An EMG-assisted model of trunk loading during free-dynamic lifting.

One of the continuing challenges in biomechanics has been to assess loading of the spine during dynamic lifting exertions. A model was developed to accurately simulate multi-dimensional spinal loads and trunk moments from measured muscle coactivity and external forces during free-dynamic lifting exertions. Model validity was demonstrated by comparing measured and predicted trunk extension moments. Its purpose was to examine realistic representations of lifting kinetics, kinematics, and dynamic trunk mechanics that may influence spinal loading, and to demonstrate that EMG-assisted modeling techniques can be applied to the analysis of free-dynamic exertions. Spinal loads and trunk moments were predicted from the muscle force vectors and external loads. Muscle tensile forces were determined from the product of normalized EMG data modulated to account for contractile dynamics, muscle cross sectional area, and muscle force per unit cross-sectional area. Model output was physiologically valid, i.e. average predicted muscle force per unit cross-sectional area of 50-65 N cm-2, and accurately predicted measured, dynamic, lifting moments, with an average R2 = 0.81 in the sagittal plane and R2 = 0.76 in the lateral plane. Results indicated that compressive and shear loading increased significantly with exertion load, lifting velocity, and trunk asymmetry.