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

Electromyography (EMG) was used to study the role of antagonist cocontraction of the quadriceps muscles of 24 normal subjects during constant velocity muscle performance tests on a KIN/COM dynamometer. The hypothesis tested the dependence of antagonist cocontraction on joint angle, limb velocity and type of contraction. Seated subjects performed two dynamic tests of resisted muscle shortening and lengthening of the knee flexors through a 65 degrees range of knee motion under two constant velocity conditions in a single session. Each test consisted of four repetitions of maximum voluntary contractions (MVC) of constant velocity muscle shortening and lengthening of the knee flexors; one test was performed at 30 degrees s(-1), the other at 90 degrees s(-1). The sequence of velocity testing was randomized. Normalized (%Max) values of peak amplitude EMG of knee extensors were measured every 5 degrees in the constant velocity phase of each test and compared in a paired manner for each angle, contraction type and velocity, by split-plot two-way analysis of variance (ANOVA). The level of antagonist quadriceps cocontraction was low, ranging from 5-8% Max between 70 degrees and 15 degrees of knee flexion for both velocities and contraction types. Antagonist quadriceps activity was found to be velocity dependent during agonist muscle shortening (P < 0.02). However, no other effects of angle, contraction type, or velocity were found for antagonist quadriceps. It is concluded that antagonist quadriceps and the previously reported hamstrings(46) respond differently under typical constant velocity test conditions, and these differences need to be accounted for when intepreting muscle performance tests.