Reaction time in bimanual simultaneous motions.

Electromyographically determined reaction times (EMG-RTs) of the finger flexor and extensor of both forearms were measured for four different motions: inward (task 1), flexion of both wrists; outward (task 2), extension of both wrists; to the left (task 3), extension of the left wrist and flexion of the right; and to the right (task 4), flexion of the left and extension of the right. The EMG-RTs were shorter and synchronization errors in terms of left to right differences of EMG-RTs were smaller in tasks 1 and 2 than in tasks 3 and 4. Comparing the flexors and the extensors, the extent of prolongation of EMG-RTs in tasks 3 and 4 differed significantly on the left side, being larger in the flexor than in the extensor, but there was no difference in the extent of prolongation between the flexor and the extensor on the right. It was suggested that the timing of initiation of movements in simultaneous motions was primarily determined by the pattern of muscle coupling in both limbs, and not by the direction of movements. The steadiness of motor function and of the right hand in right-handed subjects was also discussed in regard to hand preference.