Subthreshold Electrical Noise Applied to the Plantar Foot Enhances Lower-Limb Cutaneous Reflex Generation

Abstract

Reflex responses generated by cutaneous mechanoreceptors of the plantar foot are important for the maintenance of balance during postural tasks and gait. With aging, reflex generation, particularly from fast adapting type I receptors, is reduced, which likely contributes to impaired postural stability in this population. Therefore, improving reflex generation from these receptors may serve as a tool to improve balance performance. A mechanism to enhance reflexes may lie in the phenomenon of stochastic resonance, whereby the addition of certain intensities and frequencies of noise stimuli improves the performance of a system. This study was conducted to determine whether tactile noise stimuli could improve cutaneous reflex generation. In 12 healthy young adults, we evoked cutaneous reflex responses using a 0-50 Hz Gaussian noise vibration applied to the plantar heel. Concurrently, we applied one of six subthreshold intensities of electrical tactile noise to the plantar heel [0%, 20%, 40%, 60%, 80% or 100% (threshold)] and were able to analyze data from 0%, 20% and 40% trials. Across participants, it was found that the addition of a 20% perceptual threshold (PT) noise resulted in enhanced reflex responses when analyzed in both the time and frequency domains. These data provide evidence that cutaneous reflex generation can be enhanced via a stochastic resonance effect and that 20% PT is the optimal intensity of noise to do so. Therefore, the addition of noise stimuli may be a valuable clinical intervention to improve reflex responses associated with postural balance in populations with impairments.

Keywords: cutaneous reflex; foot sole; lower limb; stochastic resonance; subthreshold noise.

Figure 1

Experimental setup. Participants stood on a custom-built platform with a slight anterior lean (C). Two-second data excerpts of input stimuli from a 120-s trial. Gaussian vibration stimuli [B, intensity:10× perceptual threshold (PT); E, spectral power] were applied to the plantar heel of the right foot (indicated by *) to evoke cutaneous reflexes (A). Electrical noise stimuli were concurrently applied at intensities between 0% and 100% PT (A, location; B, two-second waveform excerpt; E, spectral power). (D) Two-second data excerpts of recorded electromyography (EMG) signals from a 120-s trial. EMG was recorded from the soleus (SOL) and tibialis anterior (TA).

Figure 2

Cumulant density functions generated between probe acceleration and soleus EMG (A–C) or tibialis anterior EMG (D–F). Horizontal lines represent 95% confidence interval bounds for the 0% PT condition. In figures (D–F), solid horizontal lines represent confidence intervals for the cumulant density function in the 20% noise condition (D,F) or 40% noise condition (E). Dotted horizontal lines represent confidence intervals for the cumulant density function in the 0% noise condition (D) or 40% noise condition (E,F). For all cumulant densities, peak-to-peak (PTP) amplitudes for each function were calculated between 70–110 ms (indicated by the vertical broken lines). The number of segments: SOL, 3,515 disjointed segments; TA, 2,929 disjointed segments. Segment length = 0.8192 ms, time resolution: 0.02 ms.

Figure 3

Coherence functions generated between probe acceleration and soleus EMG (A–C) or tibialis anterior EMG (D–F). Horizontal lines represent the 95% confidence limit bounds. Frequency resolution: 1.2207 Hz. *Indicates significant differences between the two plotted coherence functions at the frequency indicated. The difference in coherence frequency resolution: 1.2207 Hz.

Figure 4

Normalized soleus cumulant density PTP amplitude (A) and peak soleus coherence between 28–32 Hz (B) and normalized tibialis anterior cumulant density PTP amplitude (C) and peak coherence between 28–32 Hz (D) at each noise intensity. Dots represent individual participant data at each noise (soleus: n = 12, tibialis anterior: n = 10) and the horizontal line represents the overall mean.