This study investigated the role of the plantar cutaneous information in controlling human balance. We hypothesized that the cutaneous afferent messages from the main supporting zones of the feet have sufficient spatial relevance to inform the CNS about the body position with respect to the vertical reference and consequently to induce adapted regulative postural responses. Skin mechanoreceptors of anterior and/or posterior areas of one or both soles of 10 standing subjects were activated by superficial mechanical vibration with high frequency and low amplitude. Variations of the subject's center of pressure (CoP) were recorded. Spatially oriented whole-body tilts were observed for every subject. Their direction depended on the foot areas stimulated and was always opposite to the vibration-simulated pressure increase. These responses are found to subserve a postural regulative function and we suggest that co-processing of the various cutaneous messages followed a vector addition mode.

International audience1. In order to assess the relative contribution and the interactions of the plantar cutaneous and muscle proprioceptive feedback in controlling human erect posture, single or combined vibratory stimuli were applied to the forefoot areas and to the tendons of the tibialis anterior muscles of nine standing subjects using various vibration frequency patterns (ranging from 20 to 80 Hz). 2. The variations in the centre of foot pressure, ankle angle and the EMG activities of the soleus and tibialis anterior muscles of each subject were recorded and analysed. 3. Separate stimulation of the plantar forefoot zones or the tibialis anterior muscles always resulted in whole-body tilts oppositely directed backwards and forwards, respectively, the amplitude of which was proportional to the vibration frequency. EMG activity of ankle muscles also varied according to the direction of the postural responses. However, the same vibration frequency did not elicit equivalent postural responses: in the low frequency range, tactile stimulation induced stronger postural effects than proprioceptive stimulation, and the converse was the case for the higher frequency range. 4. Under sensory conflict conditions, i.e. foot sole-flexor ankle muscle co-stimulation, the direction of the body tilts also varied according to the difference and the absolute levels of the vibration frequencies. In all cases, the resulting postural shifts always corresponded to the theoretical sum of the isolated effects observed upon vibrating each of these two sensory channels. 5. We proposed that tactile and proprioceptive information from the foot soles and flexor ankle muscles might be co-processed following a vector addition mode to subserve the maintenance of erect stance in a complimentary way

This study was based on the assumption that the central processing of proprioceptive inputs that arise from numerous muscles contributes to both awareness and control of body posture. The muscle-spindle inputs form a "proprioceptive chain" which functionally links the eye muscles to the foot muscles. Here, we focused on the specific contribution of two links in the control of human erect posture by investigating how proprioceptive messages arising from ankle and neck muscles may be integrated by the central nervous system. Single or combined mechanical vibrations were applied to different muscle tendons at either one (ankle or neck) or both (ankle plus neck) body levels. The amplitude and the specific direction of the resulting oriented body tilts were analyzed by recording the center of foot pressure (CoP) through a force platform with four strain gauges. The results can be summarized as follows: (1) the vibration-induced whole-body tilts were oriented according to the muscles stimulated; furthermore, the tilts were in opposite directions when neck or ankle muscles on the same side of the body were stimulated; (2) except for the ankle antagonist muscles, co-vibrating adjacent or antagonist muscles at the same body level (ankle or neck) resulted in body sways, whose orientation was a combination of those obtained by stimulating these muscles separately; and (3) likewise, co-vibrating ankle and neck muscles induced whole-body postural responses, whose direction and amplitude were a combination of those obtained by separate vibration. We conclude that the multiple proprioceptive inputs originating from either one or both body levels may be co-processed in terms of vector-addition laws. Moreover, we propose that proprioceptive information from ankle and neck muscles may be used for two tasks: balance control and body orientation, with central integration of both tasks.

We investigated whether the tactile information from the main supporting areas of the foot are used by the brain for perceptual purposes, namely body posture awareness and body representation in space. We applied various patterns of tactile stimulation to one or both soles of unmoving and blindfolded subjects by a 60 micro-vibrator tactile matrix set in a force platform. The perceptual effects of the stimulation were assessed through a 3D joystick handled by the subjects. All subjects reported illusory perceptions of whole-body leaning. Both orientation and amplitude of these perceptions depended on the stimulation pattern. Additional kinesthetic illusions sometimes occurred along the longitudinal axis of the body. We conclude that foot sole input contributes to the coding and the spatial representation of body posture.

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