Criterion validation of surface EMG variables as fatigue indicators using peak torque: a study of repetitive maximum isokinetic knee extensions.

A number of studies have been published that have used variables of the electromyogram (EMG) power spectrum during dynamic exercise. Despite these studies there is a shortage of studies of the validity of surface EMG registrations during repetitive dynamic contractions with respect to fatigue. The aim of this study was to investigate if the surface EMG variables mean frequency (MNF [Hz]) and the signal amplitude (RMS [microV]) are valid indicators of muscular fatigue (defined as "any exercise-induced reduction in the capacity to generate force or power output") during maximum repeated isokinetic knee extensions (i.e. criterion validity using peak torque). Twenty-one healthy volunteers performed 100 isokinetic knee extensions at 90 degrees s(-1). EMG signals were recorded from the vastus lateralis, the rectus femoris and the vastus medialis of the right thigh by surface electrodes. MNF and RMS of the EMG together with peak torque (PT [Nm]) were determined for each contraction. MNF showed consequently higher correlation coefficients with PT than RMS did. Positive correlations generally existed between MNF and PT. The majority of the subjects had positive correlations between RMS and PT (i.e. decreases both in PT and in RMS). In conclusion, at the individual level MNF generally - in contrast to RMS - showed good criterion validity with respect to biomechanical fatigue during dynamic maximum contractions.