Gender-Based Differences in Trunk and Shoulder Biomechanical Changes Caused by Prolonged Repetitive Symmetrical Lifting

OCCUPATIONAL APPLICATIONS This study indicates that males and females altered their lifting technique equivalently over time, as they fatigued. Time-dependent changes in lifting mechanics were dependent on the phase of the lifting task. At the initiation of the lift from the floor, participants adapted to increasing trunk fatigue by altering their lower body movements to help maintain a more upright trunk posture. When placing the load on the chest height shelf, participants seemed to adapt to growing upper extremity fatigue by holding the load closer to the body, using more elbow flexion, and less upper arm extension. As a result of this adaptation, participants began to use more forward lean. Therefore, as participants fatigued, they seemed to sacrifice balance (more leaning) in order to preserve muscular capability, which was diminishing with fatigue. This suggests that fatigued lifters may not only be at an increased risk of suffering from an overexertion injury, but they may also be more likely to suffer a slip or fall. TECHNICAL ABSTRACT Background: People working in repetitive manual materials handling jobs are at an increased risk of developing spine and shoulder injuries. To address this concern, it is important to understand how different factors, such as gender, can affect the loading on the body when performing high risk repetitive lifting tasks. Purpose: The purpose of this study was to determine if males and females altered their lifting mechanics in similar ways when exposed to a prolonged fatiguing bout of lifting. Method: Thirty-one participants (n = 15 female) performed a repetitive lifting task for 75 minutes. Isometric shoulder and trunk strength were measured pre- and post-lifting, while upper body kinematics were recorded throughout. Results: Exposure time had a similar effect on lifting kinematics for males and females. At the lift origin, participants transitioned over time toward a posture that had a mean (SD) of 7.2° (2.6°) less torso flexion. At the lift destination, participants transitioned toward a posture with 5.9° (1.7°) less shoulder flexion and 10.1° (2.3°) more elbow flexion. Such changes were consistent with a goal of trying to bring the load closer to the torso. Additionally, isometric strength decreased following the 75-minute lifting protocol by 7.9 (1.2) Nm and 52.4 (10.9) Nm at the shoulder and trunk, respectively. Conclusions: Both males and females adapted their kinematics similarly over time, during repetitive lifting. This study suggests that kinematic adaptions may be driven by participants' consideration of the changing relative demands on their body (e.g., increasing fatigue). The presence of kinematic adaptions, in both males and females, invites the opportunity to consider movement analysis methods as a screening tool for detecting fatigue in the workplace. The ability to recognize and address fatigue earlier may help ergonomists in reducing overexertion-based work related injuries.