Abstract
The details of how light touch (LT) of a stable surface reduces postural sway are still not well known. We hypothesized that removal of feedback provided by muscle afferents of the touching fingertip would increase postural sway in standing subjects. Eleven participants stood upright on a force plate with eyes closed and on an unstable surface. The experimental conditions involved two different finger positions: with partial muscle afferents (PMA), which includes sensory information from the fingertip flexor muscles, and no muscle afferents (NMA), without information from either fingertip flexor or extensor muscles. In the control condition, the participants kept the same posture, but with no finger touch (NT). Postural sway in both anteroposterior (AP) and mediolateral (ML) axes were recorded. Results showed that LT decreased all sway quantifiers as compared with the NT condition. The withdrawal of information from the touch finger muscle afferents (NMA condition) did not increase postural sway. Actually, there was a small, albeit statistically significant, decrease in the variability of center of pressure displacement in the AP direction. These results indicate that in some cases, muscle afferent input may either not contribute or even worsen the overall quality of sensory feedback from a given body segment, leading to no improvement or even a slightly decreased performance of the motor control system (evaluated by means of levels of postural sway in the present investigation). The results suggest that non-spindle fingertip afferents provide the bulk of the sensory feedback associated with the fingertip that is touching a ground-referenced object during quiet standing under LT.
Highlights
The roles of sensory and motor interactions in the control of human postural sway have been largely explored
The results showed that the effects of such a “passive touch” on postural sway were dependent on the body part being touched
During light touch (LT) conditions, the mean forces in all three directions, Fx, Fy, and Fz, applied by the fingertip were below 1 N for all trials performed by the subjects
Summary
The roles of sensory and motor interactions in the control of human postural sway have been largely explored. In this vein, lightly touching an external rigid surface has been demonstrated to reduce postural sway. The most studied LT condition has been when a fingertip is kept touching a fixed object (called “active touch”), thereby providing additional sensory cues as to the gravity force and the associated body sway. In this vein, experiments from different research groups demonstrated a markedly diminished postural sway in tandem (Lackner et al, 1999), semi-tandem (Baccini et al, 2007), single-leg (Bonfim et al, 2008), and bipedal (Franzén et al, 2012) stances. Within the scope of this fingertip LT, the putative mechanisms behind the effects of LT on postural sway have been associated with proprioceptive feedback from finger and hand muscles as well as from fingertip cutaneous mechanoreceptors (Holden et al, 1994; Jeka and Lackner, 1994; Clapp and Wing, 1999; Rabin et al, 2008; Baldan et al, 2014; Martinelli et al, 2015; Moraes et al, 2018)
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