Human Arm Trajectory Formation

Humans select one of infinite number of trajectories in voluntary arm movements and it is reported that the trajectory is rationally determined from dynamical and physiological characteristics of a human arm. Accordingly, it is necessary for human arm trajectory formulation to obtain a dynamic model and an objective function in which physiological characteristics of muscle is considered. This paper proposes a dynamic model and an objective function based on characteristics of muscle viscoelasticitiy and energy expenditure. Experiments have been conducted for human arm motions in a vertical plane and the trajectories given from the experiments and proposed trajectory formation are analyzed. As a result, formulated trajectories show good agreements with measured ones and it is verified that the trajectories which humans select have physiological advantages of minimizing energy expenditure in muscles. Furthermore, different types of trajectories are observed, which vary with humans in relatively large movements. In the proposed arm model, adjusting factors in the case of similar arm dimensions are only coefficients on muscle viscoelasticity and variety of trajectories according to humans are discussed to be caused by differences of muscle viscoelasticity characteristics.