Discharge rate of selected motor units in human biceps brachii at different muscle lengths

Action potentials of selected motor units (MUs) from biceps brachii muscle were recorded and analysed at three different elbow angles: 90, 120 and 150 degrees, corresponding to short, control and long muscle length, respectively. Using branched and conventional bipolar wire electrodes, superficial and deep-situated MUs were selectively recorded at relatively equal torques (the torque values were normalized to the corresponding maximal torque for a given muscle length). A total of 138 MUs (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 increased discharge rate compensates for the reduction of twitch duration of evoked contraction at short muscle length. The other MUs were divided into three almost equal groups: two with significantly higher discharge rates at 120 and 150 degrees and one with discharge rates unaffected by the joint angle. No significant difference in the discharge rate of superficial and deep MUs at a fixed joint angle was found.