Visuomotor contribution to force variability in the plantarflexor and dorsiflexor muscles

Abstract

The visual correction employed during isometric contractions of large proximal muscles contributes variability to the descending command and alters fluctuations in muscle force. This study explored the contribution of visuomotor correction to isometric force fluctuations for the more distal dorsiflexor (DF) and plantarflexor (PF) muscles of the ankle. Twenty-one healthy adults performed steady isometric contractions with the DF and PF muscles both with (VIS) and without (NOVIS) visual feedback of the force. The target forces exerted ranged from 2.5% to 80% MVC. The standard deviation (SD) and coefficient of variation (CV) of force was measured from the detrended (drift removed) VIS and NOVIS steadiness trials. Removal of VIS reduced the CV of force by 19% overall. The reduction in fluctuations without VIS was significant across a large range of target forces and was more consistent for the PF than the DF muscles. Thus, visuomotor correction contributes to the variability of force during isometric contractions of the ankle dorsiflexors and plantarflexors.

Fig. 1

Constant-force tasks with the ankle dorsiflexors at 50% MVC (A) and plantarflexors at 10% MVC (B). The horizontal dashed line indicates the target force. The vertical dashed line indicates removal of visual feedback. The detrended force is inset below the original force trace. (C) Power spectrum averaged across forces, vision conditions, and age groups. Power is expressed in 0.49 Hz bins as a percent of the total power between 0 and 30 Hz and displayed as a smoothed line.

Fig. 2

The change (drift) in the average force (N) for vision compared with no-vision segments for the ankle dorsiflexors (A) and plantarflexors (B) for each target force. Young and elderly participants were not different and are pooled. * p < .05 for vision compared with no-vision. † p = .06 for vision compared with no-vision. Error bars are standard error of the mean (SEM).

Fig. 3

Coefficient of variation (CV) of force during constant-force contractions of the ankle dorsiflexors and plantarflexors for vision and no-vision conditions. Values are pooled across target forces and age groups. * p < .001 for main effect of visual feedback. Error bars are standard error of the mean (SEM).

Fig. 4

Coefficient of variation (CV) of force during constant-force contractions of the ankle dorsiflexors (A) and plantarflexors (B) performed across a range of target forces both with and without visual feedback. Values are pooled across age group. * p < .05 for vision compared with no-vision. † p = .06 for vision compared with no-vision. Error bars are standard error of the mean (SEM). The actual average force exerted for no-vision was different than the nominal target forces (see Fig. 2).