The Impact Of Fatiguing, Intermittent Isometric Contractions On Muscle Force Variability

Abstract

PURPOSE: To evaluate the influence of fatigue on force variability of the vastus lateralis (VL) during repeated isometric exercise performed at 30% maximal voluntary isometric contraction force (MVIC). METHODS: Twelve resistance-trained males (23.8 ± 3.6 y; 87.3 ± 10.5 kg; 176.3 ± 6 cm) completed MVIC testing and then repeated isometric contractions at 30% MVIC while tracking target force trajectories repetitively until they could no longer achieve 95% target force. Force and surface electromyographic (EMG) signals from the VL were collected during exercise. One-way repeated measures ANOVAs were used to analyze average force (FAVG), coefficient of variation of force (FCV), EMG amplitude (EMGRMS), and median power frequency (MDF) during every 25% of the repetitions (REP) completed. To examine the relationship between EMGRMS and FCV across the work bout, FCV was plotted against EMGRMS, linear regression lines were fit, and correlation coefficients were calculated for each subject. RESULTS: No main effect of REP was observed for FAVG (p = 0.07) or MDF (p = 0.29), but there was for FCV and EMGRMS (p < 0.0001). FCV increased from the first to the 50-100% REP (p ≤ 0.001-0.02), 25% to the 100% REP (p < 0.001), and the 50% to the 100% REP (p < 0.001). EMGRMS increased from the first to the 25-100% REP (p = 0.002-0.024), the 25% to the 50-100% REP (p = 0.001-0.019), the 50% to the 75-100% REP (p = 0.002-0.018) and the 75% to the 100% REP (p = 0.014). Additionally, all 12 subjects displayed strong, significant, positive relationships for FCV vs. EMGRMS across the exercise bout (Table 1). CONCLUSIONS: The increase in both FCV and EMGRMS suggest that as fatigue develops during intermittent isometric exercise, force steadiness decreases while EMG amplitude increases. It is possible that the decrease in force steadiness occurred due to increased common synaptic input to the motor neuron pool secondary to fatigue-induced increases in net excitatory input.