RELATIVE CONTRIBUTIONS OF THE LONG AND SHORT HEADS OF THE BICEPS BRACHII DURING SINGLE OR DUAL ISOMETRIC TASKS

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 supinatio n of the elbow performed separately (flexion or supination single task s) and simultaneously (flexion/supination dual tasks). EMG-torque rela tionships and EMG-EMG relationships were established. The EMG-torque r elationships of the SH and LH show a more increased activity in supina tion than in flexion single task. The highest levels of LH and SH acti vations were found in the dual tasks. For the majority of subjects, th e maximal flexion and supination torques reached in dual tasks were id entical to those developed in single tasks. In the single tasks, the L H-SH relative contributions varied from one subject to another as atte sted by the EMG LH-EMG SH relationships, whereas in dual tasks, the EM G LH-EMG SH relationship was close to the identity line for all subjec ts, indicating a higher and homogeneous activation of both heads of th e biceps. It is concluded that dual tasks are indicated in order to sp read the motor unit (MU) recruitment or to activate fully a bifunction al muscle.