Postural coordination patterns associated with the swinging frequency of arms.

Voluntary arm movements frequently perturb body equilibrium in an upright posture. The motions of leg joints need to be coordinated according to the properties of voluntary arm movements in order to maintain body equilibrium, and this may cause a change in postural pattern. The purpose of this study was to determine whether the kinematic pattern generation of upright posture is influenced by a change in the swinging frequency of arm movements and whether the pattern generation is correlated with a change in joint torque about the shoulder joint. Four male subjects in an upright posture were instructed to swing their arms at seven different frequencies, determined by the maximum swinging frequency of each subject (35%max, 40-60%max, 65%max). Segment rotations around the shoulder, hip, and ankle joints were analyzed at kinematic and kinetic levels. The results of kinematic analysis indicated that tight coupling between motions of the shoulder and hip joints was generated in lower-frequency trials (under 40-45%max), whereas tight coupling between motions of the shoulder and ankle joints was generated in higher frequency trials (more than 40-45%max). Furthermore, the results of kinetic analysis revealed that changes in the joint torque patterns about the shoulder and hip joints occurred in trials at 40-45%max. The mean value of 40-45%max was close to the eigenfrequency of each subject's arm. We concluded that (1) postural patterns associated with a gradual change in the swinging frequency of the arms can be divided into two coordination modes (a hip-shoulder in-phase mode and an ankle-shoulder in-phase mode), and (2) these two patterns may be divided by the eigenfrequency of the arm.