Without visual feedback voluntary torque is overestimated during muscle potentiation despite similar motor unit firing rate and perception of exertion.

Abstract

The purpose was to assess whether visual feedback of torque contributes to motor unit (MU) firing rate reduction observed during postactivation potentiation (PAP) of skeletal muscle. From 15 participants 23 MUs were recorded with intramuscular fine-wire electrodes from the tibialis anterior during isometric dorsiflexion contractions at 20% of maximum, with and without both PAP and visual feedback of torque. A 5-s maximal voluntary contraction (MVC) was used to induce PAP, and evoked twitch responses were assessed before and after. After the MVC twitch torque was 188% of baseline (P < 0.001). Without visual feedback of torque and with participants targeting 20% MVC, torque, MU firing rates and rating of perceived exertion (RPE) were 22.8 ± 5.3%MVC, 14.3 ± 2.6 Hz, 1.79 ± 0.93 a.u., respectively. Inducing PAP without feedback but targeting 20% MVC torque was overestimated by 50% (P < 0.001) despite similar firing rates and RPE as baseline (both P values ≥ 0.3). With visual feedback, torque was not overestimated during PAP (P = 0.14), however, firing rates and RPE were lower (13% and 20%, respectively) than baseline (both P values ≤0.008). Therefore, no compensatory modifications in MU output occurred despite muscle potentiation. This indicates lower voluntary drive, reflected additionally by reduced RPE, was responsible for the reduced firing rates so that torque did not exceed the required task, compared with modified peripheral feedback. During PAP, the motoneuron is not sensitive to alterations in the active state of the muscle unit per se, but rather compensatory adjustments to optimize contractile output are due to reductions in descending input.NEW & NOTEWORTHY Without visual feedback of torque, no modification in motor unit output occurred despite peripheral muscle potentiation. Therefore, reduced voluntary drive lowered firing rates so that torque did not exceed the task demand, rather than peripheral muscle afferent feedback. During muscle potentiation, the motoneuron is not sensitive to acute enhanced changes in the active state of the muscle unit per se, but rather compensatory adjustments in neuromuscular output are due to perceptual reductions in descending input.