Loss of large-diameter spindle afferent fibres is not detrimental to the control of body sway during upright stance: evidence from neuropathy.

Fifteen patients with Charcot-Marie-Tooth type 1A (CMT1A) disease and 46 normal controls were studied. In the patients, leg muscle strength, touch-pressure, vibration and joint position sense were reduced; lower limb tendon reflexes were absent in 12 or markedly decreased. Motor and sensory conduction velocity (CV) of leg nerves was either reduced or not measurable. The Neurological Disability Score and the Neuropathy Score were obtained from clinical and electrophysiological examination, respectively. Tilt of a supporting platform elicited short- (SLR) and medium-latency (MLR) responses to stretch in the foot muscle flexor digitorum brevis (FDB) in controls. In the patients, the former response was absent and the latter delayed. These findings are in keeping with the known loss of large-diameter myelinated fibres, with relative sparing of the smaller fibres. The MLR delay was fully accounted for by the slowed CV of the motor fibres. The MLR afferent time was similar to that in normal subjects. Body sway area (SA) during quiet stance was recorded with eyes open or closed, and with feet apart or together. Under all postural and visual conditions, SA was within normal range in the less severely affected patients, but was moderately increased in the patients with a more severe neuropathy score. Across all patients, no correlation was found between SA and muscle force, motor CV, touch pressure, vibration and joint position sense, considered either separately or as an aggregate. We suggest that: (1) functional integrity of the largest afferent fibres is not necessary for appropriate equilibrium control during quiet stance and (2) any unsteadiness is related to additional functional alterations in smaller fibres, most likely group II spindle afferent fibres.

Postural coordination in elderly subjects standing on a periodically moving platform.

OBJECTIVE: To test body segment coordination during dynamic equilibrium on a moving platform, and assess age-related differences in the performance. DESIGN: Subjects had infrared light-emitting diode (LED) markers fixed on their temporomandibularjoint, greater trochanter, and lateral malleolus. Subjects stood upright on a sinusoidally translating platform with both feet, either with eyes open (EO) or eyes closed (EC). The platform was displaced horizontally in an anteroposterior direction at frequencies of 0.2 or 0.6Hz. Each trial, for either visual condition, was repeated twice, lasted 30 seconds, with 30-second intervals, for up to 1 hour. PARTICIPANTS: Fifty-three healthy subjects, aged 17 to 89 years. MAIN OUTCOME MEASURES: Kinematics of head and hip were analyzed by measuring periodic shift of LED markers using an optoelectronic device. Standard deviation on traces of the markers were taken as a comprehensive index of segment placement. Cross-correlation between marker traces was determined to obtain measures of degrees of coupling between body segments. RESULTS: At the low translation frequency (0.2Hz), with EO, subjects behaved like a noninverted pendulum, whereby the head tended to be stabilized more than the hip, and the legs taking up most of the imposed displacement. Aging was associated with greater head stabilization, and a looser coupling between head and hip. With EC, the body attitude changed to an inverted pendulum, whereby the head overshot the platform displacement. Aging was associated with reduced head stabilization, and a stronger coupling between head and hip. When the frequency of the platform translation increased to 0.6Hz, with EC, the general picture was similar to the above, but in older subjects vision was no longer able to counteract the imposed head displacement. At this frequency, with both EO and EC, there was a poor coupling between body segments across all ages. CONCLUSIONS: Periodic shift of the support base may be a valid protocol to test the ability to control balance in the elderly, and may be a useful tool to assess age-related changes of the sensorimotor mechanisms underlying dynamic equilibrium.

Subjective perception of body sway.

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 different body sways and scores recorded under the different 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 different 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.

Standing on a continuously moving platform: is body inertia counteracted or exploited?

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.

Time course of stabilometric changes after a strenuous treadmill exercise.

OBJECTIVE: To detect the effect of a strenuous exercise on equilibrium and to quantify its time course. PARTICIPANTS AND METHODS: Body sway area, sway path, and center of foot pressure were recorded in eight young able-bodied subjects, standing quietly with feet together, with eyes open (EO) and eyes closed (EC), on a dynamometric platform, before and after treadmill walking for 25min, during which subjects approached the maximum heart rate. RESULTS: A significant increase in body sway was observed after exercise. It was present under both visual conditions and affected both sway area and sway path. Sway increased most in the initial few minutes (sway area EO=800.7mm2 [p < .005], EC=1,779.5mm2 [not significant]; sway path EO=545mm [p < .0001], EC=811.8mm [p < .05]) with respect to preexercise values (sway area EO=449.4mm2, EC=1,316.2mm2; sway path EO=369.5 mm, EC=652.5mm) and was followed by a plateau (sway area EO=609.9mm2, EC=1,567.9mm2; sway path EO=431.2mm, EC=710.9mm). Full recovery to basal values occurred within about 15min after the end of the exercise. CONCLUSIONS: Body sway is affected by prolonged fatiguing exercise, such as strenuous walking. This effect is of moderate extent and vanishes within a few minutes.

Fatigue effects on body balance.

Body sway variables (sway area and sway path) were recorded by a dynamometric platform in 13 young subjects, standing quiet with feet together, with eyes open (EO) and eyes closed (EC), prior to and following two types of physical exercise (treadmill walking and cycle ergometer pedalling). Each exercise was performed under both fatiguing (above anaerobic threshold) and non-fatiguing conditions. Following fatiguing treadmill exercise, we observed a significant increase in body sway with respect to pre-exercise values. The increase was present under both visual conditions, affected both sway area and sway path and lasted until about 15 min from the end of the exercise. The Romberg quotient (the ratio of EC/EO of sway area, or sway path) significantly increased after the fatiguing exercise with respect to the non-fatiguing exercise. The mean position of the centre of foot pressure (CFP) was unchanged after the exercise. Fatigue induced an increase in the median frequency of oscillation of the centre of foot pressure, independent of the amplitude of sway. Non-fatiguing treadmill exercise induced no significant changes in sway or in its frequency content. Following fatiguing cycle ergometer exercise, a negligible increase or a decrease (under eyes closed condition) in body sway were observed. Non-fatiguing cycling exercise induced no significant changes or a decrease in sway. Control experiments showed that simple repetition of successive stance trials (without intercalated exercise) was able by itself to induce a decrease in sway. By taking this effect into account, both types of cycling exercises revealed a mild capacity to increase sway. We concluded that body sway increased after strenuous physical exercise, but was little affected by exercise performed below the estimated anaerobic threshold. The effects of fatigue on sway were short-lasting and of moderate extent, and therefore were not liable to seriously threaten body equilibrium.

Human stance stability improves with the repetition of the task: effect of foot position and visual condition.

The effects of repetition of quiet stance trials on body sway, recorded through a stabilometric platform, were studied in 12 normal subjects. With feet together, both with eyes open (EO) and closed (EC), a progressive shift forward of the centre of foot pressure (CFP) occurred with repetition. In addition, with EC, but not with EO, a significant progressive reduction in sway area (SA) and sway path (SP) occurred. With feet 10 cm apart, initial SA and SP values were significantly smaller than with feet together, regardless of the visual condition, but repetition of trials induced no significant effects on either position of CFP or body sway under either visual condition. Results indicate the occurrence of a learning phenomenon in this simple postural task, whereby the body shifts towards a 'safer' position with a minimum energy expenditure due to reduced corrections of sway. Forward leaning and decrease in sway are two independently-occurring processes, each possibly due to a better central integration of proprioceptive input with repetition of trials.