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.

A novel stability and kinematics-driven trunk biomechanical model to estimate muscle and spinal forces.

An anatomically detailed eighteen-rotational-degrees-of-freedom model of the human spine using optimization constrained to equilibrium and stability requirements is developed and used to simulate several symmetric tasks in upright and flexed standing postures. Predictions of this stability and kinematics-driven (S+KD) model for trunk muscle forces and spine compressive/shear loads are compared to those of our existing kinematics-driven (KD) model where both translational and rotational degrees-of-freedom are included but redundancy is resolved using equilibrium conditions alone. Unlike the KD model, the S+KD model predicted abdominal co-contractions that, in agreement with electromyography data, increased as lifting height increased at a constant horizontal moment arm. The S+KD model, however, could not fully explain the CNS strategy in activating antagonistic muscles for most of the remaining tasks. Despite quite distinct activities in individual muscles, both models predicted L4-L5 intradiscal pressure that matched the in vivo data, the L4-S1 compression loads, and the sum of all trunk muscle forces. For modeling applications in ergonomics, where the compressive spine loads are of interest, the two models yielded <15% difference. In the field of rehabilitation, where detailed muscle forces are required, the S+KD model explained more properly the CNS strategy in activating the antagonistic muscles for some tasks.