Scaling anticipatory postural adjustments dependent on confidence of load estimation in a bi-manual whole-body lifting task

Abstract

Anticipatory control of motor output enables fast and fluent execution of movement. This applies also to motor tasks in which the performance of movement brings about a disturbance to balance that is not completely predictable. For example, in bi-manual lifting the pick-up of a load causes a forward shift of the centre of mass with consequent disturbance of posture. Anticipatory postural adjustments are scaled to the expected magnitude of the perturbation and are initiated well before the availability of sensory information characterising the full nature of the postural disturbance. However, when the postural disturbance unexpectedly changes, the anticipatory adjustment of joint torques is not equilibrated and may result in a disturbance to balance. In a previous study, it was demonstrated that apart from anticipatory postural adjustments, corrective responses after load pick-up are used to further compensate the postural disturbance. In this study it was examined whether the central nervous system (CNS) assembles a strategy that incorporates both anticipatory control and corrective responses, in which the magnitude of the anticipatory postural adjustments depends on the perceived level of predictability of the postural disturbance. Subjects performed series of lifts in which the magnitude of the load was never revealed to the subject. Two boxes equal in size and colour, but different in mass (6 and 16 kg), were used. Differences in expectation were created by several lifts with the 16-kg load before the 6-kg box was presented. It was observed that the number of strong corrective responses (stepping) varied with the number of 16-kg trials that formed the prior experience when the final 6-kg trial was presented. The follow-up question was whether control relied more on anticipation in the stepping trials, compared with trials in which such gross signs of imbalance were absent. In this study it was shown that subjects when stepping (i) exhibited differential anticipatory postural adjustments in comparison with 6-kg trials in which expectation was not shaped by preceding 16-kg trials, and (ii) scaled the anticipatory postural adjustments similar to those preceding lift-off of the 16-kg trial preceding it. These findings emphasise the programmed nature of the anticipatory postural adjustments and the ability of the CNS to selectively tune the anticipatory postural adjustments to stored information gained during the previous lift(s).

Biomechanical analysis of movement strategies in human forward trunk bending. II. Experimental study

“…Gait initiation, rising on tiptoes (Crenna and Frigo 1991), whole-body lifting (Stapley et al 1998;Toussaint et al 1998), or releasing a load from extended arms (Aruin and Latash 1996) are other tasks that probably start with an A-eigenmovement causing a forward CG acceleration. The goal achieved by the A-eigenmovement is however dierent according to the task: in gait initiation, rising on tiptoes, and whole-body lifting, it mainly shifts the CG forward, whereas in load release, as in trunk bending, it mainly reduces the equilibrium disturbance initiated by the prime movement, due to the dynamic interactions between segments.…”

Handle height and expectation of cart movement affect the control of trunk motion at movement onset in cart pushing

“…Unexpected sudden loading and unloading of the trunk have been considered as challenges to trunk muscle control (Toussaint et al 1998, Cholewicki et al 2000 Burg and van Diee¨n 2001, Moseley et al 2003, Lee et al 2011. Inadequate responses to the perturbation are considered to be a risk factor for low-back injury , Cholewicki et al 2005.…”

Object size effects on initial lifting forces under microgravity conditions

“…In novel objects of unknown weight, lift and grip forces stabilize at a new level within two or three trials (Gordon et al 1993). In addition, Toussaint et al (1998) found anticipatory postural adjustments in whole-body lifting movements to be adapted to new loads within four trials. Thus, the programming of parameters concerning the expected forces is updated quite fast.…”