Relationship between knee joint angle and fine-wire EMG activity of the four quadriceps femoris muscles
The purpose of the present study was to investigate the electromyography (EMG)-knee joint angle relationship and the differences in activity among the four quadriceps femoris (QF) muscles during maximal voluntary isometric knee extensions. Ten well-trained healthy male volunteers (mean ± 1 standard deviation (SD) age 33 ± 11 years, height 1.83 ± 0.07 m, body mass 79 ± 4.7 kg) performed maximal isometric knee extensions in a seated position at four different knee joint angles (90º, 65º, 40º, and 15º of knee flexion). Myoelectric activity was simultaneously recorded from rectus femoris (RF), vastus lateralis (VL), vastus medialis (VM), and vastus intermedius (VI) using fine-wire EMG. Root-mean-square (RMS) was calculated during a sustained phase of one second at each of the different knee joint angles tested. Results showed that the RMS value of the VM was significantly higher at 90º compared to 40º (P < 0.05). When comparing 90º-normalized RMS values among the four QF muscles collapsed across the three most extended positions (65º, 40º, and 15º of knee flexion), significantly lower normalized RMS value was observed for the VM compared to that of the RF (P < 0.01) and VL (P < 0.05), respectively. No significant differences in RMS values between knee joint angles were observed for any of the other QF muscles (RF, VL, and VI). These results suggest that VM muscle activity is highest at more flexed knee joint angles during a seated knee extension task. Further, as the knee gets more extended, muscle activity of the VM decreases and VM becomes less active than the VL and RF, respectively.
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