The Effect of Obesity on Trunk Kinematics and Ground Reaction Forces during Lifting

Abstract

The prevalence of obesity is increasing worldwide. In fact, in the United States, obesity has been recognized as an epidemic. Obesity has been shown to increase the risk of physical injury and illness. Among these physical disorders, low back pain (LBP) is one of the most common phenomena in both obese and non-obese individuals. However, the relationship between obesity and LBP is not fully developed and the causal link between them is insufficient. The objective of this research was to evaluate the differences between people of normal weight and obese people in measures of trunk kinematics and ground reaction force during a lifting task. The main hypothesis of this study was that obese people would have a higher mean value and a higher variability of these measures than of normal weight people.

Two subjects groups were used in this research and each group had six subjects. Group I is the normal weight subject group (BMI<25), and Group II is the obese subject group (BMI>30). In the experiment, the subjects were asked to perform a lifting task under two levels of load (10% and 25% of maximum lifting capacity) and two levels of starting asymmetric angle (0° and 45°). The Lumbar Motion Monitor was used to collect the trunk kinematics data and two force plates were used to collect the ground reaction force and moment data. To test the variability, Modified Levene's test was employed. MANOVA and ANOVA were used to assess the effects of BMI, load, and angle on these measures. The results showed that BMI is a significant effect for mean value on several kinematics parameters. From BMI<25 to BMI>30, the rotational velocity increased by 59.2%; the rotational acceleration increased by 57.6%; the sagittal velocity increased by 30.4%; sagittal acceleration increased by 50.5% (all statistically significant at the p<0.05 level). However, the results did not support the hypothesis that BMI would affect variability in these measures.

This study provides quantitative data describing lifting task performance for obese people, and has shown that obese people have higher sagittal velocity, sagittal acceleration, rotational velocity, and rotational acceleration during lifting task, which may lead to higher forces on spine, compared to people with normal weight. The results indicated that the regular safety evaluation of lifting job based on normal people may not be appropriate for obese people since they have different lifting pattern, which may increase the risk of injury. The data presented in this work are particularly important as the general workforce continues to get heavier and heavier.