Balance loss when lifting a heavier-than-expected load: effects of lifting technique.

Abstract

To compare the lifting techniques of subjects who did and did not maintain their balance with an unexpectedly heavy load, and to examine whether the balance loss increased low back loading. Repeated-measures design. A research laboratory. Fourteen healthy volunteers were assigned to 2 groups in the post hoc analysis. Group 1 (7 men; mean age, 25.6 +/- 4.2yr; height, 1.78 +/-.08m; weight, 83.0 +/- 8.5kg; lifting capacity, 63.2 +/- 8.0kg) maintained balance and was matched to group 2 (7 men; mean age, 26.3 +/- 4.1yr; height, 1.75 +/-.06m; weight, 78.2 +/- 5.3kg; lifting capacity, 64.7 +/- 4.9kg) who lost balance. Subjects lifted boxes of 5%, 20%, and 35% of their lifting capacity. Load magnitude was expected or unexpected. Center of mass (COM), lower body mechanics, ground reaction forces, and angular and horizontal momentum. Group 1 consistently showed greater lumbar flexion, less knee flexion, and a higher COM just before and after load liftoff. During the heavier-than-expected 35% lift, the trunk angular velocities lifts indicated that both groups experienced eccentric trunk extensor muscle contractions. The semisquat technique may protect against balance loss when lifting unexpectedly heavy loads. Eccentric muscle contractions and rapid increases in lumbar joint reaction moments may increase the risk of low back injury when there is a large, unexpected increase in the weight of the lifted load.