Electrically Evoked Isotonic Plantar Flexion Contractions Are Impaired Less than Voluntary After a Dynamic Fatiguing Task

Abstract

Purpose: Evaluating central and peripheral processes responsible for reduced power after dynamic fatiguing tasks are often limited to isometric torque, which may not accurately reflect dynamic contractile performance. Here, we compare voluntary and electrically evoked peak power (and its determinants: dynamic torque and velocity) and rate of velocity development (RVD) before and after a dynamic fatiguing task using concentric Plantar flexion contractions.

Methods: Young (18-32 yr) males ( n = 11) and females ( n = 2) performed maximal-effort isotonic Plantar flexion contractions using a load of 20% isometric torque until an approximately 75% reduction in peak power. Voluntary and electrically evoked (300 Hz tibial nerve stimulation) contractions loaded to 20% and 40% isometric torque through 25° ankle joint range of motion were compared before and 0, 2.5, 5, and 10 min after task termination.

Results: At task termination, peak power and RVD of voluntary contractions at both loads were reduced more (~40% to 50% reduction) than electrically evoked (~25% to 35% reduction) contractions ( P < 0.001 and P = 0.003). Throughout the recovery period, electrically evoked peak power and RVD returned to baseline sooner (<5 min) than voluntary contractions, which were still depressed at 10 min. Reductions in peak power for the 20% load were equally due to impaired dynamic torque and velocity, whereas velocity was impaired more than dynamic torque ( P < 0.001) for the 40% load.

Conclusions: The relative preservation of electrically evoked power and RVD compared with voluntary contractions at task termination and quicker recovery to baseline indicates that the reductions in dynamic contractile performance after task termination are due to both central and peripheral processes; however, the relative contribution of dynamic torque and velocity is load dependent.