Can local muscles augment stability in the hip? A narrative literature review

This paper was published in
Journal of Musculoskeletal and Neuronal Interactions 2013; 13(1):1-12

T.H. Retchford1,2, K.M. Crossley3, A. Grimaldi4, J.L. Kemp3, S.M. Cowan1

1Melbourne Physiotherapy School, University of Melbourne, Melbourne, Victoria, Australia; 2School of Community Health, Charles Sturt University, Albury, New South Wales, Australia; 3Division of Physiotherapy, School of Health and Rehabilitation Sciences, The University of Queensland, Brisbane, Australia; 4Physiotec Physiotherapy, Brisbane, Queensland, Australia

ABSTRACT

Hip pain and dysfunction are increasingly recognised as important causes of morbidity in younger and older adults. Pathology compromising the passive stability of the hip joint, including acetabular labral injury, may lead to increased femoral head translation, greater joint contact pressures and ultimately degenerative hip disease. Activation of hip muscles may play an important role in augmenting the stability in the normal and the passively unstable hip. Research at other joints suggests that the local, rather than global, muscles are well suited to provide subtle joint compression, limiting translation, with minimal metabolic cost. Based on the known characteristics of local muscles and the limited research available on hip muscles, it is proposed that the local hip muscles; quadratus femoris, gluteus minimus, gemelli, obturator internus and externus, iliocapsularis and the deep fibres of iliopsoas, may be primary stabilisers of the hip joint. Interventions aimed at restoring isolated neuromuscular function of the primary hip stabilisers may be considered when treating people with passive hip instability prior to commencing global muscle rehabilitation. Finally, further research is needed to investigate the potential association between function of the hip muscles (including muscles likely to have a role in stabilising the hip) and hip pathology affecting hip stability such as acetabular labral lesions.

Keywords: Hip Joint, Hip Instability, Hip Muscle Control, Deep Hip External Rotator Muscles, Rehabilitation

INTRODUCTION

Our understanding of hip pathology, particularly degenerative hip pathology, is increasing1. There is growing evidence that pathology affecting the hip joint’s passive stability, such as acetabular labral tears, can progress to degenerative hip pathology2-5. Improving the active control of hip muscles in people with hip pathology and compromised joint stability may be the key to optimising joint loads and function, alleviating pain, and potentially even slowing the progression of hip disease.

Joint stability refers to the resistance that musculoskeletal tissues provide at a joint and is the product of contributions from passive, active and neural subsystems. Joint instability may result from a deficit of one or more of these subsystems and lead to excessive joint translation and subsequent joint overload if the other subsystems cannot compensate6-11. Hip joint instability was previously thought to be rare and usually associated with trauma12-14, or developmental bony abnormalities such as acetabular dysplasia. Whilst the hip joint is considered to be stable due to its bony architecture and strong capsuloligametous restraints, evidence suggests that deficits in the acetabular labrum and iliofemoral ligaments may lead to increased femoral head translation16-18 and possibly to early degenerative hip pathology3. Although surgical techniques can be used to help improve symptoms and joint function19, there is currently little evidence that surgery in people with hip joint instability alters the progression to degenerative hip disease.

Capsuloligamentous laxity may be generalised or focal. Generalised laxity is associated with connective tissue disorders whilst focal laxity may result from an acute injury or from repetitive weight bearing rotational forces overloading specific parts of the capusuloligamentous system. Sporting activities involving repeated axial loading and rotation, such as gymnastics, football, tennis, ballet, martial arts, and golf may influence the development of focal laxity. Active stability of the hip joint from tension in hip muscles may augment passive stability in the normal and structurally abnormal hip10. Despite this likely important role, little is known about what muscle or muscle synergies are involved or if hip pathology has an influence on hip muscle function.

