Effects of Task Precision Demands on Behavioral and Physiological Changes During a Repetitive Asymmetric Lifting Activity.

Objective: This study investigated the effects of task precision demands on behavioral and physiological changes during repetitive asymmetric lifting. Background: Repetitive lifting encountered in manual material handling leads to muscle fatigue and is a documented risk factor for low back disorder. Method: A total of 17 healthy volunteers performed repetitive asymmetric lifting for 60 min (10 lifts/min). Task precision demands were imposed by varying the entry width onto the destination conveyor. Physiological changes were assessed using near-infrared spectroscopy obtained from the erector spinae muscles. Three-dimensional spine kinematics and moment responses were quantified to understand behavioral changes during the lifting activity. Results: Task precision demands showed no effect on erector spinae muscle oxygenation levels. Behavioral changes associated with repetitive lifting included increases in the overall lift duration, peak forward bending motion, and three-dimensional movement velocities of the spine, along with a decrease in the lateral bending moment. Relative to low precision demands, high precision demands resulted in 20% longer placement periods, which, in turn, resulted in a 12% increase in the time-integrated twisting postures and a 10% increase in the time-integrated lateral bending moments during load placement. Conclusion: The elevated risk of low back injury when lifting under greater precision demands is likely due to the sustained spine twisting and the sustained lateral bending moment on the spine in the final phase of these lifts. Application: Understanding behavioral changes to repetitive asymmetric lifting, especially for tasks requiring greater precision can be used to support injury prevention efforts.