The purpose of this study is to examine a control mechanism of a neuromuscular system and to investigate its control characteristics in reaching movements. Here, a control mechanism for achieving antagonistic behavior between muscles is newly incorporated into a neuromuscular system. In addition, a performance criterion is defined from physiological viewpoints and a procedure for producing optimal trajectories is mathematically formulated. The human arm trajectories produced under various movement conditions are compared with ones observed in experiments and significant features of a proposed neuromuscular system are evaluated. Finally, characteristics of a neuromuscular system are investigated according to the results of computer simulations.