Lifting an unexpectedly heavy object: the effects on low-back loading and balance loss

Objective. This study evaluates the effects of lifting an unexpectedly heavy object on low-back loading and loss of balance. Background. It is often suggested that lifting an unexpectedly heavy object may be a major risk factor for low-back pain. This may lead to an increase in muscle activation, stretch of ligaments and posterior disc, and loss of balance. Methods. Nine healthy male subjects were asked to pick up and lift a box as quickly as possible. The weight of the box was unexpectedly increased by 5 or 10 kg. Kinematics and force data were recorded throughout the experiment. Results. Lifting of an unexpectedly heavy box led to a decrease in maximum torque of the low back compared to lifting the same box mass with correct expectation. The maximum lumbar angle did not increase compared to the light box condition. Only the threat to balance appeared to be somewhat increased. Conclusions. The lifting of an unexpectedly heavier box appeared not to lead to an increased balance loss or a clearly increased stress of the structures of the low back, although a burst of abdominal muscle activity was found in one condition. These results do not fully clarify the assumed relation between lifting unexpectedly heavy objects and low-back injury.