Dynamic stability of rhythmic forearm movements in the vertical plane

Comfortable amplitudes of rhythmic forearm movements increase as periods increase. To explain this phenomenon in the horizontal plane, it has been suggested that the nervous system prefers a high-stability movement under a feedforward control of the musculo-skeletal system. Here we tested this hypothesis in the vertical plane. The experimental measurement and the stability analysis of the nonlinear muscle model were performed. The stability analysis of the musculo-skeletal model showed a wide range of high-stability conditions, which cannot clearly explain the experimentally observed period-amplitude relationship. Reasons for the unclear results were discussed.