A rapid step up was compared with a rapid step forward to assess the challenge they posed to postural stability in the medio-lateral (ML) direction. The center of pressure (COP) motion, and hip abductor muscle activity were compared between the two tasks in nine young healthy subjects. Analysis revealed that in the step up task there was a greater range and velocity of medio-lateral COP motion, an earlier gluteus medius onset time and a larger magnitude of gluteus medius activity than in the step forward task. The step up task appears to provide a greater challenge to ML postural stability than a step forward and thus may be an appropriate paradigm for the investigation of medio-lateral postural instability.