The relationship between electromyography and work intensity revisited: a brief review with references to...

The aim of this investigation was to re-evaluate the relationship between electromyography and work intensity during incremental work in light of highly discrepant literature. Trained male subjects participated in the study (n = 14). Each subject completed a VO2max test on a cycle ergometer. Tests started at a power output of 60 Watts with a 30 Watt.4 min-1 work increment. Each test was terminated at exhaustion. Blood was collected at the end of each work intensity for lactate and ammonia analysis. EMG were recorded from the vastus lateralis, rectus femoris and vastus medialis using pre-amplified surface electrodes. EMG were collected at each intensity over a period of 60 cycle revolutions. EMG signals were analyzed using integration and EMG spectral analysis. Gas exchange variables were recorded on-line for each test (15 second interval). Ammonia and lactate threshold points were surpassed at the same absolute work intensity (200 Watts) which was equivalent to 64-69% VO2max. When a linear model was applied to the iEMG data, coefficients of determination achieved were greater than those obtained when an exponential model was used for the vastus lateralis and medialis. Gradients of regression lines fitted to iEMG data at pre- and post-lactate/ammonia threshold work intensities were not different. Alternatively, the iEMG-work intensity relationship for the rectus femoris muscle tended to be curvilinear. Significant increases in iEMG were observed at post-lactate/ammonia threshold work intensities for the rectus femoris reflecting increases in fatigue and type II motor unit recruitment at these intensities. In general, median frequency of the EMG power spectrum function were unchanged during incremental work, although highly individualistic results were observed between some subjects and muscles. Grouped median frequency values were insensitive to changes in recruitment, metabolite accumulation and fatigue associated with the increases in work intensity. Consequently, the usefulness of EMG spectral analysis during incremental work was questioned.