, and thereafter alternate legs used) at a cadence of 3 s lowering and 3 s rising, in time with a verbal cue. Participants may have moved faster or slower in different parts of the lowering and rising phases of the squat. This was not controlled as we did not want to constrain the squat performance to an extent whereby pain-related alterations in movement patterns (and subsequent CoP measures) would not be identified. A 10 s rest period was given between sets. The SquatEO task was chosen as a dynamic task because it is a clinical measure of lower limb function suited to testing on a force platform [16]. 2.4.4. Forward stepping with eyes open (FwdStep) Participants adopted their comfortable double-leg standing position, with one foot on each force platform with their eyes open. On a verbal cue, given by the tester (JB), participants were instructed to step up (with either their right or left leg) as quickly as possible onto a 15 cm high wooden step (80 cm width  60 cm depth), located 15 cm in front of the most distal point of the hallux. Participants performed 20 forward steps (10 per leg) in a randomised order [4]. The FwdStep task was chosen as it challenges the hip abductors to control balance in the ML direction [17]. 2.4.5. Experimentally induced hip pain Hypertonic saline (1 ml, 5% NaCl) was injected into the right gluteus medius muscle of all participants, 2 cm distal to the midway point between the anterior superior iliac spine and posterior superior iliac spine. The correct location and depth of the gluteus medius muscle belly was confirmed with ultrasound (12 MHz,

Please cite this article in press as: Hatton AL, et al. Acute experimental hip muscle pain alters single-leg squat balance in healthy young adults. Gait Posture (2015), http://dx.doi.org/10.1016/j.gaitpost.2015.02.013

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Logic e, GE Healthcare, Australia). A single bolus of hypertonic saline was delivered over 5 s, using a 25 G  25 mm needle. One injection was given prior to completing all three balance tasks. Pain intensity was reported on an 11-point numerical rating scale (NRS: 0 = no pain, 10 = worst imaginable pain) during the painful trials. During the pain condition, balance data collection commenced when the pain was reported >2/10, as per [18,19]. If pain intensity was reported as <2/10 within the time required to complete all tasks, a second injection (1 ml, 7% NaCl) was administered 1 cm from the first injection site. 2.5. Data analysis Data were processed using Matlab (The Mathworks, Nathick, USA). Balance measures included CoP path range, standard deviation (SD), and velocity of movement in the ML (ML range, MLSD, ML velocity) and AP direction (AP range, APSD, AP velocity). CoP range (mm) indicates the maximum range of movement during the test in each axis, with higher values indicating that the person swayed further [20]. The SD of CoP coordinates (mm) represents variability about the average position of the CoP coordinate, with a higher value indicative of a less clustered movement pattern and potentially greater exploratory (or less controlled) behaviour [21]. CoP path velocity (mm s 1) represents the speed at which the centre of mass is moving, with a higher value indicating more rapid and potentially unstable movement [20]. For StandEC, data were analysed from the middle 10 s of the 12 s task. For SquatEO, data were analysed from the middle 2 (updown) repetitions of the task. For FwdStep, data were analysed from the initiation of weight shift between legs (determined as turning point of force from both force plates) until ‘‘foot off’’ (i.e. when force = 0 N) on the test leg force plate.

Fig. 1. Pain intensity and pain area reported for the singe-leg standing, single-leg squat and forward stepping balance tasks.

all tasks, was 3.7 (1.7)/10 and 3.6 (1.8) cm2 respectively. All participants reported pain near the injection site (Fig. 2). Referred pain was reported in the lateral thigh (n = 2), and calf (n = 1). Participants described their pain as ‘‘dull’’ (n = 10), ‘‘aching’’ (n = 8), ‘‘brief’’, ‘‘sore’’, ‘‘discomforting’’ and ‘‘hurting’’ (n = 5).

2.5.1. Pain measures Pain intensity local to the injection site was measured using an 11-point NRS (discussed above). Size of pain area was reported using a series of 10 circles ranging from 1 to 10 cm in diameter [22]. Measures of pain intensity and area were collected immediately after injection, prior to, mid-way through, and after completion, of each balance task, and were averaged to generate one score for each task. At the end of all test procedures, participants reported their area of pain [22] using a standardised 10 cm body chart, and completed the short-form McGill Pain Questionnaire [23]. 2.6. Statistical analysis Statistics were performed with SPSS version 20.0 (SPSS Inc., Chicago, IL 60606, USA). Data were examined for normality and homogeneity of variance. A one-way repeated measures ANOVA was used to identify any differences in pain intensity and area across the balance tasks. Each balance outcome measure for each task was analysed using a two-way ANOVA with Condition (no pain and pain) and Leg (painful and non-painful) as the withinsubject factors. The alpha was set to 0.05. 3. Results 3.1. Pain Four participants reported their pain intensity <2/10 prior to completion of all three experimental tasks, and a second hypertonic saline injection was provided. No significant differences were reported for pain intensity (F(1.3,12.8) = 0.536, P = 0.52) or area (F(2,20) = 0.176, P = 0.84) between the three balance tasks (Fig. 1). Mean (SD) pain intensity and area, across

Fig. 2. Injection location (black dots) was determined using palpation and ultrasound of each participant. Region of reported pain (grey) are shown. Data represent the location of injection and region of pain from a photograph of each participant, overlaid.

