ANTAGONIST COCONTRACTION OF KNEE EXTENSORS DURING CONSTANT VELOCITY MUSCLE SHORTENING AND LENGTHENING

Electromyography (EMG) was used to study the role of antagonist cocont raction of the quadriceps muscles of 24 normal subjects during constan t velocity muscle performance tests on a KIN/COM dynamometer. The hypo thesis tested the dependence of antagonist cocontraction on joint angl e, limb velocity and type of contraction. Seated subjects performed tw o dynamic tests of resisted muscle shortening and lengthening of the k nee flexors through a 65 degrees range of knee motion under two consta nt velocity conditions in a single session. Each test consisted of fou r repetitions of maximum voluntary contractions (MVC) of constant velo city muscle shortening and lengthening of the knee flexors; one test w as performed at 30 degrees s(-1), the other at 90 degrees s(-1) The se quence 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 man ner for each angle, contraction type and velocity, by split-plot two-w ay analysis of variance (ANOVA). The level of antagonist quadriceps co contraction was low, ranging from 5-8% Max between 70 degrees and 15 d egrees of knee flexion for both velocities and contraction types. Anta gonist quadriceps activity was found to be velocity dependent during a gonist muscle shortening (P < 0.02). However, no other effects of angl e, contraction type, or velocity were found for antagonist quadriceps. It is concluded that antagonist quadriceps and the previously reporte d hamstrings(46) respond differently under typical constant velocity t est conditions, and these differences need to be accounted for when in tepreting muscle performance tests.