Effects of unexpected lateral mass placement on trunk loading in lifting.

Abstract

A repeated measurements experiment of spinal loading in healthy subjects. To test whether unexpected lateral mass placement increases low back loading and trunk movement when subjects are lifting a mass in upright posture. Epidemiologic studies suggest that sudden, unexpected loading will lead to low back pain. Also, asymmetric loading is considered to be harmful to the spine. It can be anticipated that unexpected asymmetric loading will increase the risk of injury even more. Ten subjects lifted in an upright posture a crate, in which a mass of 10 kg was placed laterally at the left side either expectedly or unexpectedly. The crate reaction forces, body movements, and trunk muscle activity were measured. From these, the L5-S1 net moments and muscle forces were estimated. Unexpected lateral placement of the mass caused no clear increase in peak low back loading. The stiffness of the trunk was lower in the unexpected condition, which, in combination with inadequate net moments produced, resulted in movement of the trunk to the side of the displaced mass. Unexpected lateral mass placement does not increase the compression force. Perturbed trunk movement and lower muscle forces indicated a decreased stability of the spine, which may imply an injury risk.