The Effects of Body Weight on Spinal Loads
Using biomechanical models is one of the most effective ways in analysis and assessment of spinal loads in occupational and daily activities. Despite many advances in the spine modeling, personal factors especially body weight is ignored in most studies. Epidemiological studies show that high body weight is a very strong risk factor to low back pain (LBP) and disc degeneration. There are only few studies that evaluate the relationship between spinal loads and body weight. The goal of this study is analyzing the effect of body weight on spinal load.
In order to facilitate quick development of a wide range of musculoskeletal models, a multi-purpose software was developed. The input file of the software is written in an easy learning language (very similar to AnyBody Modeling System Scripts) and provides tools for developing musculoskeletal models consisting hundreds of muscles and joints.

A detailed musculoskeletal model of lumbar spine with 18 degree of freedom and 76 muscles was developed. This model is hybrid employing anatomical and musculoskeletal model. Recruitment algorithm can predict muscle forces that satisfy equilibrium and stability of the spine simultaneously. Therefore, the activity of antagonist muscles are predictable and muscle forces are calculated more accurately. Equilibrium and stability equations are also verified in AnyBody Modeling System and Abaqus, respectively.

 

In the next step, the model is customized for each specific body weight using previous anatomic studies. Moment arms of muscles and weight of trunk are revised regarding to body weight in the anatomical model. The results of current study show that spinal loads increase by increasing body weight (BMI>30) in the same postures. However, heavier people can perform more demanded tasks, the level of stability is lower in these people. Therefore, heavy people are vulnerable to spine disorders more than thin people whether in light or heavy tasks and postures, and that is because stability level is lower in light tasks and spinal loads are larger in heavy tasks for heavier people.