Summary Obesity is an emerging problem that seems to be involved in occupational health and safety issues. The prevalence of musculoskeletal injuries in obese individuals and indirect (non-medical) costs are higher than for workers having a healthy weight. A sustained increase in the obesity rate of workers is being seen in Canada, so it is important to be interested in the obesity-work issue. Today the risk of back injuries during work activities remains very high, and the profession producing the most disorders is still that of material handler. There is very little evidence on the impact of obesity on the way handling tasks are performed. The objective of this study is to analyze the work strategies of obese material handlers and compare them to those of material handlers having a healthy weight.The biomechanical and ergonomic impacts of 17 obese handlers and 20 handlers of healthy weight were evaluated in the laboratory. The studied tasks involved transferring boxes from a conveyor to a hand truck and vice versa. The weight of the load, the pick-up and deposit height, and the configuration of the work area were modified to examine the participants’ methods. Several biomechanical measurements were taken, notably including the moments of force on the back, posture and box movement to assess the safety and efficiency of the observed handling methods.The results clearly show that the anthropometric factors of obese material handlers result in clearly greater (>23%) maximum lumbar loading during the lifting and deposit of boxes on a hand truck or conveyor. Few differences were observed in postural attitudes due to a very present inter-individual variability within the two groups of participants. The handlers’ weight explained 57% of the variation in the transverse maximum moment of force on the back when raising a box from the ground.These results suggest that a worker’s obesity leads unavoidably to a significant additive effect on the musculoskeletal structures of the back. This biomechanical impact exposes obese material handlers to a greater risk of developing a musculoskeletal disorder during load handling.