Evoked Contractile Properties of the Quadriceps Across a Clinical Spectrum of Knee Osteoarthritis

Abstract

Evoked contractile properties of whole muscle can provide indirect information about fibre composition and may provide insight into the mechanisms of quadriceps muscle weakness in knee osteoarthritis (OA). PURPOSE: To determine if differences in evoked contractile properties exist across a clinical spectrum of knee OA. METHODS: The following parameters were measured in the knee extensors in thirty-four patients (19 female) with clinical or radiographic knee OA: 1) isometric maximal voluntary torque, 2) resting maximal twitch tension (PT), time-to-peak tension (TPT) and half-relaxation time (HRT), 3) force-frequency characteristics measured with 500 ms pulse trains of 1, 2, 5, 8, 10, 12, 15, 20, 30, 40, 50, 75 and 100 Hz, with stimulus intensity normalized to a value eliciting a torque equivalent to 20% maximal voluntary contraction at 100 Hz. In order to facilitate comparison, participants were stratified into tertiles of disease severity based on their responses to the Western Ontario and McMaster Osteoarthritis Index. The lowest, middle and highest tertiles were designated mild, moderate and severe knee OA, respectively. RESULTS: Maximal isometric strength (normalized to body mass) was significantly lower in the severe (∼45%) and moderate (∼30%) compared to mild subgroup with post-hoc testing (p<0.05). No significant differences were observed across groups for PT, TPT or HRT (p>0.05). Torque values for each subgroup were not significantly different at any of the stimulation frequencies (p>0.05). CONCLUSION: Evoked contractile properties of quadriceps were not different across a clinical spectrum of patients with knee OA, despite differences in isometric knee extensor strength. These results indicate that muscle fibre composition may not be altered in knee OA and other mechanisms may play a greater role in the observed strength losses.