It is theorised that in the human body two muscular systems exist; local and global8. Local muscles are thought to be important in joint stability by acting close to the joint axis, thereby providing predominantly joint compression rather than torque, and having the ability to stiffen the joint by virtue of their extensive attachments to key passive elements of the joint. In contrast, global muscles are more superficial muscles that can generate greater torque at joints as a result of their larger physiological cross sectional area (PCSA) and greater moment arm. There is considerable research investigating muscle function and pathological muscle dysfunction at the lumbar spine, cervical spine38, knee and pelvis41. This research has identified local muscle dysfunction in people with pain and pathology and that specific muscular retraining can restore muscle function at these joints. Currently, there is a paucity of literature investigating individual hip muscle function, or the association between hip pathology and dysfunction of the active hip stabilisers. Furthermore, with the exception of one study42, all research investigating links between hip pathology and muscle function has studied older populations suffering from osteoarthritis (OA) of the hip43-47.

A greater understanding of the relationship between hip muscle function and hip joint stability may enhance the specificity of exercise therapy interventions with potential to improve treatment outcomes. Therefore the aim of this study was to review the available literature relating to the role the hip muscles may play in the active stability of the hip.

Methodology

Literature examining hip musculature and active stability and the possible association between hip pathology and muscle dysfunction was retrieved. In addition, literature pertaining to neuromuscular function at other joints was also retrieved to provide a broad understanding of the relationship with joint pathology. The literature search was conducted in MEDLINE (PubMed) using search terms hip, joint, muscle, joint protection, stability, instability, quadratus femoris, gluteus medius, gemelli, obturator externus, obturator internus, piriformis, gluteus medius, gluteus maximus, pectineus, adductors, hamstrings, hip rotator cuff, lumbar spine, shoulder, knee, cervical spine, feed-forward mechanism, postural adjustments, motor control, muscle control, Real Time Ultrasound (RTUS), electromyography (EMG), Computer tomography (CT) and Magnetic Resonance Imaging (MRI). The reference lists of the articles were then hand searched to retrieve articles that were not identified with the computer search.

Contributors to passive hip stability and instability

The hip comprises a multiaxial ball and socket joint with six degrees of freedom48, and is important in load transference during functional activities involving both the lower and upper limbs4. During weight bearing activities (e.g. running), the hip is subjected to loads many times greater than body weight49. To maintain passive stability, the hip relies on ideal bony structure, normally formed labrum and intact and ideal capsuloligamentous support. Based on modelling studies, deficits in these passive structures may lead to increased femoral head translation, or shearing forces16-18. It is proposed that increased shearing force of the hip joint may be associated with pathology of passive soft tissue joint restraints and subsequent pathology of joint cartilage (Figure 3).

Bony structure

The hip joint is formed by the articulation between the femoral head and the acetabulum14. The acetabulum is formed by the union of the ischium, ilium, and pubis. Only the periphery of the acetabulum articulates with the femoral head48. Although the hip joint is considered congruent, the large femoral head has considerably more articular area compared to the acetabulum. As a result, during stance, parts of the anterior and superior articular cartilage of the femoral head remain exposed48. This allows for greater mobility into hip flexion but also increases the reliance on anterior soft tissues for stability13.

Bony abnormalities resulting in reduced congruence between the femoral head and acetabulum (e.g. Developmental dysplasia of the hip (DDH)) may lead to passive hip instability and increased reliance on surrounding soft tissue structures, particularly the anterior capsulolabral structures. Over time this increased stress may lead to fatigue failure of the acetabular labrum and subsequent chondropathy. Although this theory has not been directly studied, there is evidence suggesting increased severity53 and high frequencies of labral lesions with DDH and a strong association between DDH and the development of early OA.

Other bony abnormalities such as those seen in FAI (Femoro-Acetabular Impingement – abnormal morphology of the acetabulum, femoral head or neck), have been shown to be associated with an increased risk of acetabular labral pathology and hip OA59. This in itself may result in the development of hip instability50. In addition, a link between posterior hip instability and FAI has recently been made, the authors proposing that as the hip reaches end range prematurely in flexion and internal rotation, the femoral head is levered against the posterior joint structures, and may result in subluxation with only low velocity force62.

