Repeatability of electrically evoked EMG signals in the human vastus medialis muscle

The repeatability of spectral and amplitude parameters and conduction velocity estimates of M-waves was tested on the vastus medialis muscle of 9 normal subjects. Isometric contractions sustained for 30 s were elicited by stimulation of the main muscle motor point and repeated on 5 different days. The initial value and two indicators of myoelectric manifestations of muscle fatigue were computed for each variable. The test-retest Pearson correlation coefficient, the paired Wilcoxon test, and the intraclass correlation coefficient (ICC) showed that parameters of spectral variables are more repeatable than those of amplitude variables and conduction velocity. The ICC ranged from 86.4% for the initial value of median frequency to 21.6% for the initial value of the average rectified value. Fatigue indices showed ICCs in the range of 20-64%. It is concluded that: (a) initial values and fatigue indices based on spectral variables are more repeatable than those based on amplitude variables; (b) the repeatability of conduction velocity estimates is not yet sufficient for clinical application; (c) M-wave shape, rather than amplitude or width, is a characteristic of individual muscles; and (d) electrode location is critical.