An EMG frequency-based test for estimating the neuromuscular fatigue threshold during cycle ergometry

The purposes of this investigation were twofold: (1) to determine if the model used for estimating the physical working capacity at the fatigue threshold (PWC_FT) from electromyographic (EMG) amplitude data could be applied to the frequency domain of the signal to derive a new fatigue threshold for cycle ergometry called the mean power frequency fatigue threshold (MPF_FT), and (2) to compare the power outputs associated with the PWC_FT, MPF_FT, ventilatory threshold (VT), and respiratory compensation point (RCP). Sixteen men [mean (SD) age = 23.4 (3.2) years] performed incremental cycle ergometer rides to exhaustion with bipolar surface EMG signals recorded from the vastus lateralis. There were significant (p < 0.05) mean differences for PWC_FT [mean (SD) = 168 (36) W] versus MPF_FT [208 (37) W] and VT [152 (33) W] versus RCP [205 (84) W], but no mean differences for PWC_FT versus VT or MPF_FT versus RCP. The mean difference between PWC_FT and MPF_FT may be due to the effects of specific metabolites that independently influence the time and frequency domains of the EMG signal. These findings indicated that the PWC_FT model could be applied to the frequency domain of the EMG signal to estimate MPF_FT. Furthermore, the current findings suggested that the PWC_FT may demarcate the moderate from heavy exercise domains, while the MPF_FT demarcates heavy from severe exercise intensities.