Hip osteoarthritis is a progressive joint disease that results in significant functional disability and places a considerable financial burden on the healthcare system in Australia. While there are many risk factors recognized in the development and progression of osteoarthritis, many cannot be influenced by conservative measures. Muscle dysfunction, such as muscle atrophy and weakness, is one risk factor that is amenable to change. Current international guidelines recommend the use of exercise in the management of this disease. However, research based evidence for the effect of exercise on pain and physical function of patients with hip osteoarthritis has only demonstrated a small beneficial effect that is not sustained in the long term. The lack of specificity in the strengthening programs used in previous studies may be one reason for the small beneficial effect of exercise. Addressing specific hip muscle dysfunction has been proposed as a necessary focus in exercise prescription for patients with hip osteoarthritis. However evidence of muscle dysfunction in the presence of hip joint pathology has so far been limited to the postero-lateral hip muscles.
The anterior hip muscles play an important role in hip joint function and stability however little research has so far been conducted into their function. Changes in muscle function in response to functional activity, or the lack thereof, have been proposed to play a role in the development and progression of joint pathology. An imbalance in size, strength or recruitment pattern within a group of synergist muscles is proposed to alter joint movement and loading. With regards to the anterior hip muscles, it is unknown if specific muscle dysfunction occurs in response to changes in activity levels and in response to hip joint pathology. Therefore the aim of this thesis was to investigate the effect of reduced activity, increased activity and joint pathology on the individual anterior hip flexor muscles with the use of magnetic resonance imaging (MRI). As MRI is not easily accessible in physiotherapy clinics, a secondary aim of this thesis was to develop a method of measuring the size of these muscles in clinical practice to enable the assessment of specific anterior hip muscle dysfunction.
The primary aim of studies 1 and 2 was to examine the effect of reduced activity on individual anterior hip flexor muscle size as part of the first and second Berlin Bed Rest studies. In study 1 and 2, prolonged bed rest resulted in a differential pattern of atrophy among the hip flexor muscles with the iliopsoas and sartorius muscles preferentially affected at the level of the hip joint. The primary aim of study 3 was to investigate the effect of increased activity on the individual anterior hip flexor muscles in elite football players. Results indicated that iliopsoas and sartorius muscle size at the level of the hip joint increased over a season of playing football. These results were the opposite of those seen for the effect of decreased activity due to prolonged bed rest. As muscle dysfunction may occur as a result of joint pathology, study 4 aimed to examine individual anterior hip flexor muscle size and recruitment pattern and hip flexor muscle strength in subjects with and without a unilateral acetabular labral tear. Differences between groups were found for hip flexor muscle strength but not for size or recruitment pattern. Study 5 examined the validity of real-time ultrasound imaging to measure individual anterior hip muscle size in comparison to MRI. No difference was found between the measurements obtained from the two modalities.
Findings from this thesis indicate that region-specific changes in the size of individual anterior hip flexor muscles occur in response to changes in activity levels, and that hip flexor muscle dysfunction exists in the presence of intra-articular hip joint pathology. Specific muscle atrophy or hypertrophy around the hip joint or muscle weakness may affect loading of the hip joint surfaces. Over time this may contribute to hip joint injury and pathology and may have long-term implications for the development of hip osteoarthritis. Further research is needed to fully understand the relationship between specific muscle dysfunction and the progression of hip joint pathology. The findings of this thesis provide support for the importance of specificity in the assessment of the hip muscles. Assessment of individual hip muscle size and function in clinical practice may be necessary in conjunction with other clinical measures of muscle length, strength and control of recruitment in order to provide effective exercise therapy for the patient with hip joint pathology. Real-time ultrasound imaging, with further research and development, may provide a means to more specific assessment and treatment of muscle function around the hip.