Effect of body weight on spinal loads in various activities: a personalized biomechanical modeling approach.

Epidemiological studies are divided over the causative role of body weight (BW) in low back pain. Biomechanical modeling is a valuable approach to examine the effect of changes in BW on spinal loads and risk of back pain. Changes in BW have not been properly simulated by previous models as associated alterations in model inputs on the musculature and moment arm of gravity loads have been neglected. A detailed, multi-joint, scalable model of the thoracolumbar spine is used to study the effect of BW (varying at five levels, i.e., 51, 68, 85, 102, and 119 kg) on the L5-S1 spinal loads during various static symmetric activities while scaling moment arms and physiological cross-sectional areas of muscles using in vivo imaging data. The L5-S1 loads substantially increased with BW especially in flexed postures. As BW increased from 51 to 119 kg, the L5-S1 compression increased in flexed postures by ~80-147% with no load in hands and by ~46-52% in load holding tasks. In obese individuals with body mass index>30 kg/m(2) spinal loads further increased by up to 15% as lever arms for gravity loading at the waistline (T12 through L5) increased by 2 cm (for BW=102 kg) and 4 cm (for BW=119 kg). With changes in BW, spinal loads would have moderately altered (<17%) had identical muscle parameters been considered. Since scaling muscle parameters demands additional efforts in modeling, one could opt for simulation of alterations only in BW while using some averaged musculature values.