Stability limits modulate whole-body motor learning

Abstract

Our daily movements exert forces upon the environment and also upon our own bodies. To control for these forces, movements performed while standing are usually preceded by anticipatory postural adjustments (APAs). This strategy is effective at compensating for an expected perturbation, as it reduces the need to compensate for the perturbation in a reactive manner. However, it can also be risky if one anticipates the incorrect perturbation, which could result in movements outside stability limits and a loss of balance. Here, we examine whether the margin for error defined by these stability limits affects the amount of anticipation. Specifically, will one rely more on anticipation when the margin for error is lower? Will the degree of anticipation scale with the margin for error? We took advantage of the asymmetric stability limits (and margins for error) present in the sagittal plane during upright stance and investigated the effect of perturbation direction on the magnitude of APAs. We also compared anticipatory postural control with the anticipatory control observed at the arm. Standing subjects made reaching movements to multiple targets while grasping the handle of a robot arm. They experienced forward or backward perturbing forces depending on the target direction. Subjects learned to anticipate the forces and generated APAs. Although subjects had the biomechanical capacity to adapt similarly in the forward and backward directions, APAs were reduced significantly in the backward direction, which had smaller stability limits and a smaller margin for error. Interestingly, anticipatory control produced at the arm, where stability limits are not as relevant, was not affected by perturbation direction. These results suggest that stability limits modulate anticipatory control, and reduced stability limits lead to a reduction in anticipatory postural control.

Learning from the value of your mistakes: evidence for a risk-sensitive process in movement adaptation

“…The latter will result in a loss of balance, the former will not. Results from a recent study suggest that postural learning is modulated asymmetrically by stability limits, suggesting that adaptation may be risk-sensitive (Manista and Ahmed, 2012). Indeed, we propose that motor adaptation in any environment where the error and its consequence do not correspond, will be risk-sensitive.…”

Intermittent control with ankle, hip, and mixed strategies during quiet standing: A theoretical proposal based on a double inverted pendulum model

“…Multi-link models of this kind have been investigated in recent years in the framework of ''anticipatory postural adjustments'' and ''whole body reaching'' (Bouisset and Zattara, 1987;Stapley et al, 1999;Aruin, 2002;Pozzo et al, 2002;Kaminski, 2007;Morasso et al, 2010;Krishnan et al, 2011;Manista and Ahmed, 2012). However, the focus was mainly on synergy formation rather than stabilization and control.…”

Sagittal Plane Helmet Acceleration at Pole Contact of Alpine Ski Racers is Dependent on Slalom Pole Type and Skill Level

“…The skier skill level can lead to behavior that is based on risk avoidance or risk-seeking objectives [11,12]. A risk avoidance situation can be postulated from the less skilled ski racers that will have greater variability in most aspects of their skiing technique suggesting increased instability [13]. Some of the negative acceleration seen in this study could be explicated by the risk perception that occurs during the middle of the turn when the skis are pointed at their most down the hill attitude and coinciding with a possible pole contact.…”