Spinal loads during two-person team lifting: effect of matched versus unmatched standing height

Abstract

The purpose of this experiment was to compare spinal loads during two-person lifting tasks performed with team members of matched versus unmatched standing height. Twelve young healthy male subjects performed matched and unmatched team lifts with two box masses (30 and 60 kg) and three initial box heights (0, 20 and 40 cm). All lifts were performed in the sagittal plane with a self-selected lifting technique from the initial box height to standing knuckle height. The box was instrumented with force transducers that measured horizontal and vertical hand forces, whilst sagittal plane segmental kinematics were determined using a video-based motion measurement system. Dynamic L4/L5 torques were calculated and used in a single equivalent extensor force model of the lumbar spine to estimate L4/L5 compression and shear forces. No significant differences were found in the maximum torque, compression or shear forces at L4/L5 between lifts performed with team members of matched or unmatched height. However, in the last part of the unmatched lifting condition the box was located closer to the short subject, which caused the reactive L4/L5 torque to decrease for short subjects and increase for tall subjects. Average (but not peak) L4/L5 torque and compression force were therefore significantly lower for short subjects and significantly higher for tall subjects in the unmatched compared to the matched lifting condition. However, the larger spinal load incurred by the tall subjects at the end of the unmatched compared to matched lifts are unlikely to drastically alter the risk of spinal injury caused by acute loading, since they were only one-third of the maximal spinal load that occurred during the lift and were below the NIOSH Action Limit. Despite this matching team member height may still be important for decreasing cumulative loading during repetitive lifting. Relevance to industry Manual handling guidelines typically recommend that persons involved in team lifts be of similar standing height. Results of the present study indicated that matching the standing height of lifting team members may be important for preventing exposure to higher average spinal loads in tall compared to short subjects. Height matching may therefore reduce the cumulative effects of spinal loading for taller subjects.