Capsule and ligaments

The hip capsule attaches to the periphery of the acetabulum and acetabular labrum and extends down to the femoral neck48 (Figure 1). Its fibres are aligned in a circumferential manner and are considered to provide significant passive stability to the hip joint. The capsule is further reinforced by strong extra-capsular ligaments; the iliofemoral, pubofemoral and ishiofemoral ligaments. In addition to the extra-capsular ligaments, passive hip stability may also be augmented by the intra-articular ligamentum teres. The ligamentum teres is taut in external rotation of the hip and may undergo compensatory hypertrophy in passively unstable dysplastic or labral deficient hips. In addition ligamentum teres contains free nerve endings64 and attaches to the transverse acetabular ligament and thus the acetabular labrum63, suggesting a proprioceptive role.

Capsuloligamentous laxity may be generalised or focal. Generalised laxity is associated with hypermobility syndromes and often has an underlying connective tissue disorder. It is speculated that focal laxity may arise from acute high-force trauma or repetitive overload of specific areas of the capsuloligamentous complex. People who compete in sports that require repetitive weight bearing combined with hip rotation towards, or at, the limit of normal physiological movement, such as golf, tennis and football, are reported to be more likely than inactive people to develop laxity in the capsuloligamentous system of the hip, particularly in the iliofemoral ligament. In addition, evidence of focal ligamentous instability when passive instability exists is provided by a cadaveric study of fifteen male hips. The authors noted a significant increase in hip external rotation and femoral head translation after sectioning the iliofemoral ligament, suggesting the ligament may have a significant stabilising role in the hip17. Furthermore, the proposed relationship between capsuloligamentous laxity, generalised or focal, capsular redundancy and labral lesions, particularly in active people has been highlighted in a number of review papers.

Acetabular Labrum

The acetabular labrum is a fibrocartilagenous extension to the rim of the acetabulum (Figure 2). While its function is not fully understood, it is considered important in improving joint congruity (increasing joint contact area by 25-28%), helping contain the femoral head in extremes of range and enhancing joint proprioception. In addition, the acetabular labrum and the inferiorly placed transverse acetabular ligament are thought to have an important role acting as a seal, limiting fluid movement in and out of the intra-articular space67. This sealing mechanism could potentially help hydraulically distribute load evenly across the articular surfaces of the hip, thereby reducing direct hyaline cartilage contact. This sealing mechanism may also help maintain a partial vacuum in the joint, further contributing to passive stability48. Biomechanical modelling studies suggest that in hip flexion, atmospheric pressure plays a greater joint stability role than the capsuloligamentous structures68.

Disruption of the acetabular labrum is thought to “break the seal” of the hip joint and lead to increased femoral head translation16-18, greater contact pressure of the femoral head against the acetabulum, and subsequent pathology of joint cartilage3. However, due to the difficulties associated with measuring intra-articular pressure in vivo, this theory has not been proven. Key risk factors for labral pathology are capsuloligamentous laxity and bony abnormalities, particularly DDH and FAI. Based on review papers, it is proposed that hip joint laxity can compromise the ability of the labrum to provide adequate joint protection and may allow excessive femoral head translation, potentially leading to abnormal labral loading and subsequent pathology51. The link between bony abnormalities and labral pathology has been previously discussed.

Overview of muscle function, joint function and pathology

Studies on muscle function, joint function and pathology have primarily focussed on the lumbar spine, pelvis, knee and cervical spine. To date little is known of the role of the muscles acting at the hip joint and even less is known of their association with hip pathology. Knowledge of muscle function at other joints, and its association with pain or pathology may help inform understanding of hip stability and appropriate rehabilitative strategies.

Panjabi6 proposed a model of joint stability for the lumbar spine involving the coordinated interaction between the passive, neural and active subsystems. He suggested that joint instability could occur with deficits in one or more of these subsystems, resulting in excessive motion and overload to joint structures if the other subsystems cannot compensate9. Particular muscles which form part of the active subsystem are biomechanically and physiologically well placed to provide joint protection with limited metabolic cost72. Although controversy exists, the weight of evidence suggests that local muscles rather than global muscles are preferentially suited to joint protection at the lumbar37 and cervical spines73, shoulder joint74 and pelvis36. The properties of these local muscles are discussed below.

