Muscle effort is best minimized by the right-dominant arm in the gravity field

Abstract

The central nervous system (CNS) develops motor strategies that minimize various hidden criteria, such as end-point variance or effort. A large body of literature suggests that the dominant arm is specialized for such open-loop optimization-like processes, whilst the non-dominant arm is specialized for closed-loop postural control. Building on recent results suggesting that the brain plans arm movements that take advantage of gravity effects to minimize muscle effort, the present study tests the hypothesized superiority of the dominant arm motor system for effort minimization. Thirty participants (22.5 ± 2.1 years old; all right-handed) performed vertical arm movements between two targets (40° amplitude), in two directions (upwards and downwards) with their two arms (dominant and non-dominant). We recorded the arm kinematics and electromyographic activities of the anterior and posterior deltoid to compare two motor signatures of the gravity-related optimization process; i.e., directional asymmetries and negative epochs on phasic muscular activity. We found that these motor signatures were still present during movements performed with the non-dominant arm, indicating that the effort-minimization process also occurs for the non-dominant motor system. However, these markers were reduced compared with movements performed with the dominant arm. This difference was especially prominent during downward movements, where the optimization of gravity effects occurs early in the movement. Assuming that the dominant arm is optimal to minimize muscle effort, as demonstrated by previous studies, the present results support the hypothesized superiority of the dominant arm motor system for effort-minimization.

Aging decreases the lateralization of gravity-related effort minimization during vertical arm movements

“…These kinematics and electromyographic results are thought to represent the hallmarks of a gravity-related optimization processes that occurs during motor planning to minimize muscle effort (for reviews see (Berret et al, 2019;White et al, 2020). In our recent study (Poirier et al, 2022), we found that direction-dependent kinematics and negative epochs were still present during movements performed with the non-dominant arm, but significantly reduced compared to movements performed with the dominant arm. These results support the hypothesized superiority of the dominant motor system for open-loop optimization processes.…”

“…Model simulations demonstrated that the negativity of antigravity muscles reflects an optimal motor strategy where gravity force is harvested to save muscle force. Recently, we found that the amount of negativity was superior when healthy young participants reach with their dominant than with their non-dominant arm (Poirier et al, 2022). We defined negative epochs as a time interval when phasic EMG was inferior to zero minus a 95% confidence interval (computed on the integrated EMG signals from 500ms to 250ms before movement onset) for at least 50ms.…”

“…1A). In a randomized block order, movements were executed with the dominant (D) and non-dominant (ND) arms (Poirier et al, 2022). We investigated single degree-of-freedom movements to isolate and emphasize the mechanical effects of gravity on arm motion.…”

“…It must be noted, however, that some recent results may limit this theory (Takagi et al, 2020;Maurus et al, 2021). We recently investigated inter-limb differences regarding gravity-related optimization processes during vertical arm movements (Poirier et al, 2022). A large body of literature combining mathematical simulations and behavioral experiments previously demonstrated that the brain optimally takes advantage of gravity torque to minimize muscle effort during vertical arm movements (Berret et al, 2008;Crevecoeur et al, 2009;Gaveau et al, 2011Gaveau et al, , 2014Gaveau et al, , 2016Gaveau et al, , 2021.…”

“…More precisely, investigating vertical arm movements, multiple studies have reported direction-dependent kinematics where upward movements exhibit sharper acceleration profiles than downward ones (Papaxanthis et al, 2005; Gentili et al, 2007; Le Seac’h and McIntyre, 2007; Gaveau and Papaxanthis, 2011b; Gaveau et al, 2011, 2014, 2016, 2021; Yamamoto and Kushiro, 2014; Yamamoto et al, 2016, 2019; Hondzinski et al, 2016; Poirier et al, 2022, 2020). More, recent works confirm kinematics results by revealing systematic negative epochs in the phasic activity of antigravity muscles (Gaveau et al, 2021; Poirier et al, 2022). Such negativity of the phasic signal demonstrate that gravity effects are not compensated for but, instead, harvested to discount muscle effort (Gaveau et al 2021).…”