Posture is maintained through a combination of muscle tone and reflexes. The muscles that maintain posture contain a high proportion of slow-twitch fibers to allow for sustained contraction. Postural reflexes integrate inputs from proprioceptors, the vestibular system and visual system to make continuous corrections to muscle activity and maintain balance. The spinal cord, brainstem and cerebellum are involved in regulating these reflexes. Upright human posture relies on minimal muscle activity but reflex adjustments of antigravity muscles in response to sway to oppose the effects of gravity.