Knee extensor torque, work, and EMG during subjectively graded dynamic contractions

We examined knee extensor peak torque, work, and electromyogram (EMG) during dynamic contractions to perceived exertion levels in men and women. Thirty subjects performed three maximal effort isokinetic knee extensions (60 deg x s(-1)), followed by three contractions to each of nine separate levels of perceived exertion. Surface EMG of the vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF), and knee extensor peak torque and work were normalized to a percent of each respective value obtained during the maximal effort contractions. The results demonstrated a significant linear increase in voluntary knee extensor peak torque and work across perceived exertion levels. Knee extensor peak torque and work were less than 70%, 80%, and 90% maximal voluntary contraction (MVC) at perceived exertion levels 7, 8, and 9, respectively. A significant increase in VM, VL, and RF muscle EMG was observed across perceived exertion levels 1 through 9, with EMG increase highest for the VL. The findings demonstrate that dynamic contractions guided by perceived exertion are underproduced at relatively high perceptual intensities, and that reliance on VL activation occurs across submaximal torque levels. The overestimation of knee extensor peak torque and work at relatively high perceptual intensities may suggest the presence of a subconscious mechanism aiming to reduce high muscle and joint forces.