Impact of Contraction Intensity and Velocity on Vastus Lateralis SEMG Power Spectrum and Amplitude

This study investigated the effect of contraction intensity (100%-, 75%-, 50%-, and 25%- maximum voluntary contraction [MVC]) and movement velocity (00 [isometric], 1000, 2000, 3000, and 4000 s-1) on root mean square amplitude (SEMG-RMS) and median frequency power spectrum (SEMG-MNF) of the surface electromyography of vastus lateralis (VL). Ten healthy female university students (age = 23.5 ± 2.7 years, mean height = 164.8 ± 6.7 cm, mean weight = 66.2 ± 6.3 kg) with no known knee pathologies participated in the investigation. Peak torque (PT), average torque (AVT), SEMG-MNF, and SEMG-RMS were analyzed using separate repeated measures analysis of variance. Results, significant at p < 0.05 or better, were: (1) an inverse relationship between PT and AVT and movement velocity, which was consistent with the literature and the force-velocity curve; (2) greater SEMG-MNF values during all isovelocity conditions compared to isometric, with highest values occurring at 500 s-1 and during 100%- and 75%- MVC; and, (3) an interactive effect of contraction intensity and velocity on muscle SEMG-RMS such that at all contraction intensities highest values were found at 2000 s-1, with a penchant for higher SEMG-RMS values during dynamic movements compared to isometric. It would appear that dynamic muscular contractions facilitated a greater recruitment of fast-twitch fibers -- as determined by increased SEMG-MNF values -- compared to that found during isometric contractions, and that this difference was intensified at slow isokinetic velocities (50 s-1), whereas greater overall muscle activation was found at 2000 s-1.