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

Highlights

• Balance measures during the squat task were reduced during pain relative to control.

• Hip muscle pain did not alter balance measures during standing or forward stepping.

• Acute hip pain may not lead to the same balance deficits as painful hip pathologies.

Abstract

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.

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 [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 isolation from pathological changes [9], [10], [11], [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.