Differential effect of knee extension isometric training on the different muscles of the quadriceps femoris in humans

This study determined the effects of a short period of knee isometric training on the quadriceps muscles accessible to surface electromyography (EMG). For this purpose, a training (n = 9) and a control (n = 7) group were tested on five identical occasions at 1 week intervals during 4 weeks. The training group exercised three times a week by making isometric knee extensions at 80% of the maximal voluntary contraction (MVC). During the test sessions, maximal and submaximal torque and associated activations of the rectus femoris (RF), vastus lateralis (VL) and vastus medialis (VM) muscles were analysed. As a result of training, differences between MVC values of the two groups were highly significant (P < 0.001), whereas only RF-EMG showed significant differences (P < 0.05). The VL and VM did not present any significant changes in maximal activation. The EMG-torque relationships were analysed individually before and after the training period. For the control subjects, EMG-torque relationships did not present significant changes while for the training group, these relationships showed a significant increase in RF, VL, and VM maximal activation in 6, 6 and 4 subjects, respectively, and a significant decrease in 1, 2 and 5 subjects, respectively. In almost all cases, a significant downward shift of the relationship was observed. This study confirmed that the parts of the quadriceps muscle tested present different adaptation capacities and demonstrate inter-individual variability in the strategies used to enhance muscle strength. In conclusion, to analyse the neural effects resulting from training in a large and compartmentalized muscle like the quadriceps femoris, it is desirable to take into account each muscle independently. Moreover, we suggest that overall results obtained from the experiment population should be completed by an analysis on individuals.