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.

Effects of visual feedback absence on force control during isometric contraction

“…When force stability improved, explanation was given that force fluctuations appeared to be wider with visual feedback because of a greater number of force adjustments and corrections induced by visual information (Tracy 2007b). Overall, our results reveal a lower complexity (signal noise in its time domain structure) of the force variability without vision, which is consistent with earlier studies showing that the complexity of the force variability decreases when less visual information is provided (Slifkin et al 2000).…”

“…When comparing our findings with those during visual feedback removal present in the literature (Vaillancourt and Russell 2002;Baweja et al 2009;Prodoehl and Vaillancourt 2010;Tracy 2007b), somehow similar data can be found showing a force drift from the force target when visual feedback was taken away, especially at high force levels. This phenomenon became more evident as force level increased (Baweja et al 2009).…”

“…The amplitude of force fluctuation, an index of force stability, is also inversely related to the level of effort and to the number of the recruited motor units (Hamilton et al 2004), and has been shown to be influenced directly by alterations in synergistic muscle activation (Shinohara et al 2009). Overall, visual feedback removal during an isometric task either did not influence (Christou et al 2004;Vaillancourt and Russell 2002) or reduce force variability /accuracy making more stable the signal Christou 2005;Tracy et al 2007;Baweja et al 2009;Kennedy and Christou 2011;Schiffman et al 2002;Tracy 2007b). Conversely, Moreover, when visual feedback is removed, the force accuracy was compromised because the force output tended to drift away from the required target (Tracy 2007b;Vaillancourt and Russell 2002).…”

“…The control mechanisms of both static and dynamic muscle contraction have been already studied extensively (Svendsen et al 2011;Janczyk et al 2009;Harbst et al 2000;Hu et al 2011). Isometric force control has been observed to be influenced by many extrinsic and intrinsic variables, including number, size and type of active muscles (Prodoehl and Vaillancourt 2010;Tracy 2007b), force intensity (Baweja et al 2009), training status (Enoka et al 1999), fatigue (Esposito et al 2009), age (Ofori et al 2010;Kennedy and Christou 2011;Schiffman et al 2002) and visual feedback characteristics (Newell et al 2003;Tracy 2007b). Visual feedback is an essential component when attempting to minimize force fluctuations and to hold force steady (Prodoehl and Vaillancourt 2010).…”

“…Visual feedback, indeed, has been either removed during isometric contraction (Schiffman et al 2002;Tracy 2007b) or provided intermittently throughout the task (Slifkin et al 2000;Baweja et al 2009;Madeleine et al 2002). During an isometric task without visual feedback following the same task performed with the aid of visual feedback, force output can be reasonably supposed to be handled by the central nervous system in a different way because of an altered somatosensory feedback involvement.…”