Please cite this article in press as: Hatton AL, et al. Acute experimental hip muscle pain alters single-leg squat balance in healthy young adults. Gait Posture (2015), http://dx.doi.org/10.1016/j.gaitpost.2015.02.013

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3.2. Balance performance during the StandEC

3.4. Balance performance during the FwdStep

No significant main effect of Leg, Condition, or Leg  Condition interaction was found for any of the balance measures during the StandEC task (all P > 0.272) (Table 1).

No significant main effect of Condition, or Leg  Condition interaction was observed, for any of the balance measures during the FwdStep task (all P > 0.401) (Table 3). A significant main effect of Leg on ML velocity (F(1,10) = 5.997, P = 0.034) was observed, with lower mean [95% CI] values for the painful leg compared to the nonpainful leg ( 32.9 [ 62.8 to 3.0] mm s 1) (Table 3).

3.3. Balance performance during the SquatEO No significant main effect of Leg (all P > 0.182), nor Leg  Condition interaction (all P > 0.168) was found during the squat task (Table 2). However there was a significant main effect of Condition on ML range (F(1,10) = 6.568, P = 0.028), AP range (F(1,10) = 12.635, P = 0.005), APSD (F(1,10) = 10.564, P = 0.009) and AP velocity (F(1,10) = 6.228, P = 0.032) (Table 2). Significant mean differences [95% CI] for ML range ( 1.2 [ 2.3 to 0.2] mm), AP range ( 10.4 [ 16.9 to 3.9] mm), APSD ( 3.3 [ 5.5 to 1.0] mm) and AP velocity ( 2.8 [ 5.4 to 0.3] mm s 1) were observed, with each outcome measure being lower during pain compared to Control.

4. Discussion This study provides evidence that unilateral experimental hip muscle pain reduces the excursion and velocity of CoP movement bilaterally, during a single-leg squat task in healthy young adults. With pain into the right gluteus medius, we observed reductions in ML range, AP range, APSD and AP velocity. These changes may represent an overall more stable movement pattern, whereby the centre of mass moves slower, and is less likely to approach or exceed the limits of stability. Alternatively, reduced postural sway

Table 1 Repeated measures ANOVA for main effects of pain for balance measures during the single-leg standing task with eyes closed in healthy young adults (N = 12). Balance measure

ML range (mm) MLSD (mm) AP range (mm) APSD (mm) ML velocity (mm s 1) AP velocity (mm s 1)

No pain

Pain

ANOVA (P value)

Non-painful leg

Painful leg

Non-painful leg

Painful leg

Mean (SD)

Mean (SD)

Mean (SD)

Mean (SD)

41.5 10.2 59.6 12.6 49.9 49.0

42.3 10.8 62.0 12.7 51.7 52.8

38.2 9.5 53.6 11.1 48.8 47.3

42.4 10.5 60.6 12.6 47.4 51.8

(8.4) (1.9) (16.6) (2.8) (12.2) (18.8)

(9.4) (3.4) (27.9) (5.6) (11.8) (22.4)

(8.9) (2.9) (10.1) (2.4) (14.3) (13.3)

(18.8) (5.4) (30.6) (5.7) (20.2) (35.0)

Leg

Pain

Leg  Pain

0.341 0.402 0.465 0.566 0.879 0.362

0.390 0.401 0.272 0.285 0.304 0.527

0.341 0.562 0.538 0.340 0.469 0.911

ANOVA, analysis of variance; ML, medio-lateral; AP, anterior–posterior; SD, standard deviation.

Table 2 Repeated measures ANOVA for main effects of pain for balance measures during the single-leg squat task with eyes open in healthy young adults (N = 11).a Balance measure

ML range (mm) MLSD (mm) AP range (mm) APSD (mm) ML velocity (mm s 1) AP velocity (mm s 1)

No pain

Pain

ANOVA (P value)

Non-painful leg

Painful leg

Non-painful leg

Painful leg

Mean (SD)

Mean (SD)

Mean (SD)

Mean (SD)

24.2 5.9 57.3 15.7 36.5 49.2

25.5 6.4 55.5 15.1 36.4 48.6

23.9 6.0 46.4 12.1 33.9 47.1

23.4 5.8 45.7 12.1 34.3 45.0

(5.0) (1.3) (19.7) (6.4) (6.8) (11.4)

