MOTOR UNIT DISCHARGE DIFFERS WITH INTENSITY AND TYPE OF ISOMETRIC CONTRACTION PERFORMED WITH THE ELBOW FLEXOR MUSCLES

Abstract

Discharge characteristics of low-threshold motor units differ in isometric contractions when subjects maintain a target force (force task) compared with maintaining a target position (position task) (Jakobi et al, 2002). It is unknown if this difference exists at higher force levels. PURPOSE To compare changes in motor unit activity between a force task and a position task performed at high (≥ 30% MVC) and low (≤ 20% MVC) forces. METHODS Motor unit (n = 15) activity was recorded with branched wire electrodes that were inserted subcutaneously over the biceps brachii muscle in the non-dominant arm. Contractions were sustained for 60–300 s at a target force that was 5% of MVC greater than the recruitment threshold of the motor unit. The order of the tasks for each motor unit was randomized, and subjects rested for 15 min between tasks. RESULTS At low force levels, the initial mean discharge rate was similar for the force (12 ± 3 Hz) and position (12 ± 2 Hz) tasks. At high force levels, however, the initial mean discharge rate was greater during the force task (17 ± 1 Hz) compared with the position task (13 ± 2 Hz). The decline in discharge rate from the beginning to the end of the contraction was greater for the position task (low force 22% change, high force 33% change) compared with the force task (low force 12% change, high force 22% change), and was greater in both tasks for high-force contractions. Discharge rate variability increased during the contraction for both tasks and force levels; however, the increase was greater for high-force contractions (p ≤ 0.05). CONCLUSION Adjustments in the discharge rate of motor units in biceps brachii were more pronounced during high-force contractions compared with low-force contractions for both tasks. Supported by NIH grant AG09000 awarded to RME