Central and peripheral fatigue in isometric contractions with distinct rates of force development

Purpose. The purpose of this study was to determine if distinct rates of force development (30%, 60%, 90% and 100% max RFD) affect the location of fatigue (central vs peripheral) in isometric contractions. Methods. Degree of voluntary activation and surface electromyography (EMG) were evaluated to check the agreement between them. Ten currently untrained males were fatigued in a 5 condition cross-over design with distinct RFDs; 30%, 60%, 90%, 100% max RFD/sec and control condition (no fatigue protocol). The degree of voluntary activation was tested with a supramaximal superimposed stimuli before and after the fatigue protocol. Root mean square (RMS) and median frequency of the EMG signal for the VM and VL muscles were averaged for the first 5, middle 5, and last 5 repetitions of the fatigue protocol, for each condition. Results. The degree of voluntary activation did not change from the beginning to the end of the fatigue protocol (growth curve analysis). The intercepts were not different (P > 0.85) from each other across conditions and the slopes were all equal to zero ( P > 0.24). The same analysis of the RMS of the EMG signal for both muscles showed that the mean linear slope averaged over the 4 RFD conditions increased for the VM muscle in a rate of 174.42 ± 10.85 (μV/half of total repetitions) and for the VL in a rate of 169.51 ± 12.03 (μV/half of total repetitions). The median frequency had the opposite behavior; the mean average linear slopes were of −2.723 ± 2.24 (Hz/half of total repetitions) and −2.86 ± 2.18 (Hz/half of total repetitions) for the VM and VL, respectively. Conclusions. As the degree of voluntary activation did not change and the RMS increased and MF decreased from the beginning to the end of the fatigue protocol, we conclude that the RFD does not change fatigue location. Fatigue was peripheral across all RFD conditions.