Local muscles, such as lumbar and cervical multifidus, are predominantly composed of Type I slow twitch muscle fibres making them fatigue resistant and well suited to tonic muscle contraction; thus being ideal for postural control75-77. Fibre type gradients exist with type I fibres typically occupying deep and type II occupying more superficial regions. In vivo studies have demonstrated differential activity of deep and superficial fibres of lumbar multifidus in response to functional movement and provided evidence that deep fibres have a significant stabilising role, possibly through exertion of compressive forces with minimal associated torque, whilst superficial fibres contribute primarily to joint orientation72. Moseley et al.72 theorised that the deeper fibres are anatomically and biomechanically more suited to metabolically efficient stability by virtue of their proximity to the joint’s centre of rotation whilst more superficial fibres, owing to their larger CSA and moment arms, have greater torque generating capacity. At the shoulder, the rotator cuff muscles are thought to be ideally aligned to provide a net compressive force on the glenohumeral joint irrespective of shoulder position79, whilst the transverses abdominis, owing to its transversely oriented muscle fibres, is reported to significantly increase joint compression in the sacroiliac joints36.

It is rare that individual muscles act in isolation. In most instances muscle synergies exist23. Co-contraction of muscle groups, particularly agonists and antagonists, is thought to enhance joint stiffness80. Local muscle synergies have been described at the lumbar spine35, cervical spine73 and shoulder joint74. Contractions of the local muscles are considered a feed-forward strategy by the nervous system, preparing, and thus stabilizing and protecting the joint or joints for the perturbation caused by limb movement. This hypothesis is formed on the basis that these postural adjustments occur before feedback is available81 and in advance of a limb movement40.

Induced pain studies in the lumbar spine30, and cross-sectional studies of the sacroiliac joint41, the cervical spine38, and knee joint39suggest that pain alters normal feed-forward postural adjustments. Pain can also cause selective and rapid atrophy of the local muscles in the lumbar spine in response to lower back pain (LBP)27 and experimental disc or nerve root injury29. The underlying mechanism is unclear. The rapid onset may be more suggestive of pain inhibition rather than disuse atrophy27. Global muscles are also affected by joint pain, with evidence of increased activation, which may be a compensation for local muscle dysfunction.

Studies have shown that exercise therapy targeted specifically at the local stabilising muscles can improve function, reduce pain, restore the normal feed-forward response and reduce recurrence of pain in the knee40, cervical spine84, and the lumbar spine24,81,85,86 in symptomatic individuals. Specific isolated local muscle retraining is suggested to be more effective in stabilising joints than global muscle bracing36, and may lead to immediate alterations in feed-forward postural adjustments in symptomatic people81. Interventions targeting isolated tonic activation of the local muscles were found to be associated with earlier feed-forward postural activations, whereas non-specific training involving contraction of local and global muscles resulted in delayed local muscle activation. Once selective local muscle function has been restored, the use of exercises that simultaneously challenge the local and global muscles has been advocated.

Review of muscle function at the hip joint

Currently, it is unclear which muscle synergies have potential to stabilise the femoral head within the acetabulum. This is largely due to the inherent difficulties with measuring joint stability and muscle forces in vivo. The following review discusses what is known about individual muscles acting at the hip and explores their potential role in active joint stability. It is based on electromyography, modelling, cadaveric studies, MRI and RTUS studies and strongly guided by recent studies investigating the line of force87 and muscle morphology of the hip muscles88. The primary role of muscles, local or global, is considered to be influenced by multiple factors. It is speculated, however, that muscle architecture (PCSA relative to fibre length) and lines of action are perhaps the most important features in determining primary muscle roles. Muscles that can generate large forces over small changes in muscle length and muscles that have lines of forces predominately creating joint compression could be considered to be primary active stabilisers. A number of muscles impact on the hip. However, the focus of the review is on the deeper muscles of the hip due to their potential stability role and the abductors of the hip due to information available that suggests this muscle group is closely associated with joint loading patterns.

