| There is considerable evidence that sensations derived from the feet and ankles are important for the maintenance of standing balance. Patients suffering from reduced foot and ankle sensation due to peripheral neuropathies are less stable during standing and walking and are at greater risk of falling than their healthy counterparts. It remains unclear, however, to what extent specific sensory subsystems within the feet and ankles are involved in balance control. The current study is an attempt to isolate the role played by mechanoreceptors within the skin of the foot soles in the control of standing balance. Two major balance experiments were performed. In the first experiment, subjects' ability to stand as still as possible was assessed before and after their foot soles were anesthetized. In one group of subjects, anesthesia of the forefoot soles was accomplished using a novel method of iontophoretic drug delivery. In a second group of subjects, anesthesia of the weight-bearing foot soles was accomplished through intradermal injections of lidocaine. Changes in the contact forces between subjects' feet and the support surface were quantified in terms of fractal Brownian motion. In the second experiment, subjects experienced lateral accelerations of the support surface before and after anesthesia of their foot soles. Subjects' reactions to these perturbations of balance were quantified by estimates of the response torques produced at their ankles, hips, and lower spine. The major finding from the first experiment was that reduced sensation from the foot soles increased the velocity of postural sway during unperturbed stance. This effect was seen only when the balance control system was challenged by closing the eyes or standing on one foot. The magnitude of this effect suggests that loss of foot sole sensation is not the dominant contributor to the instability seen in peripheral neuropathy patients during quiet stance. Results of the second experiment, however, demonstrate that loss of foot sole sensation leads to a relative redistribution of response torque from the ankles and trunk to the hip joints. Reduced foot sole sensation may therefore be an important contributor to the balance deficits associated with peripheral neuropathies during dynamic activities. |
