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Light touch of a stable reference reduces sway during standing. However, unexpected displacement of a light touch reference leads to short-latency reactions in ankle muscles consistent with a balance reaction, that are replaced by responses in arm muscles on subsequent trials. We anticipated that excitability of sensorimotor pathways arising from finger cutaneous afferents would reflect these changes in behavior. We hypothesized that 1) interlimb cutaneous reflexes in muscles of the ipsilateral leg, derived from median nerve (MED) stimulation would be facilitated when touch was stable, but reduced when touch was unreliable, 2) intralimb MED reflexes in muscles of the homonymous arm would be facilitated when touch was unreliable and participants tracked the touch reference with arm movements, and 3) radial nerve (RAD) evoked reflexes would be unaffected, given that the RAD innervation territory is not involved in the light touch task. Cutaneous reflexes were evoked using a transcutaneous train of pulses (5 x 1.0 ms square-wave pulses; 300 Hz) and recorded using electromyography of muscles of the ipsilateral arm and leg. As hypothesized, interlimb MED reflexes recorded in soleus (SOL) were larger when touching the stable reference (mean ± sd %MVC; 4.78 ± 1.57) than when not touching a reference (1.00 ± 1.05) or when touching an unstable reference (1.07 ± 1.16). In addition, intralimb MED reflexes in anterior deltoid (AD) were larger when touching an unstable reference (4.50 ± 1.31), compared to touching a stable reference (1.34 ± 1.01) or not touching (1.50 ± 1.00). In contrast, interlimb RAD reflexes in SOL were larger when not touching (4.29 ± 4.34), compared with touching a stable (1.14 ± 1.84) or unstable reference (3.11 ± 4.15). These findings indicate that cutaneous reflexes from the hand are scaled with a rapid change in motor behavior when a touch reference becomes unstable, suggesting that spinal sensorimotor pathways are functionally reweighted based in part upon the reliability of tactile inputs.

Contains results from sensory testing, transcranial magnetic stimulation, changes in walking skill and distance after long breaks in training. (PDF 202 kb)

Lightly touching a stable reference is associated with sway reduction during standing. Unexpected displacement of the touch reference results in a false-positive balance reaction in some participants, but only with the first such disturbance. This study investigated whether light touch reduces standing sway 1) after the touch reference becomes unreliable, and 2) when participants are aware the touch reference is unreliable. Forty healthy adults, twenty that were naïve to the possibility of a touch reference displacement and twenty that were made aware prior to testing, were asked to stand while lightly touching (

The first exposure of a rapid displacement of a light touch reference induces an inappropriate balance corrective response during standing in a proportion of participants that is extinguished with repeated exposures. We hypothesized that if the spatial touch reference was critical to performing of a task the evoked response would be more consistently expressed across participants and observed with repeated exposures to the disturbance. To test this, twenty participants received either forward (N=10) or backward right touch displacements at right heel strike during motorized treadmill walking without visual feedback. Electromyographic recordings from 4 arm, 4 leg and 1 neck muscle were sampled along with joint kinematic and step cycle data. Rapid displacement of the touch surface elicited responses in all 20 participants. However, the frequency of first trial responses was not different from what was observed during standing. In contrast, responses were observed in all participants with subsequent trials. None of the participants tripped or stumbled as a result of the touch perturbations, however the step cycle duration was consistently shorter following the first forward touch displacement. A post-experiment questionnaire revealed that many participants often perceived the touch plate displacement as a disturbance to the treadmill belt speed, suggesting the disturbance was occasionally misinterpreted. The activation of ankle muscles following the unexpected slip of a touch reference during walking suggests that tactile information from the finger is a relevant sensory cue for the regulation and control of stepping and stability.

We hypothesized that standing on an unstable surface would increase the relevance of light touch to standing balance, such that unexpected displacement of a touch reference would result in more consistent expression of balance corrections, compared to standing on a firm surface. Ten healthy participants stood on a foam block atop a force plate without vision, while lightly touching a reference. The touch plate was unexpectedly displaced forwards 10 times. Responses in tibialis anterior (TA) were observed more frequently across the 10 trials compared with standing on a firm surface. However, the responses evoked in trials 2-10 were functionally distinct from those following the first trial. We suggest the first trial responses represent balance corrective responses induced by the slip of the finger relative to the reference. In contrast, the subsequent responses in TA are likely related to an arm-tracking reaction that emerges, indicating a rapid repurposing of the tactile feedback.

Responses elicited by the rapid displacement of a light touch contact surface were investigated during standing with the eyes closed. During quiet standing the touch surface was moved with an imperceptible slow (0.5 Hz), small (0.5 cm) oscillation to entrain the participant’s sway. Periodically, a rapid displacement (1.25 cm, 12.5 cm/s peak velocity, 187.5 cm/s/s peak acceleration) of the rod was applied, either forwards or backwards, at either the fore or aft position of the entrained sway. Each participant received 10 unexpected displacements of the same direction, with 20 participants receiving forward displacements and 6 participants receiving backward displacements. Electromyographic recordings from 4 arm and 2 ankle muscles were sampled along with center of pressure and joint kinematics. Rapid displacement of the touch surface consistently resulted in short-latency (