Do synergies decrease force variability? A study of single-finger and multi-finger force production

Abstract

We tested a hypothesis that force production by multi-finger groups leads to lower indices of force variability as compared to similar single-finger tasks. Three experiments were performed with quick force production, steady-state force production under visual feedback, and steady-state force production without visual feedback. In all experiments, a range of force levels was used computed as percentages of the maximal voluntary contraction force for each involved finger combination. Force standard deviation increased linearly with force magnitude across all three experiments and all finger combinations. There were modest differences between multi-finger and single-finger tasks in the indices of force variability, significant only in the tasks with steady-state force production under visual feedback. When fingers acted in groups, each finger showed significantly higher force variability as compared to its single-finger task and as compared to the multi-finger group as a whole. Fingers that were not instructed to produce force also showed close to linear relations between force standard deviation and force magnitude. For these fingers, indices of force variability were much higher as compared to those computed for the forces produced by instructed fingers. We interpret the findings within a feed-forward scheme of multi-finger control with two inputs only one of which is related to the explicit task. The total force variability reflects variability in only the task-related component, while variability of the finger forces is also due to variability of the component that is not related to the task. The findings tentatively suggest that total force variability originates at an upper level of the control hierarchy in accordance to the Weber-Fechner law rather than reflects a "neural noise" at the segmental level.

The nature of constant and cyclic force production: unintentional force-drift characteristics

“…Indeed, after the original publication, only a few studies explored this phenomenon (Vaillancourt et al 2001, Vaillancourt and Russell 2002, Shapkova et al 2008, and it was also invoked in studies of "slacking", a phenomenon observed in stroke survivors who show a drop in effort when helped by an external device (Reinkensmeyer et al 2009). The original interpretation that the phenomenon reflected a limitation in the working memory was indirectly supported by a study of patients with Parkinson's disease (Vaillancourt et al 2001) and has not been challenged until recently Jo et al 2015).…”

“…We conclude that while working memory limitations may influence cyclic F-tasks, their role in determining the force drift in constant F-tasks is limited. The results of both experiments are interpreted within the referent configuration hypothesis supplemented with an idea visual force feedback and then the feedback is turned off, a slow drop in the force is typically observed, while the subject is unaware of the force change (Slifkin et al 2000;Vaillancourt et al 2001;Vaillancourt and Russell 2002;Shapkova et al 2008). This force drift was interpreted in the cited early studies as a consequence of a working memory limitation.…”

Equifinality and its violations in a redundant system: multifinger accurate force production

“…Analysis of multifinger synergies. It is well-established that, in the absence of visual feedback, subjects show a slow, gradual decrease in the amount of force they produce even if they try to produce a constant force (Shapkova et al 2008;Slifkin et al 2000;Vaillancourt and Russell 2002). Our fifth condition (when no finger was raised) was used to control for this phenomenon.…”

“…It is well-known that, when subjects produce a constant force under visual feedback and then feedback is turned off, there is a slow decline of F TOT (Shapkova et al 2008;Slifkin et al 2000). In terms of the RC hypothesis, this decline is associated with an unintentional drift of the referent coordinate.…”

On the nature of unintentional action: a study of force/moment drifts during multifinger tasks

“…IF A PERSON is instructed to produce a constant force magnitude in isometric conditions by an effector (e.g., pressing with a finger) with the help of visual feedback, turning the feedback off results in a slow consistent drift of the force, typically to lower magnitudes (Shapkova et al 2008;Slifkin et al 2000;Vaillancourt and Russell 2002). This drop in force can reach large magnitude, up to 30% of the initial force level over 20 -30 s, without the subject being aware of the force drop.…”