SELECTIVE MUSCLE ACTIVATION FOLLOWING RAPID VARUS VALGUS PERTURBATIONS AT THE KNEE/

Knee muscles are generally divided into groups based on their function as flexors or extensors. In this study we sought to determine if musc les were selectively activated according to their potential roles as v arus or valgus stabilizers following rapid loads to the knee. While su bjects were supine, varus or valgus moments were applied to the knees of 10 human subjects using a servomotor-driven perturbation device. Du ring the experiments, electromyograms (EMG) were recorded from seven m uscles, four of which had medial moment arms relative to the knee cent er, and three of which had lateral moment arms. It was observed that, for all medial muscles, a statistically significant increase in muscle activation followed valgus loads as compared with varus loads. All la teral muscles except the vastus lateralis showed the opposite response (as expected). These results suggest that muscles can be reflexively activated independent of their roles as flexors or extensors to provid e stability to the human knee during varus or valgus loads. The timing of the reflex is consistent with that arising from joint mechanorecep tors, although polysynaptic stretch reflex may also be involved.