Postural configuration does not alter unperturbed or perturbed reach movement kinematics

Abstract

This study investigated whether postural configuration has a significant effect upon the kinematics of arm movements when humans performed unconstrained reach movements to visual targets. Eight subjects were required to reach to static visual targets (unperturbed REACH movements) or correct reach movements when the position of a target unexpectedly changed during the execution of a planned movement (perturbed reaches, or online corrections, OC). Subjects were required to execute REACH and OC movements in sitting and standing (STAND) positions. The height of the targets, distance from the right shoulder (acromion) and eccentricity in terms of the body midline were standardized between the two postural conditions before movements begun. Unperturbed REACH movements were executed to a central target placed at 130 % of outstretched arm length, along the midline (0°). Perturbed (OC) movements involved subjects initiating an arm movement to the 0° target upon its illumination. Two hundred milliseconds after the onset of the hand movement, the 0° target was extinguished and the target at 60° to the right of the midline (still at 130% outstretched arm distance) illuminated. Subjects had to correct their reach movements online to the new target. Results demonstrated that, despite evident differences in postural kinematics between the four experimental conditions (e.g. pelvis obliquity and trunk/pelvis rotation), postural configuration had little or no effect upon the endpoint kinematics of the finger. Most importantly, the STAND position, with its greater postural constraints, did not affect the time taken to initiate an OC, nor did it lengthen the time taken to complete the REACH or OC movements. Our results suggest, therefore, that postural constraints are accounted for by the central nervous system when executing complex arm reaching movements.

Constancy of Preparatory Postural Adjustments for Reaching to Virtual Targets across Different Postural Configurations

“…A lack of kinematic changes for the finger between the SIT and STAND configurations; however, suggests that the mechanical challenges associated simply with standing may be well accounted for by the CNS. This is also supported by previous work for perturbed and unperturbed reaching (Hua et al, 2013).…”

“…PVel: 2.31 -2.92). When compared to results within our own laboratory using a similar paradigm (with the notable differences of physical vs. remembered targets and greater range of target directions) movement time and peak velocities were relatively quicker and faster respectively (MT = 746 -881 ms, Leonard et al, 2009; MT = 627 -1190 ms, PVel = 1.1 -2.2 m/s, Hua et al, 2013; MT = 740 -940 ms, PVel = 1.4 -3.8 m/s, Stamenkovic and Stapley, 2016). Further comparison with studies examining reaching during classic seated scenarios and whole body reaching paradigms place our results well within reported movement speeds (seated: PVel ~ 1.8 -2.2 m/s, Kaminski et al, 1995 Ma and Feldman, 1995).…”

“…single step) and movements that require online corrections (e.g. double-step), do not differ in arm kinematics between seated and standing positions (Hua et al, 2013). Moreover, across a wider range of postures, no distinct changes in gaze trajectory or accuracy are observed despite clear differences in segmental contributions to movement (Stamenkovic et al, 2018).…”

Age-related effects of increasing postural challenge on eye movement onset latencies to visual targets

“…The distance between the initial (central) target and each participants' sternum corresponded to 130% of the length of their arm, measured from the acromion process to the right index finger tip (with the arm outstretched). As we have previously used this distance to study postural adjustments during arm movements in sitting vs. standing positions in young participants (Hua et al 2013), we sought to evaluate any possible differences in eye movement behavior using the same distance.…”

Online reach correction in 6‐ and 11‐month‐old infants

“…At first, the multiple‐MU reaches made by infants appear qualitatively distinct from adult reaches, which typically show a smooth goal‐directed trajectory consisting of a single MU. However, under high‐precision requirements or target perturbations, multiple peaks can be found in the hand speed profile of adult reaches as well, especially when the hand approaches the target (Hua et al., 2013; Milner, 1990). Hence, infant and adult reaching may be considered within a single framework, where the distinct kinematic characteristics of infant reaching arise from the relatively more demanding task requirements that are the result of less fine‐tuned motor control abilities.…”