Discharge pattern of human motor units during dynamic concentric and eccentric contractions.

OBJECTIVES: A total of 45 motor units (MUs) from the human biceps brachii muscle were investigated during isovelocity concentric and eccentric movements performed by means of a device implementing an external torque in the direction of the extension proportionally to the elbow angle changes. The effects of movement velocity on the recruitment and decruitment thresholds (RT and DT) and the corresponding discharge patterns were determined.
METHODS: A wire branched electrode placed subcutaneously was used to discriminate the potentials from a single MV.
RESULTS: The majority of MUs (91%) were recruited at lower torque values with the increase of movement velocity. The decrease of RT was statistically significant for 47% of the investigated MUs. A typical discharge pattern of short first interspike interval (ISI) followed by a longer 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 concentric movement.
CONCLUSIONS: There are differences in the muscle force control during shortening and lengthening contractions. For 82% of the investigated MUs DT was smaller at faster movements and for 21 MUs (47%) the decrease of DT was significant. The gradually declined MU discharge rate throughout the entire movement with a very long last ISI was demonstrated for 93% of the investigated MUs.