The value of time in the invigoration of human movements when interacting with a robotic exoskeleton

Abstract

Time and effort are thought to be subjectively balanced during the planning of goal-directed actions, thereby setting the vigor of volitional movements. Theoretical models predicted that the value of time should then amount to high levels of effort. However, the time-effort trade-off has so far only been studied for a narrow range of efforts. To investigate the extent to which humans can invest in a time-saving effort, we used a robotic exoskeleton to substantially vary the energetic cost associated with a certain vigor during reaching movements. In this situation, minimizing the time-effort trade-off should lead to high and low human efforts for upward and downward movements, respectively. Consistently, all participants expended substantial amounts of energy upward and remained essentially inactive by harnessing the work of gravity downward, while saving time in both cases. A common time-effort trade-off may therefore determine the vigor of reaching movements for a wide range of efforts.

Evidence for flexible motor costs in vertical arm movements: reduced gravity-related effort minimization under high accuracy constraints

“…Thus, movement duration can be considered as the outcome of the minimization of a subjective weighting between a cost of time and a cost of movement, depending of expected reward. In tasks where reward is not explicit (as in our task), multiple studies showed that movement duration can be appropriately captured by a common tradeoff between time and effort (Shadmehr et al, 2010; Berret and Jean, 2016; Berret and Baud-Bovy, 2022; Carlisle et al, 2023; Verdel et al, 2023). The existence of this cost of time can explain why our participants did not choose to move slower and adopted a more effort-consuming strategy.…”

Human movement modifications induced by different levels of transparency of an active upper limb exoskeleton

“…This could be explained by recent advances in the understanding of human movement invigoration via the minimum time-effort theory. Indeed, recent works showed that participants can be prone to spend significant amounts of energy, and withstand high interaction efforts, in order to save time ( Verdel et al, 2023b ). Here, participants reduced their speed likely in relation with the associated effort, as predicted by a time-effort trade-off ( Bastide et al, 2018 ).…”

“…Active upper-limb exoskeletons are promising devices mainly dedicated to robot-assisted rehabilitation ( Pons, 2010 ; Jarrassé et al, 2014 ; Proietti et al, 2016 ; Mehrholz et al, 2020 ) and musculoskeletal disorders prevention ( de Looze et al, 2016 ; Ajoudani et al, 2018 ; Nussbaum et al, 2019 ). The possibilities offered by active exoskeletons in general are also of interest for the investigation of human motor control theories ( Selinger et al, 2015 ; Verdel et al, 2023a ; b ). Several elements have to be taken into account when dealing with human-exoskeleton interactions, like the mechanical design of the exoskeleton and the way the exoskeleton is controlled.…”

Running, Fast and Slow: The Dorsal Striatum Sets the Cost of Movement During Foraging

“…Independently of the neural mechanisms underlying the preserved duration of their runs, the fact that rats care primarily about time is unlikely to be specific to our experimental framework. It is for instance strikingly reminiscent of a recent study on the vigor of reaching movement assisted by an exoskeleton showing that humans are willing to generate effort to reduce the duration of movements, even when they do not yield any explicit rewards [32].…”