Physical Demand but Not Dexterity Is Associated with Motor Flexibility during Rapid Reaching in Healthy Young Adults

Abstract

Healthy humans are able to place light and heavy objects in small and large target locations with remarkable accuracy. Here we examine how dexterity demand and physical demand affect flexibility in joint coordination and end-effector kinematics when healthy young adults perform an upper extremity reaching task. We manipulated dexterity demand by changing target size and physical demand by increasing external resistance to reaching. Uncontrolled manifold analysis was used to decompose variability in joint coordination patterns into variability stabilizing the end-effector and variability de-stabilizing the end-effector during reaching. Our results demonstrate a proportional increase in stabilizing and de-stabilizing variability without a change in the ratio of the two variability components as physical demands increase. We interpret this finding in the context of previous studies showing that sensorimotor noise increases with increasing physical demands. We propose that the larger de-stabilizing variability as a function of physical demand originated from larger sensorimotor noise in the neuromuscular system. The larger stabilizing variability with larger physical demands is a strategy employed by the neuromuscular system to counter the de-stabilizing variability so that performance stability is maintained. Our findings have practical implications for improving the effectiveness of movement therapy in a wide range of patient groups, maintaining upper extremity function in old adults, and for maximizing athletic performance.

Old adults preserve motor flexibility during rapid reaching

“…The repeated measures ANOVA revealed significant main effects for variability, physical demand and dexterity demand, but no interaction effects or significant age group effects. These findings are in line with our previous reaching experiment (Greve et al 2015) and together with the results on V Ratio demonstrate that both young and old adults similarly increased GEV and stabilized the V Ratio as the amount of NGEV increased with increasing physical constraints of the reaching task (Fig. 3).…”

Flexibility in joint coordination remains unaffected by force and balance demands in young and old adults during simple sit-to-stand tasks

“…Based on these well-established findings on age differences in neuromuscular functions previous studies assumed that flexibility in joint coordination might also decline with ageing. However, our current findings and previous studies on age differences in motor flexibility during STS and other tasks provide evidence that there is not a general decline in motor flexibility with ageing (Decker et al 2012 ; Eckardt and Rosenblatt 2018 ; Freitas and Duarte 2012 ; Greve et al 2013 , 2015 , Hsu et al 2013 , 2014 ; Kapur et al 2010 ; Krishnan et al 2013 ; Krüger et al 2013 ; Olafsdottir et al 2007 ; Shim et al 2004 ; Singh et al 2013 ; Skm et al 2012 ; Verrel et al 2012 ; Wu et al 2013 ; Xu et al 2013 ).…”

The development of consistency and flexibility in manual pointing during middle childhood

“…All these developmental dynamics may then result in a less stable system, thereby explaining the high V ort . To compensate for this V ort increase, V ucm may need to be increased to counter the variability in joint angles that destabilizes the index‐finger position (which has also been suggested in the literature; Black et al, ; Greve et al, ). As children grow older, both V ort and V ucm decrease, resulting in more consistent endpoint movements as well as a decreased flexibility in movement execution.…”

“…Participants were seated in a chair (Tripp Trapp, Stokke, Sweden) adjusted to their height facing the middle of the longer side of the screen. Their trunks were stabilized with a crossover harness of the upper body that was tied to the chair (Greve, Hortobàgyi, & Bongers, 2015;Valk, Mouton, & Bongers, 2016;Van der Steen & Bongers, 2011). This was done to prevent major movements of the torso.…”