Activation Of Knee Extensor Muscles During Rapid Force Development In Men And Women

Abstract

Adequacy of activation during rapid voluntary contractions is limited in young and old adults and can be shown by comparing the voluntary rate of force development (RFD) and electrically evoked RFD. Nonetheless, it is unknown whether the activation of rapid voluntary contractions differs between young men and women. PURPOSE: The purpose of this study was to compare the maximal RFD of young men and women during electrically evoked isometric contractions and rapid voluntary contractions with the knee extensor muscles across a range of torques. METHODS: Eight young adults (18-26 years; 4 men, 4 women) consented to sets of single and double pulse (10 ms interval) stimulations of the femoral nerve at maximal intensities followed by rapid voluntary isometric knee extensions at target torques matched to the electrically evoked torques. For the rapid voluntary isometric contractions, subjects were instructed to kick as fast as possible rather than as accurately as possible. Voluntary target torques ranged between 10-40% of maximal voluntary contraction (MVC) and were set to match the electrically evoked torques. Maximal RFD for each trial (voluntary and electrically evoked) was calculated as the peak values of the first derivative of the torque signal. RESULTS: Torques (between 10-40% MVC) were similar for the electrically evoked and voluntary contractions (F1,6=0.54, P=0.49) for both the men (25.0 ± 0.6 %MVC vs. 26.4 ± 0.7 %MVC) and women (26.0 ± 2.0 %MVC vs. 26.5 ± 2.7 %MVC). Although torques were similar, RFD from electrically evoked contractions in men (514.8 ± 30.4 % MVC.s−1) and women (475.1 ± 78.0 %MVC.s−1) was 36% greater than the voluntary contractions (men; 330.2 ± 18.8 vs. women; 305.9 ± 21.2 %MVC.s−1; F1.6 = 23.1, P = 0.003). The linear association between relative torque (%MVC) and electrically evoked RFD (R2 = 0.85, b = 0.92, P<0.001) was larger and steeper than the association with voluntary RFD (R2 = 0.72, b = 0.85, P<0.001) indicating that the difference in RFD between the electrically evoked and voluntary contractions increased at the higher forces. CONCLUSIONS: Young men and women had marked reductions in the ability to voluntarily generate isometric knee extension torque rapidly compared with electrically evoked contractions that were independent of the central nervous system. Supported by NIA R01 AG048262 to SKH