Balance assessments will be performed immediately before and after completion of the intervention programs. Balance perturbations will be delivered using a large (2x2m) computer-controlled motion platform. Subjects will stand at the center of the platform and a motor-driven moveable handhold system will be mounted to the front wall of the platform. In each trial, the handhold (a 10cm rod) will be controlled to move, intermittently and unpredictably, back and forth along a horizontal axis in the frontal plane, before coming to rest at one of four final locations (corresponding to visual angles of 5, 10, 15, and 20 degrees). The moving platform will deliver a small perturbation a few seconds after the final handhold location is reached. Subjects will be told to grasp the handhold as quickly as possible in response to the platform motion. During each trial, subjects will perform a concurrent visual vigilance task, which requires them to continuously look straight ahead at the center of a computer screen. This task is designed to ensure the subjects rely on peripheral vision to acquire the spatial information needed to direct the hand toward the handhold following the onset of the platform perturbation. The absence of any eye movements toward the grasp location will be confirmed via recordings from a head-mounted eye tracker. A motion-analysis system will be used to determine the speed and accuracy of the grasping reactions.