(4.4) (1.2) (20.7) (6.5) (7.5) (14.1)

(5.5) (1.5) (14.2) (4.3) (7.8) (10.8)

(4.8) (1.3) (13.0) (4.2) (6.0) (15.1)

Leg

Pain

Leg  Pain

0.456 0.182 0.429 0.586 0.830 0.591

0.028* 0.127 0.005* 0.009* 0.080 0.032*

0.328 0.168 0.697 0.504 0.826 0.646

ANOVA, analysis of variance; ML, medio-lateral; AP, anterior–posterior; SD, standard deviation. a Data is based on N = 11 as one participant rated their pain <2/10 after a second injection, prior to completing their third balance task (single-leg squat), at which point testing ceased. * P < 0.05.

Table 3 Repeated measures ANOVA for main effects of pain for balance measures during the forward stepping up task in healthy young adults (N = 11).a Balance measure

ML range (mm) MLSD (mm) AP range (mm) APSD (mm) ML velocity (mm s 1) AP velocity (mm s 1)

No pain

Pain

ANOVA (P value)

Non-Painful Leg

Painful Leg

Non-Painful Leg

Painful Leg

Mean (SD)

Mean (SD)

Mean (SD)

Mean (SD)

18.3 (11.0) 5.9 (3.6) 44.8 (16.2) 14.6 (5.8) 88.50 (78.40) 164.43 (88.87)

27.0 (18.9) 8.6 (5.9) 47.7 (14.8) 15.7 (5.4) 54.52 (36.85) 154.08 (77.95)

18.9 (10.0) 6.3 (3.2) 46.5 (18.7) 15.5 (6.5) 88.89 (61.74) 163.58 (76.41)

27.7 (17.1) 8.9 (5.5) 48.8 (19.4) 16.1 (6.5) 57.11 (38.84) 140.16 (69.07)

Leg

Pain

Leg  Pain

0.066 0.084 0.648 0.677 0.034* 0.306

0.551 0.405 0.543 0.401 0.770 0.710

0.999 0.978 0.923 0.828 0.872 0.723

ANOVA, analysis of variance; ML, medio-lateral; AP, anterior–posterior; SD, standard deviation. a Data is based on N = 11 as one participant rated their pain <2/10 after a second injection, prior to completing their third balance task (forward stepping), at which point testing ceased. * P < 0.05.

Please cite this article in press as: Hatton AL, et al. Acute experimental hip muscle pain alters single-leg squat balance in healthy young adults. Gait Posture (2015), http://dx.doi.org/10.1016/j.gaitpost.2015.02.013

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amplitude and velocity may be indicative of a protective, postural ‘stiffening’ response, as a means to preserve upright stability or reduce the potential of further injury [24]. However, this may represent a system which is less adaptable to balance threats [24]. The current study identified that unilaterally induced pain led to a bilateral reduction in CoP measures during a single-leg squat. It is possible that bilateral changes in balance may be related to alterations in attention to the task due to the presence of pain [25]. This is in line with evidence that changes in motor adaptation to pain involve both peripheral (nociceptive stimulation) and central (including fear/anticipation/attention) mechanisms [15]. Whilst impaired balance control is evident in people with hip pathology [3–5,8], the current findings suggest that acute hip pain alone, in isolation of pathological changes, does not lead to the same changes in balance as observed in clinical pain populations. This may be due to substantial alterations in muscle function (across multiple muscles), including physiological and neuromotor changes, associated with chronic hip pathology, or pain which is neuropathic rather than nociceptive in nature. Consistent with young adults with hip chondropathy [8] and older males with hip OA [26], experimental pain did not influence CoP measures for the StandEC task. Further, static single-leg standing places lower mechanical demand on gluteus medius, relative to dynamic tasks such as a single-leg squat [27], and therefore, changes in balance due to experimental hip muscle pain, may be more evident in dynamic tasks. Hirata et al. showed significant alterations in CoP movement during quiet standing when pain was induced in the knee extensors [12], calf muscles [10], or infrapatellar fat pad [11]. No effects were observed following injection into biceps femoris [12]. During unperturbed standing, individuals maintain balance by making postural adjustments about the ankle joints. As gluteus medius (hip abductor) and biceps femoris (hip extensor) cross the hip joint, it is possible that pain at these sites had minimal effect on static balance, if the hip postural strategy was not used. Therefore, variability in motor adaptations in response to pain may be dependent upon the location or function of the painful muscle, relative to the task being performed [28]. Disparities between the current findings and those of Hirata et al. [10–12] may be attributed to differences in testing procedures, specifically the base of support. Hirata et al. [10–12] investigated quiet bipedal standing over 1 min whilst the current study explored unipedal standing over 10 s. In bipedal standing positions, individuals are more likely to adjust their posture to offload the painful limb, distributing a proportion of bodyweight onto the non-painful leg, leading to substantial alterations in CoP position. During unipedal standing, other compensatory strategies, such as greater activation of surrounding hip musculature, may be used for balance control. Bipedal standing may generate less variable CoP movement [29], and thus more clearly identify an effect of experimental pain on balance, as observed by Hirata et al. [10–12]. No significant effect of pain on any balance measures were observed during the FwdStep task, which concurs with previous work comparing adults with symptomatic unilateral hip osteoarthritis to healthy controls [4]. This may be associated with the loading demands of the task, or alternatively the participants being well-trained in maintaining single-leg balance control during a stepping task which is commonly performed in everyday life. Interestingly, we observed a significant effect of Leg on ML velocity. This finding supports our suggestion that familiarity with a movement, in addition to leg dominance, may determine balance performance during stepping. For both conditions, faster ML velocity (interpreted as reduced stability) was reported when participants stood on their left leg and stepped with their right leg. Eleven participants in the current study reported right leg

