Selective engagement of long-latency reflexes in postural control through wobble board training

Abstract

Long-latency reflexes (LLRs) are critical precursors to intricate postural coordination of muscular adaptations that sustain equilibrium following abrupt disturbances. Both disturbances and adaptive responses reflect excursions of postural control from quiescent Gaussian stability under a narrow bell curve, excursions beyond Gaussianity unfolding at many timescales. LLRs slow with age, accentuating the risk of falls and undermining dexterity, particularly in settings with concurrent additional tasks. We investigated whether the wobble board could cultivate the engagement of LLRs selectively in healthy young participants executing a suprapostural Trail Making Task (TMT). A concurrent additional-task demand constituted visual precision predominantly along the anteroposterior (AP) axis and mechanical instability mainly along the mediolateral (ML) axis. We scrutinized planar center-of-pressure (CoP) trajectories to quantify postural non-Gaussianity across various temporal scales. Wobble board increased engagement of LLRs and decreased engagement of compensatory postural adjustments (CPAs), indicated by the peak in non-Gaussianity of CoP planar displacements over LLR-specific timescales (50-100 ms) and non-Gaussianity of CoP planar displacements progressively diminishing over CPA-specific timescales ([Formula: see text] ms). Engagement with TMT did not show any noticeable influence on non-Gaussian postural sway patterns. Despite aligning the unstable axis of the wobble board with participants' ML axis, thus rendering posture more unstable along the ML axis, the wobble board increased engagement of LLRs significantly more along the AP axis and reduced engagement of CPAs significantly more along the ML axis. These findings offer initial mechanistic insights into how wobble boards may bolster balance and potentially reduce the occurrence of falls by catalyzing the engagement of LLRs selectively.

Figure 1

The wobble board used to impose continuous [mechanical] postural instability along the ML axis.

Figure 2

Schematic of the experimental setup used to investigate the effects of wobble board on postural control mechanisms. We integrated a body-sized Trail Making Test (TMT) with stabilography, requiring participants to stand either directly on a force plate or atop a wobble board inducing mediolateral instability and trace a path through randomly placed numbers projected on the screen using a laser pointer held in hand. We assessed changes in the engagement of short-latency reflexes (SLRs; 20–50 ms), long-latency reflexes (LLRs; 50–100 ms), and compensatory postural adjustments (CPAs; 100–1000 ms) across four experimental conditions: (a) No wobble board, no TMT; (b) No wobble board, TMT; (c) Wobble board, no TMT; and (d) Wobble board, TMT.

Figure 3

Non-Gaussian characterization of CoP planar displacements, combining displacements across both the ML and AP axes, in participants’ upright postures under varying support and task conditions using the Multiscale Probability Density Function (PDF) analysis. (a–d) CoP planar trajectories for a representative participant for the four experimental conditions: (a) No wobble board, no TMT; (b) No wobble board, TMT; (c) Wobble board, no TMT; and (d) Wobble board, TMT. (e–h) CoP planar displacement time series corresponding to trajectories in (a–d) for each condition (top) and variance of intermittent deviations in CoP displacement time series, that is, detrended integrated CoP displacement time series for timescales , 63, and 315 ms, representative of short-latency reflexes (SLRs; 20–50 ms), long-latency reflexes (LLRs; 50–100 ms), and compensatory postural adjustments (CPAs; 100–1000 ms). (i–l) Standardized probability density functions (PDFs; in logarithmic scale) of the detrended integrated CoP displacement time series for timescales spanning SLRs: , 25, 33, and 41 ms; LLRs: 51, 63, and 79 ms; and CPAs: , 127, 159, 199, 251, 315, 397, 501, 629, 793, and 999 ms—evenly spaced in logarithmic scale (from top to bottom, where denotes SD of . Solid circles represent estimated PDFs from time series in (e–h, top), with vertical shifting for clarity. Solid lines show numerical integration of Eq. (3) for values. The dashed line is a Gaussian PDF for comparison. .

Figure 4

Wobble board increased long-latency reflexes and decreased compensatory postural adjustments, as evidenced by peaks in non-Gaussianity of CoP planar displacements, combining displacements across both the ML and AP axes, at timescales unique to long-latency reflexes and a gradual decrease in non-Gaussianity at timescales associated with compensatory adjustments. (a–d) Non-Gaussianity in CoP planar displacements vs. log-timescale, vs. , relationship corresponding to short-latency reflexes (20–50 ms), long-latency reflexes (50–100 ms), and compensatory postural adjustments (100–1000 ms) for the four experimental conditions: (a) No wobble board, no TMT; (b) No wobble board, TMT; (c) Wobble board, no TMT; and (d) Wobble board, TMT. Solid lines indicate mean and shaded regions indicate (; 48 participants 2 trials/participant). (e) Mean non-Gaussianity of CoP displacements across all timescales corresponding to short-latency reflexes, long-latency reflexes, and compensatory postural adjustments for the four experimental conditions. Each violin plot’s right and left half depict data distributions for original CoP planar displacements and corresponding IAAFT surrogates. Horizontal bars indicate mean, and white circles indicate median (; 48 participants 2 trials/participant). ***.

Figure 5

Despite aligning the unstable axis of the wobble board with the participants’ ML axis, rendering posture more unstable along the ML axis, the wobble board increased engagement of long-latency reflexes significantly more along the AP axis and reduced engagement of compensatory postural adjustments significantly more along the ML axis. (a–d) Non-Gaussianity in CoP displacements along ML and AP axes vs. log-timescale, vs. , relationship corresponding to short-latency reflexes (20–50 ms), long-latency reflexes (50–100 ms), and compensatory postural adjustments (100–1000 ms) for the four experimental conditions: (a) No wobble board, no TMT; (b) No wobble board, TMT; (c) Wobble board, no TMT; and (d) Wobble board, TMT. Solid lines indicate mean and shaded regions indicate (; 48 participants 2 trials/participant). (e) Mean non-Gaussianity of CoP displacements across all timescales corresponding to short-latency reflexes, long-latency reflexes, and compensatory postural adjustments for the four experimental conditions. Each violin plot’s left and right half depict data distributions for CoP displacements along ML and AP axes. Horizontal bars indicate mean, and white circles indicate median (; 48 participants 2 trials/participant). ***.