Maximal force during eccentric and isometric actions at different elbow angles.

The purpose of this study was to examine a course of force potentiation and/or inhibition during maximal voluntary eccentric action. Maximal voluntary force (MVC) of elbow flexion of ten healthy male volunteers was measured during isometric and isokinetic eccentric action starting from 80 degrees or 110 degrees and ending at 140 degrees elbow angle. Surface EMG was recorded from biceps brachii (BB) and brachioradialis (BR) muscles. Maximal voluntary eccentric force during the first 10 degrees of the movement was higher (P<0.001) than the maximal voluntary isometric preactivation force both in 80 degrees and in 110 degrees starting position at all three velocities (1, 2, and 4 rad s(-1)). The relative force potentiation was velocity dependent being smallest at the lowest stretching speed (P<0.01). Average EMG (aEMG) of BB and BR decreased as the joint angle increased both in eccentric and in isometric actions but the decrease in aEMG towards extension was somewhat higher in eccentric actions as compared to isometric. It was concluded that the force measured during the first 10 degrees of eccentric contraction always exceeded the maximal voluntary isometric preactivation force regardless of the joint angle or of the movement velocity. When maximal voluntary preactivation preceded the stretch, the relative force potentiation seemed to be greater at higher stretching velocities (velocity dependent) while at lower preactivation levels, the velocity dependence was not observed. Decreased muscle activation and lower maximal voluntary force towards the end of the movement suggested inhibition during maximal voluntary eccentric actions.