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Prince of Wales Medical Research Institute, Prince of Wales Hospital, Sydney, New South Wales, Australia.

1. This study investigated the sources of sensory information upon which normal subjects' ability to stand depends. 2. An 'equivalent body' was used to simulate the physical properties of each subject's body during standing. The modulation of ankle torque required to support the equivalent body in an upright position was similar to that required to support the subject's own body when standing. However, when balancing the equivalent body, vestibular inputs were excluded from directing the appropriate changes in ankle torque. Thus, stability of stance could be studied with (normal stance) and without (balancing equivalent body) modulation by vestibular inputs. Vision could be excluded by closing the eyes. Sensory input from the feet and ankles could be removed by local anaesthesia from prolonged ischaemia, induced by occluding blood flow with inflated pneumatic cuffs just above the ankles. With vestibular, visual and peripheral sensory inputs negated, standing could rely only upon remaining sensory inputs, notably those from sensory receptors in the leg muscles. 3. Unlike the human body, the equivalent body used to negate vestibular inputs is not segmented. Therefore, the effects on stability of having a segmented body were determined by splinting subjects during standing so that only ankle movement was possible. This was done in the presence and absence of visual stabilization. 4. For each experimental task, either standing or balancing the equivalent body, sway was recorded while posture was unperturbed. Root mean square values of sway amplitude and power spectra were used to compare conditions. 5. Every subject could balance the equivalent body in a stable way when the eyes were closed, and when the feet were anaesthetized.(ABSTRACT TRUNCATED AT 250 WORDS)

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