The patterns of control signals underlying elbow joint movements in humans

Position, velocity, flexor and extensor electromyographic (EMG) activity of fast, moderate and slow elbow movements to a target were recorded and simulated using a model in which reciprocal and co-activation central commands, proprioceptive feedback and mechanical properties of muscles were considered. Two hypotheses concerning the pattern of shift in the equilibrium point (EP) underlying the movements were tested. First, the nervous system specifies a constant rate of EP shift to produce movement and encodes displacement by the duration of the shift (ramp-shaped pattern). Second, in fast movements, the EP rapidly shifts towards the future final position but then shifts back and forth eventually reaching the final EP (N-shaped pattern). The ramp pattern was consistent with kinematic and EMG experimental data regardless of movement speed. In contrast, the N-shaped pattern was incompatible with the kinematic characteristics of fast movements.