RATE OF FORCE DEVELOPMENT INFLUENCES MOTOR UNIT FIRING RATES AND RECRUITMENT PATTERNS DURING ISOMETRIC CONTRACTIONS

C.J. Lund1, Marissa D. Gingrich1, Jonathan D. Miller1, Mandy E. Wray1, Trent J. Herda1 ¹Uniersity of Kansas, Lawrence, KS PURPOSE: To examine differences in motor unit (MU) mean firing rates (MFR) and action potential amplitudes (MUAPAMP) versus recruitment threshold (RT) relationships during isometric contractions of the vastus lateralis (VL) with fast (FA) or slow (SL) rates of torque development. METHODS: Nine males (age=20.7±1.1 years, height=182.2±7.4 cm, weight=92.5±42.0 kg) and six females (age=20.7±1.4 years, height=166.3±5.2 cm, weight=62.2±17.9 kg) performed 3 maximal voluntary contractions (MVCs) with the highest MVC force used to determine the target force for the 2 subsequent contractions at 40% MVC. The rate of torque development for the SL contraction was 5% MVC s-1 while the FA was 20% MVC s-1. Participants maintained force output as close as possible to a target force presented in real time on a computer monitor. Surface electromyography (EMG) was recorded from the VL via 5-pin surface sensor array. EMG signals were decomposed into action potential trains of individual MUs to yield MFRs and MUAPAMPS which were regressed against RTs separately for each contraction. Dependent samples t-test were used to analyze differences between the SL and FA ramps. RESULTS: For the MFR vs. RT relationships, dependent samples t-tests indicated the SL contractions had more negative slopes (SL=-0.504, FA=-0.313, P=0.003) than FA, however, the y-intercepts were similar between contractions (SL=24.4, FA=23.8, P=0.46). For the MUAPAMP vs. RT relationships, dependent samples t-tests indicated the slopes were greater for the SL contractions (SL=0.00575, FA=0.00366, P\u3c0.001) than FA, however, there were no differences in the y-intercepts (SL=-0.0197, FA=-0.00389, P=0.079) CONCLUSIONS: The less negative slopes for the FA contraction indicated firing rates of the higher-threshold MUs were greater at the targeted force, compared to the SL contraction. The slopes of MUAPAMP vs. RT relationships were greater for the SL, which indicated larger MUs were being recruited at lower RTs in comparison to the FA contraction. It is plausible that the slower ramp (linear increase in force) may increase coactivation of antagonist muscles in order to adhere to a task which requires finer motor control. Further research should examine co-activation of the knee flexors during isometric contractions with faster and slower rates of torque development