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

Abstract

We explored unintentional changes in forces during performance of constant and cyclic force-production tasks (F-tasks) after visual feedback removal. Based on earlier studies, we expected all force parameters to drop exponentially with time. We also explored possible role of working memory in the force drop phenomena. Healthy subjects performed constant or cyclic isometric F-tasks with the index finger under visual feedback. The cyclic task was paced by a metronome. Removing visual feedback resulted in a consistent force drop in constant F-tasks and a qualitatively similar drift in the mean force in the cyclic F-task. Both were slow with characteristic times of about 10-20 s. In contrast, force amplitude in the cyclic F-task increased quickly (within 1-2 s). When the subjects were asked to stop producing force for 5 s after the visual feedback disappeared and then resume force production, no downward force drift was seen in constant F-tasks, while in cyclic F-tasks, the drift of the mean force was present and an exaggerated increase in force amplitude was also observed. 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 of unintentional drift of referent coordinates (RC-back-coupling) induced by differences between the referent and actual body configurations.

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

“…The exploration of the magnitude of the ME and nME drifts (drifts within the UCM and ORT subspaces) has suggested no consistent differences in the magnitude of the ME and nME deviations. This result has important implications for an earlier hypothesis on the origin of the unintentional force drift (Ambike et al 2015(Ambike et al , 2016a) discussed later in this section. …”

“…This hypothesis received indirect support in an experiment with cyclical force production (Ambike et al 2016a). Cyclical force production may be viewed as a combination of setting a steady midpoint RC coordinate and a cyclic oscillation of RC about that point (cf.…”

Unsteady steady-states: central causes of unintentional force drift

“…This result suggests that two parallel processes interact to produce the observed behavior. First, there is a slow drift that generates error in the performance, possible induced by a hypothetical process addressed as RC-back-coupling (Wilhelm et al 2013; Zhou et al 2014; Ambike et al 2015; Ambike et al 2016a). Second, on a shorter time scale, there is co-variation between changes in the two referent variables, R FT and C FT that is primarily confined to the UCM and may be seen as a reflection of a synergy stabilizing the fingertip force (Latash 2008).…”

“…This may be because force drift saturates with time (Slifkin et al 2000; Vaillancourt and Russell 2002; Ambike et al 2015, 2016a; Heinen et al 2012, 2014), thus limiting the magnitude of the nME drift. However, the force drop is also proportional to the force produced when the feedback is turned off (Slifkin et al 2000; Vaillancourt and Russell 2002; Ambike et al 2015, 2016a). In future, we plan to investigate if the synergies survive higher initial forces: It is possible that the ME component would also increase with the initial force value.…”

“…In recent studies (Ambike et al 2015, 2016a), we have suggested a different conceptual framework to address the phenomena of unintentional changes in performance based on the idea of control of voluntary actions with referent coordinates (RCs) for the involved effectors (Latash 2010; Feldman 2015). According to this idea, the neural control of an action is associated with defining spatial RCs for the salient performance variable (Latash 2010; Feldman 2015).…”