Fingertip-Coupled Spindle Signaling Does Not Contribute to Reduce Postural Sway Under Light Touch

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.

Keywords: balance; center of pressure; fingertip touch; haptic information; muscle spindles; postural control.

Figure 1

Representation of the experimental setup. (A) Depiction of participant standing quietly over the foam pad, with the mechanical apparatus fixed to his/her body and showing the flat rod used as an extension of the mini-force platform (MFP), which served as the touching surface for the participants’ fingertip. (B,C) Condition PMA, with the extensor muscle of the terminal joint of the middle finger disengaged. (D,E) Condition NMA, with both flexor and extensor muscles of the terminal joint of the middle finger effectively disengaged.

Figure 2

Fingertip and ground reaction forces (GRFs) measured across the experimental conditions. (A) Vertical (Fz) and horizontal (Fx and Fy) forces (M ± SEM) applied to the MFP during the LT conditions. There were no statistical differences for the force values between the PMA and NMA conditions (p > 0.05). (B) Shows the standard deviations (SDs) of the horizontal GRFs in the AP direction (during LT and NT conditions) and the SDs of the horizontal fingertip forces in the AP direction (PMA and NMA conditions are shown in left and right panels, respectively).

Figure 3

Averaged (squares) and individual (gray lines) RMS and mean sway velocity (MSV) measurements (M ± SEM) computed from the COP signals in both AP and ML directions. * indicates significant (p < 0.05) main effects for light touch, and significant interactions between light touch and finger position.

Figure 4

Frequency-domain analyses showing the power spectral densities (PSDs) of the COP signals in different conditions. (A) Mean power spectra (n = 11) of COPap. (B) Mean power spectra of COPml. (C) Bar plots show the areas under the COPap PSDs divided into low-frequency (LF, 0.05–0.5 Hz) and high-frequency (HF, 0.5–2 Hz) bands. (D) The same as in C, but for COPml. (E) Areas under the PSDs during LT in the PMA and MNA conditions measured from the COPap. (F) The same as in E, but for the COPml. * indicates significant differences (p < 0.05).