DISCHARGE RATE OF SELECTED MOTOR UNITS IN HUMAN BICEPS BRACHII AT DIFFERENT MUSCLE LENGTHS

Action potentials of selected motor units (MUs) from biceps brachii mu scle were recorded and analysed at three different elbow angles: 90, 1 20 and 150 degrees, corresponding to short, control and long muscle le ngth, respectively. Using branched and conventional bipolar wire elect rodes, superficial and deep-situated MUs were selectively recorded at relatively equal torques (the torque values were normalized to the cor responding maximal torque for a given muscle length). A total of 138 M Us (74 superficial and 64 deep) were investigated. The mean interspike intervals were significantly shorter at 90 degrees for the majority ( 52.2%) of the investigated MUs than at the other two angles. This incr eased discharge rare compensates for the reduction of twitch duration of evoked contraction at short muscle length. The other MUs were divid ed into three almost equal groups: two with significantly higher disch arge rates at 120 and 150 degrees and one with discharge rates unaffec ted by the joint angle. No significant difference in the discharge rat e of superficial and deep MUs at a fixed joint angle was found. (C) 19 98 Elsevier Science Ltd. All rights reserved.