Abstract
The effect of contraction intensity (100%-, 75%-, 50%-, and 25%-maximum voluntary contraction [MVC]) and movement velocity (0°, 100°, 200°, 300°, and 400°/s) on the root mean square amplitude(SEMG-RMS) and median frequency power spectrum (SEMG-MPF) of the surface electromyography of the vastus medialis (VM) and rectus femoris (RF) muscles during isokinetic extension movements was investigated. Eight healthy female university students (age=23.1 ± 2.6 yrs., mean height=161.8 ± 6.8 cm, mean weight=64.6 ± 6.1 kg) with no known knee pathologies participated in the study. Peak moment (PM), SEMG-MPF, and SEMG-RMS were analyzed using separate repeated measures analysis of variance (ANOVA). Results indicated (p⩽ 0.05): (1) an inverse relationship between PM and movement velocity, which was consistent with the literature and the force-velocity curve; (2) an increase in VM and RF SEMG-RMS across all isovelocity movements compared to isometric, with highest values occurring at 200°/s; (2) a linear increase in SEMG-RMS as force levels increased from 25%-MVC to 100%-MVC; and, (3) an increase in SEMG-MPF values during isovelocity movements compared to that found during isometric contractions, with highest values at 50°/s, suggesting isovelocity movements facilitated a greater recruitment of fast-twitch fibers. It appears that the degree to which the central nervous system activates muscle is based on the interplay of both contraction intensity and velocity needed to complete the required motor task.