The characteristics of postural response in upright stance to the floor vibration

Abstract

This study was designed to investigate the anticipated postural control and the dynamic postural control ability through the postural response to the floor vibration during maintaining the standing posture. A vibration table with a force platform, on which the subjects maintained the standing posture with eyes open or closed, was vibrated sinusoidaly in anteroposterior or right-left directions under the condition of 2.5 cm amplitude and various frequencies (0.1, 0.5, 1.0 and l.5 Hz). The fluctuations of the center of foot pressure (CFP) and various body portions (vertex, lumbale and caput fibulae) were measured using the force platform and potentiometers to which the silk threads connected the movement of the body portions. By frequency spectrum analysis, the amplitude and phase of each fluctuation in vibration frequency were obtained. The stability was evaluated by the amplitude of the CFP fluctuation. Subjects were ten male trained gymnasts and ten untrained male university students . The results were summarised as follows ; 1) The fluctuation of the body in an absolute coordinate system decreased in amplitude and increased in phase delay with the increase in frequency of vibration. Such trend was especial1y remarkable in the higher portions of the body. 2)In the vibration of 0.1Hz, the phase advance of the CFP fluctuation to the floor vibration was observed in about a half of the subjects, and the stability of standing posture in open eyes was higher in the subjects with phase advance than those with phase delay. The phase advance of the CFP fluctuation couldn't be explained by the physical properties and was accepted as the evidence of the feed forward postural control. 3) In the vibration of 0.5Hz or higher, the phase of the CFP fluctuation to the floor vibration was delay in all the subject, and the greater the phase delay, the higher the stability of standing posture. It was suspected that the stability was augmented by actively controlling the phase delay which caused by the physical properties of the body. 4) In the vibration of 0.5 and 1.0Hz, the stability of standing posture was much higher in the trained than in the untrained and with eyes open than with eyes closed. It was suggested that those vibration frequencies were suitable for the evaluation of dynamic postural control ability.