We developed and validated a fully articulated model of the thoracolumbar spine in OPENSIM that includes the individual vertebrae, ribs, and sternum. To ensure trunk muscles in the model accurately represent muscles in vivo, we used a novel approach to adjust muscle cross-sectional area (CSA) and position using computed tomography (CT) scans of the trunk sampled from a community-based cohort. Model predictions of vertebral compressive loading and trunk muscle tension were highly correlated to previous in vivo measures of intradiscal pressure (IDP), vertebral loading from telemeterized implants and trunk muscle myoelectric activity recorded by electromyography (EMG).

Trunk muscle attenuation is lower in older adults than in younger adults in both women and men, but such age-related differences vary widely between muscle groups. The reasons that some muscles exhibit larger age-related differences in fat content than others should be further explored to better understand age-related changes in functional capacity and postural stability.

Vertebral fractures occur most frequently in the mid-thoracic and thoracolumbar regions of the spine, yet the reasons for this site-specific occurrence are not known. Our working hypothesis is that the locations of vertebral fracture may be explained by the pattern of spine loading, such that during daily activities the mid-thoracic and thoracolumbar regions experience preferentially higher mechanical loading compared to other spine regions. To test this hypothesis, we used a female musculoskeletal model of the full thoracolumbar spine and rib cage to estimate the variation in vertebral compressive loads and associated factor-of-risk (load-to-strength ratio) throughout the spine for 119 activities of daily living, while also parametrically varying spine curvature (high, average, low, and zero thoracic kyphosis models). We found that nearly all activities produced loading peaks in the thoracolumbar and lower lumbar regions of the spine, but that the highest factor-of-risk values generally occurred in the thoracolumbar region of the spine because these vertebrae had lower compressive strength than vertebrae in the lumbar spine. The peaks in compressive loading and factor-of-risk in the thoracolumbar region were accentuated by increasing thoracic kyphosis. Activation of the multifidus muscle fascicles selectively in the thoracolumbar region appeared to be the main contributor to the relatively high vertebral compressive loading in the thoracolumbar spine. In summary, using advanced musculoskeletal modeling to estimate vertebral loading throughout the spine, this study provides a biomechanical mechanism for the higher incidence of fractures in thoracolumbar vertebrae compared to other spinal regions.