Effects of Fatigue on Voluntary Electromechanical and Relaxation Electromechanical Delay

The purposes of the present study were to examine: 1) the effects of fatigue on electromechanical delay from the onsets of the electromyographic signal to force production (EMD_E-F), the onsets of the electromyographic to mechanomyographic signals (EMD_E-M), the onsets of the mechanomyographic signal to force production (EMD_M-F), as well as the cessations of the electromyographic to force production (R-EMD_E-F), cessation of the electromyographic to mechanomyographic signals (R-EMD_E-M), and cessations of the mechanomyographic signal to force production (R-EMD_M-F); and 2) the relative contributions from EMD_E-M and EMD_M-F to EMD_E-F as well as R-EMD_E-M and R-EMD_M-F to R-EMD_E-F from the vastus lateralis in non-fatigued and fatigued states. The values EMD_E-F, EMD_E-M, EMD_M-F, R-EMD_E-F, R-EMD_E-M and R-EMD_M-F were calculated during maximal voluntary isometric contractions, before and after 70% 1-repetition maximum leg extensions to failure. There were significant pretest to posttest increases in EMD_E-F (73%;p<0.01), EMD_E-M (99%;p<0.01), EMD_M-F (60%;p<0.01), R-EMD_E-F (101%;p<0.01) and R-EMD_M-F (368%;p<0.01)_, but no significant change in R-EMD_E-M (25%;p=0.46). Fatigue-induced increase in EMD_E-F indicated excitation-contraction coupling failure (EMD_E-M) and increases in the compliance of the series elastic component (EMD_M-F). Increases in R-EMD_E-F were due to increases in relaxation time for the series elastic component (R-EMD_M-F), but not changes in the reversal of excitation-contraction coupling (R-EMD_E-M).