The effects of obesity on lifting performance.

Obesity in the workforce is a growing problem worldwide. While the implications of this trend for biomechanical loading of the musculoskeletal system seem fairly straightforward, the evidence of a clear link between low back pain (LBP) and body mass index (BMI) (calculated as whole body mass in kilograms divided by the square of stature in meters) has not been shown in the epidemiology literature addressing this topic. The approach pursued in the current study was to evaluate the lifting kinematics and ground reaction forces of a group of 12 subjects -- six with a BMI of less than 25 kg/m(2) (normal weight) and six with a BMI of greater than 30 kg/m(2) (obese). These subjects performed a series of free dynamic lifting tasks with varied levels of load (10% and 25% of capacity) and symmetry (sagittally symmetric and 45 degrees asymmetric). The results showed that BMI had a significant effect (p<0.05) on trunk kinematics with the high BMI group exhibiting higher peak transverse plane (twisting) velocity (59% higher) and acceleration (57% higher), and exhibiting higher peak sagittal plane velocity (30% higher) and acceleration (51% higher). When normalized to body weight, there were no significant differences in the ground reaction forces between the two groups. This study provides quantitative data describing lifting task performance differences between people of differing BMI levels and may help to explain why there is no conclusive epidemiological evidence of a relationship between BMI and LBP.