Neuromuscular control of medio-lateral postural stability in unilateral hip osteoarthritis (2003)

The incidence of hip osteoarthritis (OA) increases markedly after age 50. At the same time, the neuromuscular system is showing signs of decline suggesting a possible link between the two. The forces acting on the hip in the frontal plane are large and are predominantly the result of the hip abductors, which must control motion of the pelvis and trunk. The frontal plane orientation of the trunk and pelvis motion also influences the forces acting on the hip. A reduction in the ability to control frontal plane trunk and pelvic motion, as a result of aging, may cause the hip to sustain excessive loading and contribute to joint destruction. The overall aim of this thesis is to investigate aspects of neuromuscular function during a task challenging medio-lateral postural stability in a group of subjects with clinical unilateral hip OA, to determine whether there is any consistent dysfunction associated with the osteoarthritis process. An initial aim of this thesis was to investigate the effect of aging on neuromuscular control of frontal plane trunk and pelvis motion. Once this had been established, subjects with hip OA could then be investigated to determine whether specific deficits of trunk and pelvis frontal plane control were present. Nineteen young, 19 older and 19 hip OA subjects participated in 6 studies: five comparative and one correlational. The first study investigated two step tasks to determine which task provided the greater challenge to frontal plane postural control. The next two studies used the step up task to investigate how aspects of frontal plane postural control differed between young and older adults. These revealed a loss of hip abductor muscular control, with a delay in activation (p< 0.000) and a reduction in magnitude (p=0.009) of the gluteus medius in older adults. Further kinematic investigation revealed older adults also showed deficits in measures of frontal plane trunk motion (p=0.031). The fourth study demonstrated an increase in gluteus medius activity in hip OA subjects (p=0.014), while the fifth showed an increased frontal plane velocity of centre of mass motion (p=0.013). The final study sought to relate clinical and laboratory measures, to determine whether any clinical measures were associated with any of the key laboratory findings. Some hip ranges of motion were associated with delays in hip abductor activation and centre of mass velocity. This study also attempted to predict the level of pain and disability in the hip OA subjects using clinical and laboratory data. Both a clinical (internal rotation range) and laboratory (kinematic trunk motion) variable were predictive of components of the level of patient physical disability. A reduction in the range of hip flexion predicted a component of the patients pain score. This thesis has identified specific age-related changes in frontal plane postural control, which are relevant to the trunk, hip and pelvis. Furthermore, hip OA subjects were found to have specific deficits in frontal plane control when compared to older adults. These findings indicate that frontal plane control of trunk motion deteriorates with age and is consistent with the possibility that such changes may play a role in development or progression of hip OA. This information provides a basis for further research and is relevant to the clinical physiotherapist treating hip OA.