DISCHARGE PATTERN OF HUMAN MOTOR UNITS DURING DYNAMIC CONCENTRIC AND ECCENTRIC CONTRACTIONS

Objectives: A total of 45 motor units (MUs) from the human biceps brac hii muscle were investigated during isovelocity concentric and eccentr ic movements performed by means of a device implementing an external t orque in the direction of the extension proportionally to the elbow an gle changes. The effects of movement velocity on the recruitment and d ecruitment thresholds (RT and DT) and the corresponding discharge patt erns were determined. Methods: A wire branched electrode placed subcut aneously was used to descriminate the potentials from a single MV. Res ults: The majority of MUs (91%) were recruited at lower torque values with the increase of movement velocity. The decrease of RT was statist ically significant for 47% of the investigated MUs. A typical discharg e pattern of short first interspike interval (ISI) followed by a longe r one was observed for 93% of all MUs. After the first 2-3 spikes the rate of the MU discharge was approximately constant regardless of the fact that the muscle force gradually increased until the end of the co ncentric movement. Conclusions: There are differences in the muscle fo rce control during shortening and lengthening contractions. For 82% of the investigated MUs DT was smaller at faster movements and for 21 MU s (47%) the decrease of DT was significant. The gradually declined MU discharge rate throughout the entire movement with a very long last IS I was demonstrated for 93% of the investigated MUs. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.