Quadratus femoris, obturator internus and externus and gemelli

The deep external rotators (quadratus femoris, obturator internus and externus and the gemelli) have been proposed as key active stabilisers of the hip and, along with the internally rotating gluteus minimis, are often described as the “rotator cuff” of the hip. Previous research on these muscles has been limited to anatomical modelling studies and descriptive cadaver studies90-93. The quadratus femoris, gemelli and obturator externus and internus are described as external rotators of the hip48, however their rotational force producing capacity, particularly in the weight bearing leg, is likely to be minimal given their small PCSA and moment arms22. These muscles do however have a favourable ratio between PSCA and fibre length, potentially making them suited to stabilising the femoral head in the acetabulum. Ward88 speculates that the deep external rotators may play a role in modulating hip joint stiffness and providing subtle positional adjustments to the hip joint. Modelling studies suggest that the deep external rotators, with the exception of piriformis, have a nearly horizontal line of force, which is advantageous for producing external rotation, but perhaps more importantly, compression of the joint surfaces87. As such, their morphology and proposed role is very much analogous to the rotator cuff muscles of the shoulder, particularly infraspinatus and teres minor87.

Indirect evidence of the stabilising role of these muscles comes from studies showing increased rate of prosthetic dislocation and functional deficits following resection of the external rotator muscles with posterior surgical approach94. When the external rotators and capsule were spared on a posterior approach using a capsular-enhanced repair, dislocation rates dropped dramatically.

Further indirect evidence of the dynamic stabilising role of quadratus femoris comes from a bed rest study by Miokovic et al.97 who demonstrated significant preferential atrophy of the quadratus femoris muscle when investigating the effects of unloading on the postero-lateral hip muscles in 24 male subjects. Interestingly, the other deep external rotators studied (obturator internus and externus) did not demonstrate significant changes in muscle size after sixty days of bed rest. Previous bed rest studies have demonstrated preferential atrophy of the local stabilising muscles when investigating the muscles of the trunk with preservation of global muscle size25.

To date, no human studies have investigated the fibre types of the hip cuff muscles. However, several animal studies have reported high proportions of slow twitch fibres in hip cuff muscles (up to 69.9% in quadratus femoris of mice). It is surmised that this high percentage of slow twitch fibres may imply a high spindle density and therefore an important proprioceptive role at the hip98.

The lack of information on the deep hip external rotator muscles, particularly EMG data, may be explained by the depth of the muscles and their proximity to the sciatic nerve22. While RTUS and MRI studies may provide a pathway to greater understanding of deeper muscles, no studies have investigated these muscles in symptomatic and asymptomatic individuals.

Iliocapsularis

Iliocapsularis is a muscle not well described in anataomical texts. Ward et al.100 described the muscle as originating from the anteromedial hip capsule as well as the inferior border of the anterior inferior iliac spine, inserting just distal to the lesser trochanter, based on observations of 20 human cadavers. Iliocapsularis’ extensive attachments to the hip capsule may provide potential to tighten the anterior aspect of the capsule, enhancing joint stability. An MRI study by Babst et al.101 reported greater cross sectional area, greater partial volume and less fatty infiltrate of iliocapsularis muscles of subjects with hip dysplasia compared to subjects with excessive acetabular coverage. The findings suggest that hypertrophy of iliocapsularis may represent a compensatory strategy to improve active hip joint stability in the presence of passive hip instability100.

Piriformis

The piriformis may be important in stability of the hip with evidence of lower dislocation rates when the piriformis is preserved following insertion of a prosthetic hip via a posterior approach. This may imply an important role in stabilising the hip, however it should be noted that these studies looked at piriformis in conjunction with quadratus femoris94, and obturator internus89. Piriformis is most active in resisted external rotation of the hip102. Like the other deep external rotators, the piriformis muscle has a high ratio of PSCA: fibre length suggesting a potential stability role, however unlike the other external rotators of the hip, the line of force of the piriformis muscle is not as favourable to enhance joint compression87.

