Control of stance during lateral and anterior/posterior surface translations.

The purpose of this study was to compare and contrast postural responses to lateral and A/P surface translations by quantifying joint positions, bilateral three-dimensional (3-D) ground reaction forces, and lower limb and trunk muscle electromyographic (EMG) activity. Subjects stood on a movable platform which was randomly translated in four different directions. The kinematic patterns in response to lateral and anterior/posterior (A/P) surface translations were similar in that there was a sequential displacement and reversal of the shank/thigh and then trunk segments. While the body center of mass (CoM) was displaced equally in response to lateral and A/P translations, equilibrium was maintained by redistributing the vertical forces and changing the shear forces exerted against the support surface. These force responses were bilaterally symmetrical for A/P translations but not for lateral translations. With respect to EMG activity, the first muscle activated was the proximal tensor fascia latae for lateral translations whereas the distal muscles were recruited first for A/P translations. Results from this study suggest that control of postural equilibrium may be similar for A/P and lateral translations, although specific differences in patterns may reflect various biomechanical constraints of the trunk and the lower extremities associated with the two planes of movement.