Title: Effects of muscle vibration on human postural responses

Abstract

Human upright posture is maintained by the central nervous system via integration of afferent and efferent signals resulting in postural orientation and postural equilibrium. Small changes in body orientation with respect to gravity cause posture destabilization manifested as body deviations from the equilibrium position. To control body sway, the postural system requires multimodal integration of visual, vestibular, and proprioceptive information. When influencing individual sensory inputs, a sensory conflict occurs, followed by a disturbance of the internal representation of the body vertical. By experimental stimulation of sensory systems, it is possible to study their relative roles in maintaining balance, and simultaneously monitor the impact of sensorimotor dysfunctions caused by age, disease or injury. One of the most effective ways of experimental stimulation of the proprioceptive system is the vibration of postural muscles or tendons, which leads to the so-called direction-dependent postural response. In our studies, we examined the responses to lower leg muscles vibration with different frequency and duration in young and elderly, as well as Parkinson's patients or patients after anterior cruciate ligament reconstruction. We also investigated effect of vibratory-visual, and vibratory-galvanic co-stimulations and evaluated postural responses during the stimulation and immediately after the vibration offset. We recorded body sway and kinematics of the trunk using a force plate, inertial sensors and motion capture system.Our results indicate that magnitude of postural responses increases linearly with increasing frequency and duration of vibration. We found that vision availability is crucial in responding to stimulus offset, with absent vision resulting in a greater postural response regardless of age. Significant differences due to age and Parkinson's disease were manifested as increased velocity of body sway and trunk tilts. A significant overshoot of the center of pressure over the initial equilibrium position suggests increased rigidity of movements in response to proprioceptive alteration. This work was supported by the grants VEGA 2/0080/22 and APVV-20-0420.