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 (MUAP AMP ) 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 (SLOW 40 ) and one at 20% MVC/s (FAST 40 ) 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 MUAP AMP 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 SLOW 40 (− 0.491 ± 0.101 pps/%MVC) than FAST 40 (− 0.322 ± 0.109 pps/%MVC) and the slopes of the MUAP AMP vs. RT relationships ( P = 0.022, SLOW 40 = 0.0057 ± 0.0021 mV/%MVC, FAST 40 = 0.0041 ± 0.0023 mV/%MVC) and muscle activation of the extensors ( P