THE PATTERNS OF CONTROL SIGNALS UNDERLYING ELBOW JOINT MOVEMENTS IN HUMANS

Position, velocity, flexor and extensor electromyographic (EMG) activi ty of fast, moderate and slow elbow movements to a target were recorde d and simulated using a model in which reciprocal and co-activation ce ntral commands, proprioceptive feedback and mechanical properties of m uscles were considered. Two hypotheses concerning the pattern of shift in the equilibrium point (EP) underlying the movements were tested. F irst, the nervous system specifies a constant rate of EP shift to prod uce movement and encodes displacement by the duration of the shift (ra mp-shaped pattern). Second, in fast movements, the EP rapidly shifts t owards the future final position but then shifts back and forth eventu ally reaching the final EP (N-shaped pattern). The ramp pattern was co nsistent with kinematic and EMG experimental data regardless of moveme nt speed. In contrast, the N-shaped pattern was incompatible with the kinematic characteristics of fast movements.