Quantification of back motion during asymmetric lifting.

The objective of this study was to determine how trunk motion characteristics (in all three planes of the trunk) change as a free dynamic lifting task becomes more asymmetric. Trunk motion characteristics included range of motion, velocity (peak and average), and acceleration. Previous studies have shown that trunk motion characteristics affect trunk strength as well as the action of the trunk musculature. These trunk motion characteristics were quantified as a function of seven task asymmetries and three task weights. The experimental task required the subject to lift materials in positions commonly seen in the workplace. The range of motion, peak velocity, average velocity, and peak acceleration in each plane of the body were documented during the tasks. Generally, trunk motion characteristics in all three planes increased with an increase in task asymmetry. However, with an increase in task weight all the sagittal plane parameters and one transverse plane parameter decreased. Models were constructed to predict trunk motion characteristics given a task asymmetry and weight. When these motion components were compared to dynamic strength estimates from previous studies it was found that dynamic asymmetric lifts could reduce available strength up to 21% of maximum static strength. The results provide new insight into factors associated with the risk of developing low back disorders.