Gluteus minimus

Gluteus minimus, the deepest part of the abductor synergy, is an abductor, rotator and flexor of the hip103. However, its primary function is considered to be as a stabiliser of the hip and pelvis103-105. Its fibres run parallel to the neck of the femur104, and it has attachments to the superior aspect of the capsule106, supporting the contention that gluteus minimis is an important stabiliser of the femoral head in the acetabulum. A cadaveric study by Beck et al.103 of 16 hips found the gluteus minimus had extensive attachments to the hip joint capsule. Gluteus minimus may therefore be important in stabilising the hip by being able to modulate joint capsule stiffness; it may also help prevent anterior dislocation and superomedial migration of the femoral head, as well as providing a proprioceptive role. A recent fine wire EMG study has provided support for the role of the gluteus minimus as a stabiliser in their demonstration that the anterior portion of gluteus minimus is active in both prone hip extension and in late stance phase, acting presumably to provide anterior support to the joint, rather than as a hip extensor for which is has no moment arm107.

Gluteus medius

Gluteus medius is the primary abductor of the hip and important stabiliser of the pelvis and hip, preventing the pelvis from dropping in single leg stance. It is has three segments; anterior, posterior, middle or superficial. Each segment is separately innervated and has unique fibre orientation. Electromyographic analysis suggests that the amplitude of activity in any of the segments is highly dependent upon the task and gluteus medius activation is not always consistent across the segments109. Based on anatomical and surface electromyographic studies, Gottschalk et al.104 propose that during gait the posterior portion of gluteus medius is an important stabiliser of the femoral head in the acetabulum whilst the middle subdivision helps initiate hip abduction and the anterior subdivision contracts to cause pelvic rotation. Other gait studies suggest that the gluteus medius plays an important stabilising role of the pelvis on the hip by contracting prior to and after foot contact to prevent adduction of the hip. This activity does not seem to change with increased speed. In contrast to Gottschalk et al.104 these studies did not individually test the three subdivisions of gluteus medius. A fine wire EMG study investigating the activation of the three segments of gluteus medius during non weight bearing hip movements, found the anterior portion of the muscle to be highly active during hip extension, perhaps suggesting a stability role in this position to minimise anterior femoral head translation110. Anatomical modelling studies indicate that gluteus medius may act as a hip stabiliser on the basis of a high ratio of PSCA: fibre length, however its large moment arm for abduction makes it better suited to produce force, advantageous for stabilising the pelvis in weight bearing, rather than optimal positioning of the femoral head in the acetabulum during functional activities.

Iliopsoas

Iliopsoas has two main portions, psoas major and iliacus, which are separately innervated. Both are active throughout hip flexion. Psoas major has been found to have a greater percentage of fast twitch than slow twitch muscle fibres, particularly in its caudal portion based on muscle biopsies of 15 male subjects78, whereas an animal histology study suggested iliacus may contain a large proportion of slow twitch fibres98. A fine wire EMG study by Andersson et al.114 investigating 11 subjects, supports the role of iliacus as a stabiliser of the hip, particularly in late stance phase of gait. Lewis et al.115 surmised that the iliacus and psoas muscles may play a role similar to that of the rotator cuff muscles at the shoulder by being able to influence joint stability not only by its insertion but also by tension in musculotendinous units as they pass over the anterior aspect of the hip joint.

A prolonged bed rest study by Mendis et al.116 investigated the effect on the anterior hip muscles of 8 weeks of bed rest, with results showing reduced CSA of the deep fibres of iliopsoas at the level of the head of femur, suggesting preferential atrophy.

Discussion

Active hip stability is likely to be primarily modulated by the deep local muscles

If the passive stability mechanisms of the hip are inadequate, due to local pathology or insufficiency, the muscular system will be needed to augment stability. The local muscles of the hip including gluteus mimimis, quadratus femoris, gemelli, obturator internus and externus, iliocapsularis and possibly the deep fibres of iliopsoas are anatomically, biomechanically and physiologically well suited to provide dynamic stabilisation of the femoral head in the acetabulum, helping reduce shearing forces on the joint. These muscles share many of the characteristics of other local muscles of the lumbar spine, pelvis, shoulder and knee. Although most have relatively small PCSA, they have short muscle fibre lengths and are therefore able to produce significant forces over small changes in muscle length. They also have advantageous lines of force to provide compression of the head of the femur in the acetabulum. They may also contain predominantly slow twitch muscle fibres, making them suited to tonic contractions and providing fatigue resistance and have direct capsular attachments, suggesting a significant proprioceptive role.

