The effect of elevated body mass index and sex on L5-S1 forces and lower body kinematics during symmetrical manual lifting

In the U.S., 65 % of workers in the Transport, Warehousing, and Utilities sector meet the body mass index (BMI) criterion for obesity, contributing significantly to work-related musculoskeletal injuries among manual workers. However, recommended weight limits (RWL) for manual lifting tasks are based on the general population, overlooking potential adverse effects of BMI and its interaction with sex. This study examined the impact of BMI and sex on L5-S1 forces and lower body kinematics in 30 adults performing a symmetrical box-lifting task designed in accordance with the National Institute for Occupational Safety and Health (NIOSH) guidelines. L5-S1 compression and shear forces, knee and hip angles, and center of mass displacement were measured. A two-way ANOVA assessed biomechanical differences across BMI classifications and sex, while linear regressions examined BMI correlations. Results showed that BMI significantly influenced L5-S1 forces (p < 0.001), with positive relationships found between BMI and both compression (r² = 59.4 %) and shear forces (r² = 61.3 %). Kinematic analysis showed that obese participants flexed their hips less and had a greater center of mass displacement than non-obese participants. Female participants exhibited lower spinal loads and greater hip flexion compared to males. Notably, all obese participants exceeded the safety limits at a lifting index of 1, compared to 70 % of overweight and normal-weight participants who also surpassed these limits. These findings expand the understanding of the recommended NIOSH lifting equation (RNLE) as an ergonomic risk assessment tool by incorporating a broader BMI spectrum and its interaction with sex.