Tactile Control of Balance

Somatosensory function declines with age, and such changes have been associated with diminished motor performance. Input noise can enhance sensory and motor function. We asked young and elderly participants to stand quietly on vibrating gel-based insoles, and calculated sway parameters and random-walk variables. In our 27 participants, application of noise resulted in a reduction in seven of eight sway parameters in young participants and all of the sway variables in elderly participants. Elderly participants showed greater improvement than young people in two variables, mediolateral range (p=0.008), and critical mean square displacement (p=0.012). Noise-based devices, such as randomly vibrating insoles, could ameliorate age-related impairments in balance control.

1. Thresholds for the perception of postural sway induced by gentle perturbations were determined for five normal standing subjects. In this context we understand 'perception' to mean 'able to give a subjective report'. The thresholds for the perception of movements that were equivalent to sway in velocity and amplitude were determined when the available sensory input was limited to only one, or a pair, of the vestibular, visual, and proprioceptive systems. To examine vestibular inputs alone, vision was excluded and the whole body was moved with the ankles in a fixed position. To examine visual inputs alone, the body was kept stationary and a 'room' was moved around the subjects to simulate the relative visual-field movement that occurs during standing. To limit the available sensory input to proprioception from the legs, subjects were held stationary and balanced a load that was equivalent to their own body using their ankles. In this situation, perturbations were applied to the 'equivalent body' and these could only be perceived from the resulting ankle movements. Thresholds for perceiving ankle movements were also determined in the same posture, but with the leg muscles bearing no load. 2. The thresholds for the perception of sway during standing were very small, typically 0.003 rad at a velocity of 0.001 rad s-1, and even smaller movements were perceived as the mean velocity of the sway increased up to 0.003 rad s-1. No difference was found between the thresholds for perceiving forward sway and backward sway. Eye closure during standing did not affect the threshold for perceiving sway. 3. When sensory input was limited to proprioception from the legs, the thresholds for the perception of passive ankle movements were equivalent to the thresholds for the perception of sway during standing with all sensory inputs available. When the leg muscles were relaxed, the thresholds for perceiving ankle movements increased approximately twofold. 4. The visual thresholds for perceiving movement were higher than the proprioceptive thresholds at slower velocities of movement, but there was no difference at higher velocities. 5. Both the proprioceptive and visual thresholds were sufficiently small to allow perception of the sway that was recorded when the subjects stood normally in a relaxed manner. In contrast, the vestibular thresholds were an order of magnitude greater than the visual or proprioceptive thresholds and above the largest sway movements that were recorded during normal standing.(ABSTRACT TRUNCATED AT 400 WORDS)

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). 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. 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. 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. 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 complementary way.