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Purpose of this study was to investigate activity of lower extremity muscles in response to single-leg stance on a training device, destabilizing the forefoot while the rearfoot stands on a fixed plate and vice versa compared with a balance pad and the floor. Twenty-seven participants performed three single-leg quiet stance trials under one stable (floor) and 5 different unstable balance conditions. Surface electromyography was used to record activity of 6 muscles of the lower extremity. Sagittal knee joint angles were controlled using 2D video analysis. The majority of lower extremity muscles were significantly more active when the forefoot was destabilized while the rearfoot remained stable compared with the stable and the other unstable conditions. Sagittal knee joint angles were significantly increased under the same conditions, indicating an altered strategy to maintain equilibrium.