Reduced muscle strength is regarded as a risk factor for pain and disability in osteoarthritis (OA). Currently, various indices for muscle strength are used when assessing determinants of pain and disability. The goal of the present study was to evaluate these indices of muscle strength.
Isometric muscle strength was measured for 16 muscle actions around the knees and hips in 52 patients with OA of the hip and 70 patients with OA of the knee. Various indices of muscle strength were derived from these measurements, applying five alternative approaches. These approaches ranged from a single overall index to a set of 16 separate indices. The internal consistency of these indices was determined (Cronbach's alpha), and it was determined to what extent they could reveal the association between reduced muscle strength on the one hand and pain and disability on the other hand.
Internal consistency was satisfactory for all indices (Cronbach's alpha >0.74). As expected, reduced muscle strength was associated with increased disability, but no clear relationship could be established between muscle weakness and pain. The strength of these associations did not depend on the approach used to derive the indices for muscle strength.
The indices did not show major differences with regard to internal consistency or the extent to which the association with pain and disability could be revealed. For reasons of parsimony, approaches resulting in few indices appear to be most useful. However, muscle strength was found to be significantly reduced around affected joints, compared with muscle strength around unaffected joints. Therefore, the most suitable approach for reducing muscle strength data into indices is one that results in as few indices as possible, but with separate indices for muscle strength around affected and unaffected joints.