Autonomy versus forcing in the organization of human rhythmic forearm movements.

In biological systems, obviously dissipative, some injection of muscle force is required in order to sustain rhythmic movement. As the movement frequency increases, the way the muscle-force-to-movement relationship evolves (in timing and amplitude) can be used to characterize some fundamental control properties, including whether the observed system is autonomous or forced. In the case of a simple rhythmic, biological movement (single-joint horizontal forearm movement), this question can be addressed by assuming that the processed electromyographic activity (EMG) is related to the muscle torques. In this case, 2 interesting phenomena can be observed as the frequency increases. The first is that the phase lag between the force and movement remains constant (40 degrees), and the second is that the co-contraction of the agonist and antagonist muscle groups increases with the square of the frequency. These results showed that the contribution of muscle forces to movement organization cannot be regarded in terms of an escapement in an autonomous system, nor in terms of a forcing function in a forced system.