Bipedal distribution of human vestibular-evoked postural responses during asymmetrical standing

• Published in: The Journal of physiology Vol. 542; no. 1; pp. 323 - 331
• Main Authors: Marsden, J. F., Castellote, J., Day, B. L.
• Format: Journal Article
• Language: English
• Published: Oxford, UK The Physiological Society 01-07-2002; Blackwell Publishing Ltd; Blackwell Science Inc
• Subjects: Adult; Electromyography; Foot - physiology; Functional Laterality - physiology; Gravitation; Head Movements - physiology; Humans; Isometric Contraction - physiology; Leg - physiology; Linear Models; Male; Original; Posture - physiology; Space life sciences; Vestibule, Labyrinth - physiology
• ISSN: 0022-3751; 1469-7793
• DOI: 10.1113/jphysiol.2002.019513

Galvanic vestibular stimulation (GVS) evokes responses in muscles of both legs when bilateral stimuli are applied during normal
stance. We have used this technique to assess whether asymmetrical standing alters the distribution of responses in the two
legs. Subjects stood either asymmetrically with 75 % of their body weight on one leg or symmetrically with each leg taking
50 % of their body weight. The net response in each leg was taken from changes in ground reaction force measured from separate
force plates under each foot. The net force profile consisted of a small initial force change that peaked at ∼200 ms followed
by an oppositely directed larger component that peaked at ∼450 ms. We analysed the second force component since it was responsible
for the kinematic response of lateral body sway and tilt towards the anode. In the horizontal plane, both legs produced lateral
force responses that were in the same direction but larger in the leg ipsilateral to the cathodal ear. There were also vertical
force responses that were of equal size in both legs but acted in opposite directions. When subjects stood asymmetrically
the directions of the force responses remained the same but their magnitudes changed. The lateral force response became 2-3
times larger for the more loaded leg and the vertical forces increased 1.5 times on average for both legs. Control experiments
showed that these changes could not be explained by either the consistent (< 5 deg) head tilt towards the side of the loaded
leg or the changes in background muscle activity associated with the asymmetrical posture. We conclude that the redistribution
of force responses in the two legs arises from a load-sensing mechanism. We suggest there is a central interaction between
load-related afferent input from the periphery and descending motor signals from balance centres.