Development and validation of a subject-specific integrated finite element musculoskeletal model of human trunk with ergonomic and clinical applications.

Biomechanical modeling of the human trunk is crucial for understanding spinal mechanics and its role in ergonomics and clinical interventions. Traditional models have been limited by only considering the passive structures of the spine in finite element (FE) models or incorporating active muscular components in multi-body musculoskeletal (MS) models with an oversimplified spine. To address those limitations, we developed a subject-specific coupled FE-MS model of the trunk and explored its applications in ergonomics and surgical interventions. A parametric detailed FE model was constructed, integrated with a muscle architecture, and individualized based on existing datasets. Our comprehensive validation encompassed tissue-level responses, segment-level mechanics, and whole-spine behavior across multiple subjects and loading conditions, demonstrating satisfactory performance in ergonomics (i.e., wearing exoskeleton) and clinical interventions (nucleotomy and spinal fusion). The model accurately predicted tissue-level stresses (in uni- and biaxial loading), whole-spine motion (i.e., moment rotation response was in agreement with in vitro measurements), intradiscal pressures (RMSE = 0.12 MPa; R² = 0.72), and muscle activities (matching EMG trends across 19 subjects during forward flexion). Wearing an exoskeleton reduced intradiscal pressures (1.9 vs. 2.2 MPa at L4-L5) and peak von Mises stresses in the annulus fibrosus (2.2 vs. 2.9 MPa) during forward flexion. Spinal fusion (at L4-L5) increased the intradiscal pressure in the upper adjacent disc (1.72 MPa vs. 1.58 MPa), but nucleotomy had a minimal effect on the intact intradiscal pressures. Nucleotomy substantially affected the load transfer at the same level by increasing facet contact loads and annulus radial strains. Unlike conventional MS models with simplified spine, and in contrast to passive models (without active components), this model provides crucial outputs such as strain/stress fields in discs/facets (essential for a comprehensive risk analysis). This integrated approach enables more accurate surgical planning, workplace safety design, and personalized rehabilitation strategies, helping reduce spine-related injuries by identifying risk factors and optimizing interventions for individual patients.