Muscle weakness impairs the proprioceptive control of human standing.

The leg muscles have two distinct roles in human standing. They are a principle source of the sensory input used to detect body sway and they also produce the contractile force that corrects body sway. In this population study, we provide evidence for a link between these contractile and sensory functions of muscle. In subjects classified as having weak or strong leg muscles, we compared body sway with and without vision. Subjects (17) with weakness through prior-polio were compared with age-matched controls (34) and from 174 subjects aged 60-69, those classified weak (<15 Nm ankle dorsiflexion) were compared with the strong (>or=15 Nm). The weaker and stronger groups from these populations had equivalent visual acuity and lower-limb sensory function. However, the weaker swayed disproportionately more than the stronger on closing the eyes. Strength alone could not cause this increased sway of the weaker subjects because they were as stable as the strong subjects when the eyes were open. This effect of strength was not apparent in an older group (>or=70 years, n = 276), where eye closure increased sway by similar amounts in the weak and strong. This appears to be related to visual and somatosensory impairments and increased morbidity in the weak of this group, an association not present in the younger groups. We conclude that there is a relative failure of proprioceptive postural control associated with muscle weakness. This indicates a functional link between contractile and sensory muscular processes and shows that multiple sensory inputs are more important for people with muscle weakness.