The behavior of rectus femoris and vastus lateralis during fatigue and recovery: an electromyographic and vibromyographic study

Eleven subjects performed isometric knee extensor contractions at maximum voluntary contraction (MVC), and at 70% of MVC preceding and following a fatigue test. The fatigue test consisted of a continuous isometric knee extensor contraction at 70% of MVC. The purpose of this study was to describe and compare the behavior of electromyographic (EMG) and vibromyographic (VMG) signals of of rectus femoris (RF) and vastus lateralis (VL) muscles during a fatigue-recovery protocol by analyzing the median frequency (MDF) and the root mean square (RMS) values obtained from these signals. EMG and VMG signals of RF and VL muscles were recorded using bipolar surface electrodes and miniature accelerometers, respectively. The MDF of EMG and VMG signals of the two muscles decreased during muscle fatigue and returned to the pre-fatigue values during the fifteen minute recovery period. Assuming a relation between the decrease of the MDF of the EMG signals and a decrease in the conduction velocity of the action potentials in the muscle fibers with fatigue, the subsequent restoration of the MDF indicates that muscle fiber conduction velocity returned to a pre-fatigue state within a short time. The decrease in the MDF of the VMG signals seems to be associated with low-frequency tremor that typically occurs towards the end of exhausting isometric contractions. The RMS of the EMG increased in both RF and VL throughout fatigue and recovery, and may be attributed to increased muscle activation. The RMS values of the VMG do not appear to change in a systematic way during the fatigue and recovery periods.