Motor Planning of Vertical Arm Movements in Healthy Older Adults: Does Effort Minimization Persist With Aging?

Abstract

Several sensorimotor modifications are known to occur with aging, possibly leading to adverse outcomes such as falls. Recently, some of those modifications have been proposed to emerge from motor planning deteriorations. Motor planning of vertical movements is thought to engage an internal model of gravity to anticipate its mechanical effects on the body-limbs and thus to genuinely produce movements that minimize muscle effort. This is supported, amongst other results, by directiondependent kinematics where relative durations to peak accelerations and peak velocity are shorter for upward than for downward movements. The present study compares the motor planning of fast and slow vertical arm reaching movements between 18 young (24 ± 3 years old) and 17 older adults (70 ± 5 years old). We found that older participants still exhibit strong directional asymmetries (i.e., differences between upward and downward movements), indicating that optimization processes during motor planning persist with healthy aging. However, the size of these differences was increased in older participants, indicating that gravity-related motor planning changes with age. We discuss this increase as the possible result of an overestimation of gravity torque or increased weight of the effort cost in the optimization process. Overall, these results support the hypothesis that feedforward processes and, more precisely, optimal motor planning, remain active with healthy aging.

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

“…Our experimental protocol was similar to those of previous studies investigating gravityrelated arm movement control (Gaveau et al 2014(Gaveau et al , 2016Gentili et al 2007;Poirier et al 2020;Le Seac'h and McIntyre 2007). Participants were asked to perform unilateral single-degree-of-freedom vertical arm movements (rotation around the shoulder joint) in a parasagittal plane with their dominant (D) and non-dominant (ND) arms.…”

“…CC-BY-NC-ND 4.0 International license made available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprint this version posted July 14, 2021. ; https://doi.org/10.1101/2021.07.13.452146 doi: bioRxiv preprint investigating single-degree-of-freedom vertical arm movements, to isolate gravity effects, revealed specific features that include: i) direction-dependent kinematics (e.g., durations to peak acceleration and peak velocity are shorter for upward than for downward movements; Gentili et al, 2007;Le Seac'h and McIntyre, 2007;Crevecoeur et al, 2009;Gaveau et al, , 2014Gaveau et al, , 2016Gaveau et al, , 2021Yamamoto and Kushiro, 2014;Yamamoto et al, 2016Yamamoto et al, , 2019Hondzinski et al, 2016;Poirier et al, 2020) and ii) negative epochs on the phasic activity of antigravity muscles during the acceleration and deceleration of downward and upward movements respectively (Gaveau et al 2021). Optimal control model simulations explained these features as a strategy that takes advantage of gravity effects to minimize muscle effort (Berret et al 2008;Crevecoeur et al 2009;Gaveau et al 2014Gaveau et al , 2016Gaveau et al , 2021.…”

“…Investigating motor planning and control of the dominant arm/hemisphere, several studies proposed that the brain uses an internal representation of gravity to take advantage of its mechanical effects (Berret et al 2008;Crevecoeur et al 2009;Gaveau et al 2014Gaveau et al , 2016Gaveau et al , 2021. More specifically, studies investigating vertical arm reaching movements reported direction-dependent arm kinematics (Gaveau et al , 2014(Gaveau et al , 2016(Gaveau et al , 2021Gentili et al 2007;Hondzinski et al 2016;Papaxanthis et al 2005;Poirier et al 2020;Le Seac'h and McIntyre 2007;Yamamoto et al 2016Yamamoto et al , 2019Yamamoto and Kushiro 2014). Particularly, the time to peak acceleration and time to peak velocity were shorter and the curvature was greater for upward than for downward movements.…”

Deterioration, Compensation and Motor Control Processes in Healthy Aging, Mild Cognitive Impairment and Alzheimer’s Disease

“…Poirier et al (2020) [ 59 ] investigated vertical arm movements in young and older adults. The authors used a paradigm allowing them to test how motor planning adapts motor patterns to the gravitational environment.…”

“…Examples of behavioral modifications: slower and more variable movements [ 46 , 47 ] and loss of muscular strength and power [ 45 , 54 ]. Examples of functional compensations: increased reliance on sensory predictions [ 55 , 56 , 57 ], increased internal model recalibration [ 58 ], increased minimization of muscle effort [ 59 ], and overactivation of cortical areas during motor tasks [ 26 ].…”