Relative contributions of the long and short heads of the biceps brachii during single or dual isometric tasks

Abstract

Electromyograms (EMG) of the long head (LH) and the short head (SH) of the biceps brachii were recorded during linearly increasing isometric elbow torques. These ramp contractions concerned flexion or supination of the elbow performed separately (flexion or supination single tasks) and simultaneously (flexion/supination dual tasks). EMG-torque relationships and EMG-EMG relationships were established. The EMG-torque relationships of the SH and LH show a more increased activity in supination than in flexion single task. The highest levels of LH and SH activations were found in the dual tasks. For the majority of subjects, the maximal flexion and supination torques reached in dual tasks were identical to those developed in single tasks. In the single tasks, the LH-SH relative contributions varied from one subject to another as attested by the EMG LH-EMG SH relationships, whereas in dual tasks, the EMG LH-EMG SH relationship was close to the identity line for all subjects, indicating a higher and homogeneous activation of both heads of the biceps. It is concluded that dual tasks are indicated in order to spread the motor unit (MU) recruitment or to activate fully a bifunctional muscle.