Directionality of anticipatory activation of trunk muscles in a lifting task depends on load knowledge

We investigated to what extent subjects base anticipatory activity patterns of trunk muscles before lifting a load on knowledge of the inertial proper ties of the load. Eight healthy male subjects performed rapid arm lifts of a load with a varying center of mass position in the frontal plane. In one set of trials subjects were familiar with the center of mass position, in a nother set of trials they were not. In both cases trunk extensor muscles we re active before the onset of Lift force applied to the load. In the trials with load knowledge this anticipatory activity was specific with respect t o center of mass position. In the absence of load knowledge left and right extensor muscles were equally active before the lift and the rate of liftin g was reduced. Thus anticipatory control of trunk muscles appears specifica lly tuned to counteract the expected perturbation. In the absence of load k nowledge trunk stiffness is increased by bilateral activity and the perturb ation is attenuated since the rate of lifting is reduced.