Antagonist cocontraction of knee flexors during constant velocity muscle shortening and lengthening.

Electromyography (EMG) was used to study the role of antagonist cocontration of the hamstring muscles of 24 normal subjects during constant velocity muscle performance tests on a KIN COM (™) dynamometer. The hypothesis tests whether antagonist cocontraction varies dependent on joint angle, limb velocity, and type of contraction. Seated subjects performed two dynamic tests of resisted quadriceps muscle shortening and lengthening through a 65° range of knee motion under two constant velocity conditions in a single session. Each test consisted of four repetitions of maximum effort constant velocity muscle shortening and lengthening of the knee extensors; one test performed at 30° s(-1), the other at 90° s(-1). The sequence of velocity testing was randomized. Normalized values of peak amplitude EMG of knee flexors were measured every 5° in the constant velocity phase of each test and compared in a paired manner between each angle, contraction type, and velocity, by split-plot 2-way ANOVA. Antagonist hamstrings cocontraction was found to be contraction type, and velocity dependent. Cocontraction was greater at larger angles of knee flexion and at higher velocity. These results do not support the hypothesis that such activity is a function of muscle moment-arm, but are consistent with the idea that antagonist muscle function is controlled by muscle spindles and perhaps Golgi tendon organs. Because antagonist cocontraction ranged up to a mean of 32% of maximum agonist activity in normal individuals, it should be taken into consideration when interpreting clinical muscle performance tests.