Comparisons of voluntary and evoked rate of torque development and rate of velocity development during isokinetic muscle actions

BACKGROUND: The rate of torque development (RTD) and the rate of velocity development (RVD) have previously been described as related; however, a direct comparison has not been performed.

OBJECTIVE: The purposes of this study were to compare voluntary and evoked RVD and RTD during the same maximal isokinetic leg extensions muscle actions and to indirectly explore the influence of motor unit discharge frequency on these variables.

METHODS: Sixteen men completed three maximal voluntary and three maximal evoked isokinetic leg extension muscle actions at 60°·s $^{-1}$ . Peak RVD, general RVD, peak RTD, and electromechanical delay (EMD) were calculated from the voluntary and evoked muscle actions. Voluntary and evoked RTD and RVD were also calculated for each 10 ms epoch up to 200 ms of the muscle actions.

RESULTS: There was no interaction between voluntary and evoked RVD across time (p=0.12), but there was an interaction for RTD (p<0.01). However, peak RTD occurred prior to the isokinetic load range. Peak RTD(p<0.001), peak RVD (p< 0.01), general RVD (p< 0.01), isokinetic load range (p<0.001), EMD (p<0.001), and PT (p<0.001) were greater for voluntary than evoked muscle actions, which was expected due to the influence of voluntary motor unit firing rates.

CONCLUSIONS: Overall, these results suggested that the calculation of RTD during the acceleration phase of an isokinetic muscle action may not be valid due to the unknown load and increasing velocity. Furthermore, the RVD may be influenced by motor unit firing rate, but to a lesser extent than RTD.