STABLE HUMAN STANDING WITH LOWER-LIMB MUSCLE AFFERENTS PROVIDING THE ONLY SENSORY INPUT

1. This study investigated the sources of sensory information upon whi ch normal subjects' ability to stand depends. 2. An 'equivalent body' was used to simulate the physical properties of each subject's body du ring 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 t he appropriate changes in ankle torque. Thus, stability of stance coul d be studied with (normal stance) and without (balancing equivalent bo dy) modulation by vestibular inputs. Vision could be excluded by closi ng the eyes. Sensory input from the feet and ankles could be removed b y local anaesthesia from prolonged ischaemia, induced by occluding blo od flow with inflated pneumatic cuffs just above the ankles. With vest ibular, visual and peripheral sensory inputs negated, standing could r ely only upon remaining sensory inputs, notably those from sensory rec eptors in the leg muscles. 3. Unlike the human body, the equivalent bo dy used to negate vestibular inputs is not segmented. Therefore, the e ffects on stability of having a segmented body were determined by spli nting subjects during standing so that only ankle movement was possibl e. This was done in the presence and absence of visual stabilization. 4. For each experimental task, either standing or balancing the equiva lent 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 s table way when the eyes were closed, and when the feet were anaestheti zed. Therefore, they could perform a task that was equivalent to stand ing but when only sensory information from the leg muscles was availab le to detect the sway. However, although subjects were stable, sway wa s greater than when they were standing normally with all sensory input s available. 6. In all situations, sway increased significantly when t he eyes were shut, or when splinting prevented the movement of body se gments above the ankles. In contrast, there was a small increase in sw ay when sensory inputs from the feet and ankles were excluded. Sway wa s similar for standing with the body splinted (vestibular inputs avail able) and for balancing the equivalent body (vestibular inputs exclude d). 7. It is concluded that, for normal subjects: (i) proprioceptive s ignals from receptors in the leg muscles are sufficient to maintain a stable upright stance; (ii) visual inputs are necessary for maximal st ability; (iii) the effect of the segmented body is to significantly im prove stability; (iv) sensory information from the feet and ankles has a smaller but significant effect on stability; and (v) during normal standing, vestibular inputs are not responsible for modulating activit y in the leg muscles to assist stability.