Activation linearity and parallelism of the superficial quadriceps across the isometric intensity spectrum

The purpose of this study was to assess neuromuscular activation of the thr ee superficial portions of the quadriceps femoris muscles during linearly i ncreasing isometric contraction intensities. Thirty healthy volunteers were assessed for isometric electromyographic (EMG) activity of the vastus medi alis (VM), vastus lateralis (VL), and rectus femoris (RF) muscles with the knee at 60 degrees of flexion. For 5 s, subjects performed isometric contra ctions equivalent to 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, and 90% of the average of three maximal voluntary contractions (MVC), in random order. Fu ll-wave rectified and integrated EMG signals over the middle 3 s of each co ntraction were expressed as a percentage of the activity recorded during th e three averaged MVCs. One sample t-tests and 95% confidence intervals were calculated at each relative torque level. A two-factor analysis of varianc e (muscle by intensity) with repeated measures was performed to evaluate pa rallel activation across the intensity levels. Activation linearity was ass essed via regression analysis for each muscle. VM activation was shown to b e significantly lower than expected at 20-70% MVC. VL and RF activations we re significantly higher than expected at 10% MVC, and RF EMG was less than expected at 40-70% MVC. EMG of VM was shown to increase significantly more than VL and RF from 80% to 90% MVC. Significant linear and quadratic relati ons were also demonstrated for all three muscles. Parallel activation of th e superficial quadriceps muscles occurred from low to moderate intensities, whereas convergence was noted at near maximal intensities. (C) 2000 John W iley & Sons, Inc. Muscle Nerve 23: 393-398, 2000.