Anticipatory Postural Adjustments and kinematic arm features when postural stability is manipulated

Abstract

Beyond the classical paradigm that presents the Anticipatory Postural Adjustments (APAs) as a manner to create forces that counteract disturbances arising from the moving segment during a pointing task, there is a controversial discussion about the role APAs to facilitate the movement and perform a task accurately. In addition, arm kinematics features are classically used to infer the content of motor planning for the execution and the control of arm movements. The present study aimed to disentangle the conflicting role of APAs during an arm-pointing task in which the subjects reach a central diode that suddenly turns on, while their postural stability was manipulated. Three postures were applied: Standing (Up), Sit without feet support (SitUnsup) and Sit with feet support (SitSup). We found that challenging postural stability induced an increase of the reaction time and movement duration (observed for the SitUnsup compared to SitSUp and Up) as well as modified the upper-limb velocity profile. Indeed, a greater max velocity and a shorter deceleration time were observed under the highest stability (SitSup). Thus, these Kinematics features reflect less challenging task and simple motor plan when the body is stabilized. Concerning the APAs, we observed the presence of them independently of the postural stability. Such a result strongly suggests that APAs act to facilitate the limb movement and to counteract perturbation forces. In conclusion, the degree of stability seems particularly tuned to the motor planning of the upper-limb during a pointing task whereas the postural chain (sitting vs. standing) was also determinant for APAs.

Anticipatory Postural Adjustments in the Frail Older Adults When Postural Stability is Manipulated

“…All kinematic and EMG analysis were previously described in by the authors [4] and are summarized bellow.…”

“…Subjects were instructed to point to a central diode attached to a horizontal bar xed in their front, with their index ngers, as soon as it turned on. Protocol followed previous recommendation [4]. Subjects carried out ten-trial randomized blocks in each posture, with a 5-minute interval between them.…”

“…The study assessed surface electromyographic (EMG) data, from their dominant-side leg muscles using disposable self-adhesive electrodes (Medtrace® 200 -Kendall, Canada): tibialis anterior (TA), soleus (SOL), rectus femoris (RF), semitendinosus (ST) and Deltoids anterior (DEL). Lower limb muscles were chosen due to its role in the ankle and knee control, especially during sitting without feet support and previously demonstrated to be related to APAs [4]. Such data were obtained by using two EMG equipment (EMG System ®, EMG System do Brasil, Brazil), using a sampling rate of 2 KHzper channel and a passband ranging from 20 to 500 Hz.…”

“…We found an increase of the reaction time and movement duration when the body was less stabilized (standing posture), which re ects a more challenging task and complex motor plan. APAs were present even with the body stable (seated with feet support), what suggests an additional APA role, independently of the postural stability, beyond the feedforward control of the other body parts (i.e., to accelerate, or to facilitate the pointing movement) [4].…”

“…During arm movements, self-induced body perturbations are expected by the central nervous system (CNS) and anticipatory strategies are generated [1,2]both to counteract these perturbations and maintain dynamic balance but also to create necessary momentums to initiate movements toward the target [3][4][5].These strategies are known as anticipatory postural adjustments (APAs) and are programmed as a feedforward control mechanism, consisting in changes in the activity of postural muscles, 100 to 150 ms prior to the focal muscle [1].…”