The effect of rate of torque development on motor unit recruitment and firing rates during isometric voluntary trapezoidal contractions.

It is common practice to examine motor unit (MU) activity according to mean firing rate (MFR) and action potential amplitude (MUAPAMP) vs. recruitment threshold (RT) relationships during isometric trapezoidal contractions. However, it is unknown whether the rate of torque development during the linearly increasing torque phase affects the activity of MUs during such contractions. Sixteen healthy males and females performed two isometric trapezoidal muscle actions at 40% of maximum voluntary contraction (MVC), one at a rate of torque development of 5% MVC/s (SLOW40) and one at 20% MVC/s (FAST40) during the linearly increasing torque phase. Surface electromyography (EMG) was recorded from the vastus lateralis (VL) via a 5-pin surface array sensor and decomposed into action potential trains of individual MUs, yielding MFRs and MUAPAMP which were regressed against RT separately for each contraction. Surface EMG amplitude recorded from leg extensors and flexors was used to quantify muscle activation and coactivation. MFR vs. RT relationship slopes were more negative (P = 0.003) for the SLOW40 (- 0.491 ± 0.101 pps/%MVC) than FAST40 (- 0.322 ± 0.109 pps/%MVC) and the slopes of the MUAPAMP vs. RT relationships (P = 0.022, SLOW40 = 0.0057 ± 0.0021 mV/%MVC, FAST40 = 0.0041 ± 0.0023 mV/%MVC) and muscle activation of the extensors (P < 0.001, SLOW40 = 36.3 ± 7.82%, FAST40 = 34.0 ± 6.26%) were greater for SLOW40 than FAST40. MU firing rates were lower and action potential amplitudes were larger in relation to recruitment thresholds for a contraction performed at a slower rate compared to a faster rate of isometric torque development. Differences in MU activity can exist as a function of rate of torque development during commonly performed isometric trapezoidal contractions.