Implementation of the equilibrium trajectory hypothesis for movement generation in the arm

Summary form only given. A mathematical model of the neuromuscular system is built to describe some of the consequences of the equilibrium trajectory hypothesis (ETH) regarding the role of spinal control structures in movement. This model builds on the assumption that the spring-like reaction of the arm to small disturbances is mainly due to the length-tension properties of the muscles and not the length-dependent spinal reflexes. In order to explore point-to-point movements, a two-joint model of the arm is constructed, and its inverse dynamics are solved to predict movement trajectories for developed muscular forces. ETH suggests that movement is controlled by the central nervous system through gradual shifting of the arm's equilibrium point. A minimum jerk criterion function is used to define this virtual trajectory. Since ETH defines the virtual trajectory only based on kinematics, it appears to be ill-suited for assigning neuronal activation rates: such a program must also take into account the nature of the task, e.g., the stability of the system in motion. An algorithm is suggested for assigning firing rates for a given virtual trajectory. To determine the role of the spinal reflexes, the model is tested in the case where no afferent information is available, so the virtual trajectory serves as the only source of neuromuscular activation. The deafferented arm is nevertheless capable of reaching the goal, although its time for completion of the task is much longer.