Spinal coordination of bilateral leg muscle activity during balancing.

While subjects were standing and balancing on two separate seesaws, the EMG of the leg muscles and the positions of the two seesaws were recorded. The spontaneous balancing movements with predominant oscillations of 4-5 Hz, and the accompanying bursts of EMG activity in the leg muscles occurred quite symmetrically on the two sides. After a displacement, induced either by stimulating the tibial nerves, or by a brisk anterior tilt of one seesaw, the EMG responses of the tibialis anterior muscles started with the same latency (about 50 ms) on both sides, and with similar amplitudes, even when only one side was displaced. It is concluded that this symmetrical leg muscle activation is mediated by a spinal coordinating mechanism the function of which depends on the actual motor task.