Co-contraction of local muscles is theorised to occur in the lumbar spine, shoulder and knee. It is plausible that local muscles act with synergy to provide hip joint stability, perhaps with the coordinated contraction of the deep internal and external rotators.

More research is needed to elucidate the effect of pathology on these local muscles. Many of the seminal articles investigating the function and dysfunction of muscles such as trans-versus abdominis, lumbar multifidus72, and gluteus medius117 have used fine wire EMG to demonstrate changes in the timing of the muscle contractions. Unfortunately the inaccessibility of the deeper stabilising muscles, particularly those lying posterior to the hip joint, makes them difficult to assess. Although fine wire EMG studies are likely to give the most definitive data, new technologies such as RTUS and dynamic MRI, may provide a less invasive method of collecting data.

Future directions in hip rehabilitation

Hip muscle strengthening exercises, particularly hip abductor strengthening, are the most commonly prescribed intervention by physiotherapists in patients with hip pain but current evidence suggests that joint stability may be enhanced via retraining of deep hip stabilisers. Although most clinicians advocate for the use of functional rehabilitation exercises, there is some evidence to suggest that this alone is inadequate for the effective retraining of normal feed-forward postural activity81. Much akin to the current rationale of strengthening the local muscles at the lumbar spine and pelvis, cervical spine, and shoulder joint prior to addressing the more superficial global muscles, it could be argued that effective therapeutic exercise programs for the pathological hip should initially target local stabilising muscles using low load tonic exercises. Specific exercises for retraining the local muscles of the hip are commonly started in positions of low postural load such as prone or sidelying. The patient can be taught to monitor their motor performance by careful palpation. In the case of a patient presenting with concurrent aberrant lumbopelvic motor control, co-contraction of the deep hip stabilisers and lumbopelvic stabilisers can be taught. Clinically such an approach appears to be effective however there is currently no evidence to support its use as it has not been evaluated. One difficulty facing clinicians is reliably measuring the function of the local muscles of the hip. RTUS is now commonly used by physiotherapists to assess and retrain muscles of the abdominal wall and lumbar spine. This technology may prove to be a reliable and valid tool for measuring local hip muscle function and for providing feedback on motor performance whilst performing rehabilitation exercises. To date there has only been one study validating the use of RTUS for measuring the size of anterior hip muscles, with findings that this clinical tool is reliable compared to MRI119. More research is needed to validate the use of RTUS as a measuring tool in other active stabilising muscles. Hand held dynamometry has been utilized to reliably determine muscle function in previous studies examining hip OA120, FAI42, and groin pain121. This may provide some insight, but further research is required to elucidate tests that are more specific for assessing deep muscle function. Testing the ability to actively move into inner range, for which the deep musculature has a better lever arm, and to tonically hold an inner range contraction have previously been suggested as important motor control assessments and retraining strategies for lumbopelvic stabilisation34 but these have not been well tested around the hip.

Once isolated contraction of the deep external rotator muscles is successfully achieved, progression can be made to the rehabilitation of secondary stabilisers and prime movers of the hip, particularly the gluteus maximus, initially using non-weight bearing exercises and progressing to weight bearing exercises once motor control and strength allows. Pre-activation of the deep external rotators may make these exercises more effective. Deficits in flexibility and proprioception should also be addressed at this stage. Once adequate hip muscle strength and endurance is achieved, functional and sports specific exercises can then be implemented.

Furthering our understanding of the role of muscles and muscle synergies at the hip may provide insight into the development of more specific assessment and treatment protocols, ensuring adequate hip joint stability in people with hip pain or pathology.

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