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 ele ctromyogram (EMG) power spectrum during dynamic exercise. Despite these stu dies there is a shortage of studies of the validity of surface EMG registra tions during repetitive dynamic contractions with respect to fatigue. The a im of this study was to investigate if the surface EMG variables mean frequ ency (MNF [Hz]) and the signal amplitude (RMS [mu V]) are valid indicators of muscular fatigue (defined as "any exercise-induced reduction in the capa city 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 9 0 degrees s(-1). EMG signals were recorded from the vastus lateralis, the r ectus femoris and the vastus medialis of the right thigh by surface electro des. MNF and RMS of the EMG together with peak torque (PT [Nm]) were determ ined for each contraction. MNF showed consequently higher correlation coeff icients with PT than RMS did. Positive correlations generally existed betwe en MNF and PT. The majority of the subjects had positive correlations betwe en 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 durin g dynamic maximum contractions. (C) 2000 Elsevier Science Ltd. All rights r eserved.