Individuals with mild-to-moderate hip osteoarthritis walk with lower hip joint contact forces despite higher levels of muscle co-contraction compared to healthy controls

Abstract

Purpose: Conservative non-pharmacological interventions are recommended for the clinical management of hip osteoarthritis (OA), though no existing treatments can modify structural disease progression. Evidence-based treatment targets are required to inform the development of disease-course altering interventions. Abnormal hip joint loading is potentially modifiable (e.g. gait and/or neuromuscular retraining) and may play an important role in hip OA progression. No previous study has investigated whether individuals with mild-to-moderate hip OA exhibit altered hip joint contact forces (HJCF) during gait, or identified any relevant contribution of altered hip muscle co-contraction to hip joint loading in this patient population. This study aimed to compare: (1) HJCF between individuals with mild-to-moderate hip OA and healthy controls during walking using an electromyography (EMG)-informed modelling approach; and (2) hip muscle co-contraction levels between groups. Methods: Eighteen participants (age=65±7 yrs; body mass index=27.6±4.8 kg.m-2; 72% females) with mild-to-moderate hip OA and 23 healthy controls without hip pain or radiographic hip OA (age=60±8 yrs; body mass index=25.1±4.8 kg.m-2; 74% females) performed 10 over-ground walking trials at a self-selected speed. Three-dimensional lower-body motion, ground reaction forces, and surface EMG from sixteen lower-limb muscles were synchronously acquired.Hip joint contact forces were solvedby balancing the external joint moments with internal muscle forces estimated using a calibrated EMG-informed neuromusculoskeletal model. Hip muscle contributions and co-contraction ratios (abduction/adduction and flexion/extension) were determined. Demographic, spatiotemporal, and primary outcome measures were compared between groups using independent t-tests. A secondary analysis of HJCF peaks was performed usingaone-way analysis of covariance with body mass index as a covariate (p<0.05). Results: Height, body mass, body mass index, and walking speed did not differ between groups. Participants with hip OA walked with an 11% lower first peak (p=0.01) and 28% lower second peak (p<0.01) HJCF compared to controls (Fig. 1). Differences remained significant when adjusted for body mass index. Participants in the hip OA group exhibited higher levels of hip muscle co-contraction in both flexion/extension (p<0.01) and abduction/adduction (p=0.04) during all phases of stance compared to controls. Hip muscle forces (p<0.01), peakhip extension (p<0.01) and peak hip adduction (p=0.03) joint angles, and peak hip flexion (p=0.01) and peak hip extension (p<0.01) joint moments were also lower in the hip OA compared to control group. Conclusions: Individuals with mild-to-moderate hip OA walk with lower HJCF, despite exhibiting higher levels of hip muscle co-contraction, compared to healthy controls. These observations contradict previous assumptions regarding the effect of higher levels of muscle co-contraction on joint loading in this cohort.This is the first study to demonstrate that under-loading is characteristic of individuals with early hip OA, and may suggest that interventions aimed at reducing hip joint loading are not appropriate for this patient group. Future studies are required to evaluate the effects of under-loading on hip cartilage health, with the aim of identifying modifiable treatment targets to slow disease progression in individuals with hip OA.