Evidence for subjective values guiding posture and movement coordination in a free-endpoint whole-body reaching task

Abstract

When moving, humans must overcome intrinsic (body centered) and extrinsic (target-related) redundancy, requiring decisions when selecting one motor solution among several potential ones. During classical reaching studies the position of a salient target determines where the participant should reach, constraining the associated motor decisions. We aimed at investigating implicit variables guiding action selection when faced with the complexity of human-environment interaction. Subjects had to perform whole body reaching movements towards a uniform surface. We observed little variation in the self-chosen motor strategy across repeated trials while movements were variable across subjects being on a continuum from a pure ‘knee flexion’ associated with a downward center of mass (CoM) displacement to an ‘ankle dorsi-flexion’ associated with an upward CoM displacement. Two optimality criteria replicated these two strategies: a mix between mechanical energy expenditure and joint smoothness and a minimization of the amount of torques. Our results illustrate the presence of idiosyncratic values guiding posture and movement coordination that can be combined in a flexible manner as a function of context and subject. A first value accounts for the reach efficiency of the movement at the price of selecting possibly unstable postures. The other predicts stable dynamic equilibrium but requires larger energy expenditure and jerk.

Influence of dual-task constraints on the interaction between posture and movement during a lower limb pointing task

“…On the other hand, the postural component started earlier than the focal component of motor behavior to minimize the perturbation of equilibrium caused by the oncoming focal movement (Frank and Earl 1990;Aruin and Latash 1995b;Conceição et al 2016). This behavior was increased in the dual-task condition, as participants needed more time to reach an adequate postural stability allowing them to perform the movement as efficiently as possible, in accordance with previous studies (Rios et al 2015;Hilt et al 2016).…”

“…This reflected a significant body inclination on that axis which was more consistent for the forward displacement during the backswing phase. This allowed reaching an adequate stability to perform the movement, even in situations in which there is an increased equilibrium constraint requiring to adopt a different strategy of COM motion (Horak and Nashner 1986;Crenna et al 1987;Hilt et al 2016).…”

Sensorimotor Reorganizations of Arm Kinematics and Postural Strategy for Functional Whole-Body Reaching Movements in Microgravity

“…This feature is typical of a “ hip strategy ” (Horak and Nashner, 1986 ), through which the postural component supporting the focal part of movement is also used to prevent falling (Massion, 1992 ). This posturokinetic organization would thus reduce the displacement of the CoM by using compensatory mechanisms (Massion, 1992 ; Vernazza et al, 1996 ) and would favor equilibrium maintenance at the expense of mechanical energy minimization and joint smoothness maximization (Hilt et al, 2016 ).…”

The adaptation to standing long jump distance in parkour is performed by the modulation of specific variables prior and during take-off

“…This leads to have a more vertical trajectory when reaching the targeted distance, while minimizing the horizontal speed at landing. This type of postural vertical strategy can generally be seen as the safest way to maintain the balance (Hilt, Berret, Papaxanthis, Stapley, & Pozzo, 2016). When practicing parkour, not falling upward or backward when landing on a wall can be of particular importance when the jump is performed from a certain height.…”