Scaling of lifting forces in relation to object size in whole body lifting.

Subjects prepare for a whole body lifting movement by adjusting their posture and scaling their lifting forces to the expected object weight. The expectancy is based on visual and haptic size cues. This study aimed to find out whether lifting force overshoots related to object size cues disappear or persist over a number of repeated lifts. In addition, the influence of the degree of alternation between load sizes, and the influence of knowledge of actual object weights prior to the lifts, were investigated with regard to their effect on force overshoots. Four experiments were performed using a large and a small box, each of 8.4 kg weight, and varying degrees of alternation between boxes. In two of the experiments, subjects were informed about the weight of the objects, while in the other two experiments they were not informed about the weight of the objects. When boxes were lifted 15 times before switching to the other box, rapid diminishing of force scaling errors was observed. However, when boxes were alternated each lift or after three lifts, persisting force scaling overshoots were found in lifting the large box compared to the small one. When participants were given information regarding the actual object weight, force overshoots in the first pair of large and small box lifts were not different from overshoots in experiments where subjects were not informed about the weight of the objects. This shows that, for occupational lifting, risks related to force overshoots in lifting large objects can persist despite experience in lifting the objects and despite the use of labels indicating the weight of the objects.