AUTONOMY VERSUS FORCING IN THE ORGANIZATION OF HUMAN RHYTHMIC FOREARMMOVEMENTS

In biological systems, obviously dissipative, some injection of muscle muscle force is required in order to sustain rhythmic movement. As th e movement frequency increases, the way the muscle-force-to-movement r elationship evolves (in timing and amplitude) can be used to character ize some fundamental control properties, including whether the observe d system is autonomous or forced In the case of a simple rhythmic, bio logical movement (single-joint horizontal forearm movement), this ques tion can be addressed by assuming that the processed electromyographic activity (EMG) is related to the muscle torques. In this case, 2 inte resting phenomena can be observed as the frequency increases. The firs t is that the phase lag between the force and movement remains constan t (40 degrees), and the second is that the co-contraction of the agoni st and antagonist muscle groups increases with the square of the frequ ency. These results showed that the contribution of muscle forces to m ovement organization cannot be regarded in terms of an escapement in a n autonomous system, nor in terms of a forcing function in a forced sy stem.