Anticipatory control of vertical lifting force and momentum during the squat lift with expected and unexpected loads

Study Design: Mixed, repeated measures design. Objectives: To determine if previous experience with loads of similar weight influences the anticipatory lifting force and postural adjustments during the squat lift. To examine a multijoint, functional task for coordination between stability and movement. To determine if lifting unexpected loads results in trunk loading patterns associated with greater injury risk. Background: Workers are increasingly exposed to variability in materials handling thereby increasing the risk of injury, Understanding the control processes underlying lifting will support clinical decision making for preparing injured workers to return to realistic working conditions. Methods and Measures: Subjects (19 men, 4 women; mean age, 25.4 +/- 3.5 y) lifted a series of boxes weighing 5%, 20%, and 35% of their maximal lifting capacity. The loads were delivered via a gravity conveyor. The identical-appearing loads were ordered so that the subjects lifted several loads of similar weight, immediately followed by a lighter or heavier than expected load based on the previous lift. Results: Generally, the vertical lifting force, force rate, horizontal momentum, and angular momentum increased with an increase in expected load. Higher peak lumbar extensor moments occurred with lighter than expected loads (expected 20% and lifted 5% load 238.2 +/- 91.2 N-m; expected 35% and lifted 5% load = 278.2 +/- 101.8 N-m) compared to expected loads of similar weight (expected and lifted 5% load 205.0 +/- 73.2 N-m). Heavier than expected loads led to eccentric trunk movements when there were large mismatches between the expected and actual loads. Conclusions: The vertical lifting forces and whole body momentum were predictively scaled according to the expected load. The loading phase relationships indicated coordination between the lifting force, force rate, horizontal momentum, and angular momentum. Trunk loading during the lifts with unexpected loads showed patterns associated with increased injury risk.