Effects of plantar cutaneo-muscular and tendon vibration on posture and balance during quiet and perturbed stance.

Modulation of lower limb somatosensory information by tendon or plantar vibration produces directionally specific, vibration-induced falling reactions that depend on the tendon or the region of the sole that is vibrated. This study characterized the effects of different patterns of plantar cutaneo-muscular vibration and bilateral Achilles tendon vibration (ATV) on the postural strategies observed during quiet and perturbed stance. Twelve healthy young participants stood barefooted, with their vision blocked, on two sets of plantar vibrators placed on two AMTI force plates embedded in a moveable support surface. Two other vibrators were positioned over the Achilles tendons. Participants were randomly exposed to different patterns of plantar cutaneo-muscular and ATV. Tilts of the support surface in the toes-up (TU) and toes-down (TD) directions were given 5-8s after the beginning of vibration. Body kinematics in 3D and ground reaction forces were recorded. Bilateral ATV applied with or without rearfoot vibration (RFV) during quiet stance resulted in a whole-body backward leaning accompanied by an increase in trunk extension and hip and knee flexion. RFV alone produced a forward whole-body tilt with increased flexion in trunk, hip, and ankle. When stance was perturbed by TU tilts, the center of mass (CoM) and center of pressure (CoP) displacements were larger in the presence of RFV or ATV and associated with increased peak trunk flexion. TD tilts with or without ATV resulted in no significant difference in CoM and CoP displacements, while larger trunk extension and smaller distal angular displacements were observed during ATV. RFV altered the magnitude of the balance reactions, as observed by an increase in CoP displacements and variable response in trunk displacement. Significant interactions between ATV and RFV were obtained for the peak angular excursions for both directions of perturbations, where ATV either enhanced (for TU tilts) or attenuated (for TD tilts) the influence of RFV. Manipulating somatosensory information from the plantar cutaneo-muscular and muscle spindle Ia afferents thus results in altered and widespread postural responses, as shown by profound changes in body kinematics and CoM and CoP displacements. This suggests that the CNS uses plantar cutaneo-muscular and ankle spindle afferent inputs to build an appropriate reference of verticality that influences the control of equilibrium during quiet and perturbed stance.