Kinematic analysis of the hip and trunk during bilateral stance on firm, foam, and multiaxial support surfaces

Objective. To differentiate hip and trunk motion during double-leg stance. Design. Trunk and hip angular position variances were measured on different support surfaces with and without vision. Background. Postural control results from motion about the hips and trunk during bilateral stance. While the hip joint has been studied extensively, information concerning relative amounts of hip and trunk motions during postural control is limited. Methods. Trunk flexion/extension, trunk lateral flexion, right and left hip flexion/extension and abduction/adduction angular position variances were assessed in 14 normal subjects using an electromagnetic tracking system during bilateral stance on firm, foam, and multiaxial support surfaces with and without vision. Results. Significantly greater amounts of motion occurred at all joints for the multiaxial-eyes closed condition compared to all other surface–vision conditions. No significant differences were found between any other surface–vision conditions. Within the multiaxial-eyes closed condition, right and left hip flexion/extension and abduction/adduction magnitudes were significantly greater than those of trunk flexion and lateral flexion, and left hip flexion/extension motion was significantly greater than that of the right hip. Conclusions. Postural control mechanisms involve similar amounts of motion at the hips and trunk, except for conditions under which a rigid base of support becomes unstable and vision is eliminated.