Ultrasound measures of muscle thickness may be superior to strength testing in adults with knee osteoarthritis: a cross-sectional study

Evaluation of muscle strength as performed routinely with a dynamometer may be limited by important factors such as pain during muscle contraction. Few studies have compared formal strength testing with ultrasound to measure muscle bulk in adults with knee osteoarthritis (OA). We investigated the muscle bulk of lower limb muscles in adults with knee OA using quantitative ultrasound. We analyzed the relationship between patient reported function and the muscle bulk of hip adductors, hip abductors, knee extensors and ankle plantarflexors. We further correlated muscle bulk measures with joint torques calculated with a hand held dynamometer. We hypothesized that ultrasound muscle bulk would have high levels of interrater reliability and correlate more strongly with pain and function than strength measured by a dynamometer. 23 subjects with unilateral symptomatic knee OA completed baseline questionnaires including the Western Ontario and McMaster Universities Arthritis Index (WOMAC) and Lower Extremity Activity Scale. Joint torque was measured with a dynamometer and muscle bulk was assessed with ultrasound. Higher ultrasound measured muscle bulk was correlated with less pain in all muscle groups. When comparing muscle bulk and torque measures, ultrasound-measured muscle bulk of the quadriceps was more strongly correlated with measures of pain and function than quadriceps isometric strength measured with a dynamometer. Ultrasound is a feasible method to assess muscle bulk of lower limb muscles in adults with knee OA, with high levels of interrater reliability, and correlates negatively with patient reported function. Compared with use of a hand held dynamometer to measure muscle function, ultrasound may be a superior modality.