Response of electromyographic variables during incremental and fatiguing cycling

Purpose: Surface electromyographic (EMG) variables allow study of the electrical activity of motor units within active skeletal muscle. Recent methodological advances permit the estimation of muscle fiber-conduction velocity (MFCV) from EMG signals recorded during dynamic exercise. However, EMG responses to incremental and fatiguing cycling are poorly understood. Methods: Twenty males completed an incremental cycling protocol (20, 40, 60, and 80% of peak power output (PPO)) on two occasions separated by 5 d. The final stage was 6 min long, to induce muscle fatigue. EMG signals were recorded from the vastus lateralis and vastus medialis muscles, and root mean square (RMS) and MFCV values were estimated. Results: For incremental exercise, EMG data were reliable between trials, especially for MFCV in the vastus lateralis at 40, 60, and 80% PPO (e.g., coefficient of variation < 6.3%; bias < 0.4 m.s(-1); limits of agreement < 1.2 m.s(-1)). Significant positive correlations were observed between RMS and MFCV (r = 0.79-0.83), as both measures increased with power output (repeated-measures analysis of variance all P = 0.001). For fatiguing exercise, many EMG variables showed systematic bias between trials. RMS (P = 0.009 to 0.05 1) but not MFCV (P = 0.236-0.578) significantly increased during fatiguing cycling. Conclusions: During dynamic cycle exercise, EMG variables are reliable and increase with power output. During fatiguing exercise, EMG variables may be sensitive to learning effects in the execution of the task. MFCV and RMS are correlated with varying power output in the nonfatigued muscle, but not during the development of fatigue.