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Joint instability is a recognised mechanism of abnormal loading that may play a role in the progression of osteoarthritis. Although the hip is considered a stable joint, the hip is one of three most affected joints by osteoarthritis. Recent evidence suggests individuals with hip pathology may have rotational (i.e., ‘giving way’) and/or micro (i.e., excessive femoral head translation) hip instability, which could contribute to early onset osteoarthritis. The deep hip muscles are a group of small and deeply situated muscles that are considered important for active hip stabilisation. Some rehabilitation programs target the deep hip muscles with the aim of improving joint stability. However, given their small physiological cross-sectional areas and moment arms, their force and moment generating potential is limited and may render them insufficient to contribute to hip stability or loading. Insight into the function of the deep hip muscles will establish whether targeted training of these muscles is a viable rehabilitation strategy for improving hip stability, and potentially, slowing osteoarthritis progression. The contribution of the deep hip muscles to joint stability and loading during dynamic tasks can be estimated using neuromusculoskeletal modelling approaches. Informing muscles in neuromusculoskeletal models using individual electromyography (EMG) signals (i.e., EMG-informed modelling) can provide estimates of joint loading close to measured values using instrumented implants. However, EMG signals of the deep hip muscles can only be acquired using intramuscular measurements due to the muscles’ deeply situated positions. This thesis aimed to uncover the potential function of the deep hip muscles in relation to joint stability and loading. Furthermore, we aimed to investigate whether intramuscular EMG signals of the deep hip muscles are required for investigations into hip loading and deep hip muscle function when using an EMG-informed neuromusculoskeletal modelling approach. Study 1 determined the contribution of the deep hip muscles to hip stability. Hip stability was defined as rotational hip stiffness in the sagittal plane estimated using EMG-informed modelling. A generic musculoskeletal model was adjusted to include all deep hip muscles and employed to calculate hip rotational stiffness in the sagittal plane during walking in 12 participants. Rotational hip stiffness was compared between three model configurations that differed in deep hip muscle excitations but had identical excitations for all other muscles. Results showed that the deep hip muscles contributed little to sagittal plane rotational hip stiffness during walking and had little potential to change hip rotational stiffness. These results cast doubt on the assumed function of the deep hip muscles to stabilise by stiffening the hip. Study 2 investigated the potential of the deep hip muscles to change the direction of hip loading in the acetabulum. The direction of hip contact force was calculated during walking and squatting in 12 participants using an EMG-informed neuromusculoskeletal model configured with different activation levels for the deep hip muscles. Results showed that the deep hip muscles have the potential to redirect hip contact force towards the centre of the acetabulum. Redirecting hip loading towards the centre of the acetabulum may be effective for individuals with microinstability and/or individuals who present with unfavourable regional loading and/or cartilage damage. Future investigations to hip loading should include the deep hip muscles in the neuromusculoskeletal model due to their contribution to the direction of hip loading. Study 3 determined the need for intramuscular EMG measurements of the deep hip muscles when investigating hip loading and deep hip muscle function using EMG-informed neuromusculoskeletal modelling. Hip contact forces were calculated during walking, squatting, and squat-jumping in 17 participants using an EMG-informed neuromusculoskeletal model with and without intramuscular EMG of the deep hip muscles. Hip contact force magnitude and direction were not affected by the exclusion of intramuscular EMG of the deep hip muscles. Additionally, the synthesised configuration was able to estimate high and low magnitudes of v deep hip muscle excitations, however, synthesised excitation patterns were likely too imprecise to provide valuable insight into deep hip muscle function. Taken together, the results of this thesis provide new evidence of the function of the deep hip muscles and put forward new ideas and directions for future research. Contrary to existing clinical assumptions, these muscles have limited ability to stiffen the hip but have the potential to redirect hip loading towards the centre of the acetabulum. Future studies investigating deep hip muscle function may benefit from informing the deep hip muscles with intramuscular EMG to assure accurate estimation of deep hip muscle forces. However, excitation patterns of the deep hip muscles may be synthesised in studies investigating hip loading only. Based on the results of this thesis, strengthening the deep hip muscles in rehabilitation programs may be ineffective for individuals with rotational instability, but could be beneficial for individuals who present with microinstability, unfavourable regional loading, and/or cartilage damage. |
