SCALING ANTICIPATORY POSTURAL ADJUSTMENTS DEPENDENT ON CONFIDENCE OF LOAD ESTIMATION IN A BI-MANUAL WHOLE-BODY LIFTING TASK

Anticipatory control of motor output enables fast and fluent execution of movement. This applies also to motor tasks in which the performanc e of movement brings about a disturbance to balance that is not comple tely predictable. For example, in bi-manual lifting the pickup of a lo ad causes a forward shift of the centre of mass with consequent distur bance of posture. Anticipatory postural adjustments are scaled to the expected magnitude of the perturbation and are initiated well before t he availability of sensory information characterising the full nature of the postural disturbance. However, when the postural disturbance un expectedly changes, the anticipatory adjustment of joint torques is no t equilibrated and may result in a disturbance to balance. In a previo us study, it was demonstrated that apart from anticipatory postural ad justments, corrective responses after load pick-up are used to further compensate the postural disturbance. In this study it was examined wh ether the central nervous system (CNS) assembles a strategy that incor porates both anticipatory control and corrective responses, in which t he magnitude of the anticipatory postural adjustments depends on the p erceived level of predictability of the postural disturbance. Subjects performed series of lifts in which the magnitude of the load was neve r revealed to the subject. Two bares equal in size and colour, but dif ferent in mass (6 and 16 kg), were used. Differences in expectation we re created by several lifts with the 16-kg load before the 6-kg box wa s presented. It was observed that the number of strong corrective resp onses (stepping) varied with the number of 16-kg trials that formed th e prior experience when the final 6-kg trial was presented. The follow -up question was whether control relied more on anticipation in the st epping trials, compared with trials in which such gross signs of imbal ance were absent. In this study it was shown that subjects when steppi ng (i) exhibited differential anticipatory postural adjustments in com parison with 6-kg trials in which expectation was not shaped by preced ing 16-kg trials, and (ii) scaled the anticipatory postural adjustment s similar to those preceding lift-off of the 16-kg trial preceding it. These findings emphasise the programmed nature of the anticipatory po stural adjustments and the ability of the CNS to selectively tune the anticipatory postural adjustments to stored information gained during the previous lift(s).