LOAD KNOWLEDGE AFFECTS LOW-BACK LOADING AND CONTROL OF BALANCE IN LIFTING TASKS

This study investigated the effect of the presence or absence of load knowledge on the low-back loading and the control of balance in liftin g tasks. Low-back loading was quantified by the net sagittal plane tor que at the lumbo-sacral joint. The control of balance was studied by t he position of the centre of gravity relative to the base of support, the horizontal and vertical momentum of the centre of gravity and the angular momentum of the whole body. In a first experiment, 8 male subj ects lifted a rather heavy load (22% of body mass), using a leglift an d a backlift, while they were familiar with the load mass. To countera ct the threat to balance, imposed by picking up a load in front of the body, the subjects performed specific preparations, based upon the kn own load mass: prior to load pick-up, profound changes in the horizont al and angular momentum were found. The preparations were technique sp ecific. Preserving balance seemed easier while picking up a load with a backlift than with a leglift. In the second experiment, 25 male subj ects lifted a 6 kg box, which they expected to be 16 kg, because, in a series of lifts, the load mass was changed from 16 to 6 kg without th eir knowledge. Despite the 10 kg difference in actual load mass, the n et torque at the lumbo-sacral joint was not different between lifting 6 and 16 kg, until 150 ms after box lift-off. Moreover, lifting of the overestimated load mass caused a disturbance of balance in 92% of the trials. The postural reactions aimed at regaining balance were not ac companied by an increased low-back loading. It was concluded that the absence of load knowledge, and the following overestimation of the loa d mass to be lifted, lead to an increased mechanical load on the lumba r spine and to an increased risk of losing balance in lifting tasks. B oth events may contribute to a higher risk of low-back injury in manua l materials handling tasks.