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The costs of positon, velocity and force requirements in optimal control induce triphasic muscle activation during reaching movement


 The nervous system activates a pair of agonist and antagonist muscles to determine the muscle activation pattern for a desired movement. Although there is a problem with redundancy, it is solved immediately, and movements are generated with characteristic muscle activation patterns in which antagonistic muscle pairs show alternate bursts with a triphasic shape. To investigate the requirements for deriving this pattern, this study simulated arm movement numerically by adopting a musculoskeletal arm model and an optimal control based on the minimization of neural input. The simulation reproduced the triphasic electromyogram (EMG) pattern observed in a reaching movement using a cost function that considered three terms: end-point position, velocity, and force required. The first, second and third bursts of muscle activity were generated by the cost terms of position, velocity and force, respectively. Thus we concluded that the costs of position, velocity and force requirements in optimal control can induce triphasic EMG patterns. Therefore we suggest that the nervous system may control the body by using an optimal control mechanism that adopts the costs of position, velocity and force required, which serve to initiate, decelerate and stabilize movement, respectively.