Anticipatory postural adjustments in a bimanual, whole body lifting task with an object of known weight

Abstract

Anticipatory postural adjustments (APA) were studied in a dynamic multi-joint movement, in which the legs serve both a focal and a postural role. Eight male subjects bimanually lifted a barbell (20% of body mass) after several unloaded movement cycles using two distinct lifting techniques. Picking up a load induces a perturbation to balance, because the centre of mass (CoM) of the combined body and load shifts forward at the moment of load pick-up. Furthermore, the inertia of the load decelerates the backward rotation of the body towards the erect posture. Both perturbations were found to be counteracted by APA in kinematics, kinetics and leg muscle activity patterns. The APA were characterized by an increase in the backward directed horizontal CoM momentum, an increase in the backward directed whole body angular momentum and a forward shift of the centre of foot pressure (CoP) for both techniques. Anticipatory adjustments in activity of muscles crossing the ankle joint were shown to control the CoP position, which was important to accomplish the required combination of anticipatory changes in horizontal and angular momenta. The APA in kinematics and kinetics were modulated according to the dynamic requirements of each lifting technique, although it could have been expected that picking up and lifting the same load at equal speed would have yielded a similar perturbation in each condition.