Rate of Torque Development during a Maximal Voluntary Isometric Contraction as a Surrogate Outcome for Isokinetic Power

Quadriceps function has been demonstrated to predict disability in a variety of pathological patient populations (chronic obstructive pulmonary disorder, knee osteoarthritis, diabetes). Recent evidence suggests that knee extensor power may be more closely associated with disability than typical measures of strength. Unlike isometric strength assessments, evaluating power requires the use of cumbersome and expensive equipment. However, rate of torque development (RTD) during a maximal voluntary isometric contraction (MVIC) may closely associate with power outcomes; if so, RTD may be a surrogate outcome for power that can be assessed more easily in clinical settings. PURPOSE: Determine the association between knee extensor RTD and peak isokinetic power at 60, 180 and 240°/s after accounting for isometric strength. METHODS: The dominant limb of 26 healthy volunteers was secured to an isokinetic dynamometer for testing (21 females, 5 males; Age = 20.1±0.95 yr; Height = 170.4±7.7m; Mass = 66.0±10.44kg). Peak MVIC torque (strength) and peak RTD outcomes were collected from the mean of two MVIC at 90° of knee flexion, where individuals were instructed to generate an MVIC as fast as possible. Peak isokinetic power was extracted during the concentric knee extension phase at each of the contraction speeds (60, 180 and 240°/s). Linear regression models were used to determine the amount of variance in peak power (at each contraction speed) that could be explained by RTD. To determine the change in R2 due to adding RTD in to each of the peak power speeds, separate models were fitted hierarchically with MVIC only, and then with MVIC and RTD jointly. RESULTS: Together, MVIC and RTD explained R2=0.54 (P<0.001), R2=0.48 (P=0.001), R2=0.57 (P<0.001) at 60, 180 and 240°/s, respectively. After accounting for MVIC, RTD accounted for ΔR2= 0.08 (P=0.06), ΔR2= 0.17 (P=0.01), ΔR2= 0.09 (P=0.04), of the variance in peak power at 60, 180 and 240°/s, respectively. DISCUSSION: Together, MVIC and RTD predicted between 48% and 57% of the variance in peak power. RTD significantly predicted between 9% and 17% of the variance in peak power after accounting for MVIC. RTD may not be an adequate surrogate for peak power outcomes. Future studies should determine if RFD, MVIC, and isokinetic power provide unique information regarding muscle dysfunction.