"Sensory steering" of the foot

NewsBot · Sep 10, 2014

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Cutaneous stimulation of discrete regions of the sole during locomotion produces "sensory steering" of the foot.
Zehr EP, Nakajima T, Barss T, Klarner T, Miklosovic S, Mezzarane RA, Nurse M, Komiyama T.
BMC Sports Sci Med Rehabil. 2014 Aug 9;6:33.

BACKGROUND:
While the neural and mechanical effects of whole nerve cutaneous stimulation on human locomotion have been previously studied, there is less information about effects evoked by activation of discrete skin regions on the sole of the foot. Electrical stimulation of discrete foot regions evokes position-modulated patterns of cutaneous reflexes in muscles acting at the ankle during standing but data during walking are lacking. Here, non-noxious electrical stimulation was delivered to five discrete locations on the sole of the foot (heel, and medial and lateral sites on the midfoot and forefoot) during treadmill walking. EMG activity from muscles acting at the hip, knee and ankle were recorded along with movement at these three joints. Additionally, 3 force sensing resistors measuring continuous force changes were placed at the heel, and the medial and lateral aspects of the right foot sole. All data were sorted based on stimulus occurrence in twelve step-cycle phases, before being averaged together within a phase for subsequent analysis.
METHODS:
Non-noxious electrical stimulation was delivered to five discrete locations on the sole of the foot (heel, and medial and lateral sites on the midfoot and forefoot) during treadmill walking. EMG activity from muscles acting at the hip, knee and ankle were recorded along with movement at these three joints. Additionally, 3 force sensing resistors measuring continuous force changes were placed at the heel, and the medial and lateral aspects of the right foot sole. All data were sorted based on stimulus occurrence in twelve step-cycle phases, before being averaged together within a phase for subsequent analysis.
RESULTS:
The results demonstrate statistically significant dynamic changes in reflex amplitudes, kinematics and foot sole pressures that are site-specific and phase-dependent. The general trends demonstrate responses producing decreased underfoot pressure at the site of stimulation.
CONCLUSIONS:
The responses to stimulation of discrete locations on the foot sole evoke a kind of "sensory steering" that may promote balance and maintenance of locomotion through the modulation of limb loading and foot placement. These results have implications for using sensory stimulation as a therapeutic modality during gait retraining (e.g. after stroke) as well as for footwear design and implementation of foot sole contact surfaces during gait.
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NewsBot · Jul 8, 2017

The firing characteristics of foot sole cutaneous mechanoreceptor afferents in response to vibration stimuli.
Strzalkowski ND et al
J Neurophysiol. 2017 Jul 5:jn.00647.2016. doi: 10.1152/jn.00647.2016.

Single unit microneurography was used to record the firing characteristics of the four classes of foot sole cutaneous afferents (fast and slowly adapting type I and II; FAI, FAII, SAI, SAII) in response to sinusoidal vibratory stimuli. Frequency (3-250Hz) and amplitude (0.001-2mm) combinations were applied to afferent receptive fields through a 6mm diameter probe. The impulses per cycle, defined as the number of action potentials evoked per vibration sine wave, were measured over one second of vibration at each frequency-amplitude combination tested. Afferent entrainment threshold (lowest amplitude at which an afferent could entrain 1:1 to the vibration frequency) and afferent firing threshold (minimum amplitude for which impulses per cycle was greater than zero) were then obtained for each frequency. Increases in vibration frequency are generally associated with decreases in expected impulses per cycle (p < 0.001), but each foot sole afferent class appears uniquely tuned to vibration stimuli. FAII afferents tended to have the lowest entrainment and firing thresholds (p < 0.001 for both); however, these afferents seem to be sensitive across frequency. In contrast to FAII afferents, SAI and SAII afferents tended to demonstrate optimal entrainment to frequencies below 20Hz, and FAI afferents faithfully encoded frequencies between 8-60Hz. Contrary to the selective activation of distinct afferent classes in the hand, application of class specific frequencies in the foot sole is confounded due to the high sensitivity of FAII afferents. These findings may aid in the development of sensorimotor control models or the design of balance enhancement interventions.
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NewsBot · Feb 6, 2020

The sensory role of the sole of the foot: Review and update on clinical perspectives.
Viseux FJF
Neurophysiol Clin. 2020 Jan 29

The feet constitute an important sensory structure in the mechanisms of postural control. As a direct and often only interface between the body and the ground, the feet allow us to sense and interact with our environment. Sensory information provided by muscle and cutaneous afferents in the foot contribute to our ability to stand upright, and postural sway is necessary to detect both position and motion of the body in space. A decline in foot sole skin sensitivity occurs naturally with aging and as a result of neurological disorders, including different peripheral neuropathies, the commonest etiologies of which are diabetes mellitus or effects of chemotherapy. This decline in sensitivity is frequently associated with poorer postural control and increased risk of falls in these populations. The purpose of this comprehensive review is to summarize the evidence that supports a functional role of foot sole sensory tactile and muscular feedback in standing balance, and the postural consequences when this feedback is impaired with aging or disease. This brings new clinical perspectives on the development of intervention strategies to improve the quality of foot sole cutaneous feedback. It also seems to be a promising approach in the management of patients with balance disorders, with specific chronic pain syndromes, with neurologic diseases or those at risk of falling.
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