Removal of visual feedback alters muscle activity and reduces force variability during constant isometric contractions

Abstract

The purpose of this study was to compare force accuracy, force variability and muscle activity during constant isometric contractions at different force levels with and without visual feedback and at different feedback gains. In experiment 1, subjects were instructed to accurately match the target force at 2, 15, 30, 50, and 70% of their maximal isometric force with abduction of the index finger and maintain their force even in the absence of visual feedback. Each trial lasted 22 s and visual feedback was removed from 8–12 to 16–20 s. Each subject performed 6 trials at each target force, half with visual gain of 51.2 pixels/N and the rest with a visual gain of 12.8 pixels/N. Force error was calculated as the root mean square error of the force trace from the target line. Force variability was quantified as the standard deviation and coefficient of variation (CVF) of the force trace. The EMG activity of the agonist (first dorsal interosseus; FDI) was measured with bipolar surface electrodes placed distal to the innervation zone. Independent of visual gain and force level, subjects exhibited lower force error with the visual feedback condition (2.53 ± 2.95 vs. 2.71 ± 2.97 N; P < 0.01); whereas, force variability was lower when visual feedback was removed (CVF: 4.06 ± 3.11 vs. 4.47 ± 3.14, P < 0.01). The EMG activity of the FDI muscle was higher during the visual feedback condition and this difference increased especially at higher force levels (70%: 370 ± 149 vs. 350 ± 143 μV, P < 0.01). Experiment 2 examined whether the findings of experiment 1 were driven by the higher force levels and proximity in the gain of visual feedback. Subjects performed constant isometric contractions with the abduction of the index finger at an absolute force of 2 N, with two distinct feedback gains of 15 and 3,000 pixels/N. In agreement with the findings of experiment 1, subjects exhibited lower force error in the presence of visual feedback especially when the feedback gain was high (0.057 ± 0.03 vs. 0.095 ± 0.05 N). However, force variability was not affected by the vastly distinct feedback gains at this force, which supported and extended the findings from experiment 1. Our findings demonstrate that although removal of visual feedback amplifies force error, it can reduce force variability during constant isometric contractions due to an altered activation of the primary agonist muscle most likely at moderate force levels in young adults.

Effects of visual feedback absence on force control during isometric contraction

“…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).…”

“…This phenomenon became more evident as force level increased (Baweja et al 2009). Vaillancourt and Russell (2002) and Tracy et al (2007) reported that the direction of force drift depends on the level of force intensity.…”

“…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.…”