We describe the characteristics of displacement of the head and hip in normal young subjects standing on a moving platform undergoing continuous sinusoidal horizontal translation in the antero-posterior direction, at frequencies ranging from 0.1-1 Hz. The head, hip and malleolus were marked by light-emitting diodes (LEDs), and the displacement of each LED was quantified by (1) the measure of the shift during each cycle of translation, (2) the standard deviation (SD) of the path travelled during the whole trial, (3) the power spectrum (PS) of the signal and (4) the cross-correlation (CC) between pairs of LED signals. At each frequency of translation, with eyes open (EO), the displacement of head was smaller than that of hip, and the displacement of hip was smaller than that of malleolus. With eyes closed (EC), this order was reversed. The peak value of the CC functions of the pairs malleolus/head, malleolus/hip and hip/head decreased by passing from low to high frequency of translation, under both visual conditions, and decreased more for the pair malleolus/head than malleolus/hip. The lags between body segment displacements ranged between 30 ms and 150 ms, on average, the former segment of each pair preceding the latter. The fast Fourier transformation of hip and head displacement showed a power spectrum peak at the frequency imposed by the platform translation. The peak was larger with EC than EO. With EC, another peak appeared at 0.2 Hz, possibly corresponding to the respiratory frequency. We conclude that, when vision was allowed, subjects behaved as a non-rigid, noninverted pendulum, and stabilised head in space. When vision was denied, head oscillated more than the platform, especially at low translation frequencies. Therefore, the strategy of balance control shifted from a pendulum to an inverted-pendulum behaviour, passing from active head-and-trunk control to maximal body compliance to the perturbation.

Objectives and method-The relation between body sway recorded through a stabilometric platform and the subjective report of steadiness was studied in 20 young and 20 elderly subjects and 20 neuropathic and 20 parkinsonian patients standing upright. The trials were performed under two stances (feet apart, feet together) and two visual conditions (eyes open, eyes closed). At the end of each trial, subjects scored their performance on a scale from 10 (complete steadiness) to 0 (fall). Results-In all subjects, independently of the stance conditions, the larger the body sway the smaller the reported score. The function best fitting this relation was linear when sway was expressed on a logarithmic scale. The scoring reproducibility proved high both within and across subjects. Despite the diVerent body sways and scores recorded under the diVerent visual and postural conditions (eyes closed >eyes open, feet together>feet apart) in all groups of subjects and patients, the slopes of the relations between sway and score were broadly superimposable. In the normal subjects, the scores were slightly higher during eyes open than eyes closed trials for corresponding body sways. This was interpreted as a sign of perception of greater stability when vision was allowed. Parkinsonian patients swayed to a similar extent as normal subjects, and their scores were accordingly similar, both with eyes open and eyes closed. Neuropathic patients swayed to a larger extent than normal subjects, and their scores were matched appropriately. Although the slope of their relation with eyes closed was not diVerent from that of normal subjects, with eyes open it was steeper and similar to that with eyes closed, suggesting that these patients did not feel more stable when they could take advantage of vision. Conclusions-The subjective evaluation of body sway, irrespective of stance condition, age, neuropathy, and basal ganglia disease, reflects the actual sway, and is inversely proportional to the logarithm of the sway value. The remarkable similarity of the relation between score and sway across the various groups of subjects with eyes closed indicates a common mode of sway evaluation, possibly based on integration of several sensory inputs. All groups except neuropathic patients seem to take advantage of the redundancy of the inputs. Basal ganglia integrity does not seem to have a role in the evaluation of sway. (J Neurol Neurosurg Psychiatry 1999;66:313-322)

1. In standing humans, toe-up rotation of a platform induces a short-latency (SLR) and a medium-latency response (MLR) in both soleus (Sol) and flexor digitorum brevis (FDB) muscles. Toe-down rotation evokes a MLR in the tibialis anterior (TA). The SLR is the counterpart of the monosynaptic stretch reflex, but the origin of the MLR is still debated. By means of tizanidine (an a2 -adrenergic receptor agonist) we tested the hypothesis that the MLR is relayed by group II afferent fibres, since animal data indicate that tizanidine or stimulation of monoaminergic brainstem centres decrease the excitability of spinal interneurones supplied by those fibres. In addition, we compared the effect of the drug on these responses with that induced by stabilization of posture.2. Eight subjects received tizanidine (150 jtg kg-' orally) or placebo, in a single-blind design.Platform rotations were delivered prior to administration and for 3 h afterwards. Both TAand FDB-MLRs decreased in size, starting from about 1 h after tizanidine administration. Sol-SLR was unaffected. Response latencies were unchanged. Placebo induced no changes in any response. In each subject, the extent of TA-MLR depression induced by holding onto a frame and by tizanidine was superimposable. 3. The selective effect of tizanidine on MLR supports the notion that it is relayed through group II afferent fibres. The similar effects of holding and tizanidine on the response suggests that it is modulated by monoaminergic centres.

During upright stance, foot dorsiflexion induced by the movement of a supporting platform elicits a short- (SLR) and a medium-latency response (MLR) in both the soleus and the flexor digitorum brevis muscles; foot plantarflexion elicits a MLR in the tibialis anterior. The SLR is the counterpart of the stretch reflex, but no general agreement exists about the origin of the MLR, though recent results suggest that it is transmitted through group II afferent fibres. Animal studies have shown that group II fibres impinge on interneurones projecting contralaterally as well as ipsilaterally, whereas group I fibres impinge on interneurones which project mainly ipsilaterally. Therefore, we compared the changes in amplitude and latency of the SLRs and MLRs in the right and left limb during postural perturbations induced while subjects maintained both feet on the platform (both-on condition) or while they maintained only one foot on the platform and the other on firm ground (one-on condition). Under the both-on condition, the pattern of EMG responses described above occurred bilaterally. Under the one-on condition, both SLRs and MLRs occurred in the displaced leg. However, whereas the SLRs did not change in amplitude compared with the both-on condition, the MLRs decreased in amplitude to about 50%. MLRs were also present in the non-displaced leg. They were not preceded by any SLR but showed a further decrease in size with respect to the corresponding responses in the perturbed leg. Latency of the MLRs of the perturbed leg increased by about 5 ms passing from the both-on to the one-on condition. In the latter condition, a further increase of 5 ms was observed in the nonperturbed leg with respect to the displaced one. The occurrence of the MLRs but not of the SLRs in the contralateral non-displaced leg is in keeping with the notion that crossed neural pathways fed by spindle group II afferent fibres subserve the MLRs. The changes in latency of the MLRs under the one-on condition compared with both-on give a cue about the synaptic delays along the neural circuit and the time taken by the afferent impulses to cross the spinal cord.