Hip osteoarthritis (OA) is a progressive joint disorder that causes pain and motor dysfunction, which imposes a substantial health burden on the individual and health care system. The clinical end-point for severe hip OA is a total hip replacement. However hip replacement is an invasive and costly medical procedure, so efforts to prolong time to joint replacement through interventions that reduce pain and maximise function are required. A necessary requirement for developing efficacious interventions to manage hip OA is a thorough understanding of the musculoskeletal alterations associated with the disease so that they can be more directly targeted. Research to date has predominantly focused on characterising these deficits in advanced stages of the disease. These prior studies suggest that muscle weakness, altered femoral geometry and increased BMI, total body fat and intramuscular fat content are features of advanced hip OA, which together have the potential to alter the mechanical and metabolic environment of the hip joint. A need therefore existed to better understand the musculoskeletal deficits associated with earlier stages of hip OA, and how they change over time. The general aim of this thesis was therefore to investigate muscle, bone and fat characteristics in individuals with mild-to- moderate hip OA and age-matched controls at baseline and at 12-months follow- up.

A systematic review of the literature was initially conducted to determine how muscle strength, size and quality were altered in the affected leg in unilateral hip OA compared to the contralateral leg and healthy controls (Chapter 3). The final yield consisted of 13 articles. Overall findings suggested that unilateral hip OA results in generalised muscle weakness in the affected limb, which was underpinned by a combination of factors including muscle atrophy, muscle inhibition and decreased muscle quality. The included studies were primarily in individuals with advanced hip OA, and so it remained unclear how muscle characteristics were affected in mild-to-moderate stage hip OA.

Lower limb muscle strength and volume were subsequently evaluated bilaterally in individuals with symptomatic and radiographic mild-to-moderate hip OA (n = 23) and healthy age matched controls (n = 23) (Chapter 4). The maximum voluntary isometric strength of the hip and knee flexors and extensors and hip abductors and adductors were assessed using an isokinetic dynamometer and muscle volume of major lower limb muscles was assessed using high resolution Magnetic Resonance Imaging scans. We found no evidence of limb asymmetry in muscle strength or volume within the mild-to-moderate unilateral hip OA group. However the hip OA groups were weaker and had less muscle volume than those in the healthy control group. These findings were consistent with deficits in lean mass, muscle area and muscle density compared to healthy controls reported in Chapter 5 and confirmed that deficits in muscle strength and quality are present in earlier stages of hip OA than previously reported.

In the final study (Chapter 6), muscle, bone and fat characteristics were evaluated in individuals with mild-to-moderate hip OA (n = 18) and age-matched controls (n = 15) at 12-months follow-up from baseline. Although no significant group differences in change scores over the 12-month follow-up period were detected for any variables assessed, knee flexor and extensor strength and hip flexor and abductor strength were significantly decreased over time withinthe hip OA group but not the control group. The average amount of strength reduction across muscle groups was approximately 15% in the hip OA group and 5% in the control group. The main practical implication of these findings is that early intervention appears warranted in mild-to-moderate hip OA to prevent or slow the decline of lower limb muscle strength.

Overall findings from these studies indicate that generalised bilateral lower extremity muscle weakness is a feature of mild-to-moderate symptomatic and radiographic hip OA. Although it is not known whether muscle weakness is a factor that contributes to or is a consequence of the disease, findings from these studies support the need to address muscle weakness in the early management of hip OA.