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dominance: it appears this leg provided greater ML stability during the FwdStep relative to the non-dominant side. As singleleg balance challenges become more dynamic in nature, leg dominance may play a greater role in determining the level of postural stability achieved [30]. 4.1. Study limitations All participants performed ‘no pain’ balance tests prior to the ‘pain’ condition. However, we are confident our findings reflect a true effect of pain on squat performance, rather than a time/ learning effect. We conducted further analyses to consider the change in each balance measure, between the first and third SquatEO trial of the control condition (two-way ANOVA with Leg [painful and non-painful] and Trial [control 1 and control 3] as within-subject factors). No effect of time (all P > 0.160) or leg (all P > 0.147) was observed. 5. Conclusion Experimentally induced hip muscle pain reduces CoP movement during a single-leg squat task in healthy young adults. These changes do not reflect the same balance impairments as exhibited in individuals with hip pathology. Therefore, the nociceptive component of these pathologies may not directly drive balance impairments. Bilateral changes in dynamic balance may indicate global physiological or cognitive responses to acute pain. This evidence is the first step in determining the effects of hip pain alone on balance control. Funding None. Conflict of interest statement None declared. References [1] Dawson J, Linsell L, Zondervan K, Rose P, Randall T, Carr A, et al. Epidemiology of hip and knee pain and its impact on overall health status in older adults. Rheumatology 2004;43:497–504. [2] Kemp JL, Makidissi M, Schache AG, Pritchard MG, Pollard TCB, Crossley KM. Hip chondropathy at arthroscopy: prevalence and relationship to labral pathology, femoroacetabular impingement and patient-reported outcomes. Br J Sports Med 2014;48:1102–7. [3] Tateuchi H, Ichihashi N, Shinya M, Oda S. Anticipatory postural adjustments during lateral step motion in patients with hip osteoarthritis. J Appl Biomech 2011;27:32–9. [4] Sims KJ, Richardson CA, Brauer SG. Investigation of hip abductor activation in subjects with clinical unilateral hip osteoarthritis. Ann Rheum Dis 2002;61:687–92. [5] Kiss RM. Effect of the degree of hip osteoarthritis on equilibrium ability after sudden change in direction. J Electromyogr Kinesiol 2010;20:1052–7. [6] Byrd JW, Jones KS. Traumatic rupture of the ligamentum teres as a source of hip pain. Arthroscopy 2004;20:385–91. [7] Allen D, Beaule O, Ramadan O, Douchette S. Prevalence of associated deformities and hip pain in patients with cam-type femoroacetabular impingement. Bone Joint Surg Br 2009;91:589–94. [8] Hatton AL, Kemp JL, Brauer SG, Clark RA, Crossley KM. Dynamic single-leg balance performance is impaired in individuals with hip chondropathy. Arthritis Care Res 2014;66:709–16. [9] Bennell K, Hinman RS. Effect of experimentally induced knee pain on standing balance in healthy older individuals. Rheumatology 2005;44:378–81. [10] Hirata RP, Arendt-Nielsen L, Graven-Nielsen T. Experimental calf muscle pain attenuates the postural stability during quiet stance and perturbation. Clin Biomech 2010;25:931–7. [11] Hirata RP, Arendt-Nielsen L, Shiozawa S, Graven-Nielsen T. Experimental knee pain impairs postural stability during quiet stance but not after perturbations. Eur J Appl Physiol 2012;112:2511–21. [12] Hirata RP, Ervilha UF, Arendt-Nielsen L, Graven-Nielsen T. Experimental muscle pain challenges the postural stability during quiet stance an unexpected posture perturbation. J Pain 2011;12:911–9.

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Please cite this article in press as: Hatton AL, et al. Acute experimental hip muscle pain alters single-leg squat balance in healthy young adults. Gait Posture (2015), http://dx.doi.org/10.1016/j.gaitpost.2015.02.013