Interindividual variability of surface EMG changes during cycling exercise in healthy humans

We studied surface electromyogram (SEMG) changes during 1-h endurance cycling in 12 healthy subjects of whom five were involved in mountain bike training programme. The work load was set at 50% of the predicted maximal heart rate. The surface EMG and the compound evoked muscle action potential (M-wave) from the vastus lateralis muscle were recorded at rest, during the 1-h cycling period, and the 20-min recovery period. The root mean square (RMS) and the median frequency (MF) of SEMG power spectrum were computed. In all subjects, there was no shift in the median frequency throughout the cycling period and the increase in RMS remained stable. In subjects untrained to endurance cyclism, the M-wave duration increased at the end of the cycling period and these changes persisted for a consecutive 15-min period during recovery of exercise. By contrast, in trained mountain bikers the M-wave duration decreased after 2 min of exercise--the effect persisting for 2 min during recovery. These data suggest that the interpretation of M-wave changes during cycling must take into consideration the sport practices of the subjects and also that SEMG power spectrum and M-wave explore different electrophysiological events.