Co-activation of quadriceps and hamstring muscles during concentric and eccentric isokinetic exercise

The purpose of this study was to investigate co-activation of the quadriceps and hamstring musculature during concentric and eccentric isokinetic actions at 30° s -I. Eight female sedentary subjects performed maximal concentric and eccentric knee flexion/extension actions on an isokinetic dynamometer while simultaneous recordings of moment, joint position and agonist/antagonist electromyographic (EMG) activity from the rectus femoris and semitendinosus muscles were computer processed. Mean integrated electromyographic (IEMG) activity for agonist and antagonist muscle groups and mean moment data were calculated for three discrete knee angle ranges; outer (25-34°), mid (45-54°) and inner (65-74°) for both concentric and eccentric actions. Individual IEMG activity was corrected to account for background activity in the unstressed muscle at 30°, 50° and 70° of knee flexion/extension. The results of this study show that under concentric and eccentric actions the hamstrings were more active during knee extension than the quadriceps were during knee flexion at all angle ranges studied. The corrected mean per cent IEMG activity of the co-contracting hamstrings throughout the joint range were 14 and 21 % for concentric and eccentric actions respectively, for co-contracting quadriceps these values were 2.5 and 3.5% respectively. All per cent IEMG values were expressed as a percentage of the same muscle group and action (shortening/lengthening) during agonist action. No significant difference was seen between concentric and eccentric co-contraction per cent IEMG activity at any knee angle range studied. The data suggests that the hamstrings are used to a much greater extent than the quadriceps for limb deceleration and that in particular, hamstring antagonist muscle tension should be considered when analysing agonist quadriceps isokinetic moment curves.