Muscle Receptors of a Finger Fail to Contribute as Expected to Postural Sway Decrease During Light Touch

It is well known that light touch (LT) of a stable surface with a fingertip reduces postural sway, but its mechanisms are still being studied. Generally, it is accepted that feedback provided by muscle afferents related to a finger used for LT helps decrease postural sway in standing subjects. Eleven participants stood upright on a foam pad set on a force plate with eyes closed. The experimental conditions involved two different finger positions, P1: included sensory information from the fingertip muscle flexors; P2: had no information from either fingertip muscle flexors or extensors. In the control condition, the participants kept the same stance, with no finger touch (NT). The stabilogram area (estimated from center of pressure (COP) signals measured by a force plate) was used as a quantifier of postural sway. Results showed that LT decreased COP area in comparison to NT. The elimination of feedback from the touching finger muscle afferents (P2 condition) induced similar reductions in postural sway as in P1. These results indicate that muscle afferent input (activated by movement of the distal phalanx of finger) is not able to generate an enhanced overall sensory feedback so as to induce a more pronounced decrease in postural stability as compared to the condition in which cutaneous fingertip afferents act alone.