In the U.S. it is estimated that over 5 million women have one or more vertebral fractures. These fractures are associated with pain, loss of height, limitation of movement, and even premature death. The etiology of vertebral fractures can be studied by examining a factor of risk. The factor of risk is defined as the applied load over the load required for failure of the vertebra. Although previous research has examined failure of the vertebrae, to our knowledge, the applied loads that are associated with vertebral fractures have not been studied previously. Thus, data are needed on the forces experienced when vertebral fracture occurs and the magnitude of these forces relative to vertebral fracture loads.

As part of an ongoing project, a set of programs has been developed which can be used to predict the compressive forces on the spine during various activities of daily living. Using these predictions, force estimates can be made for patients who have experienced vertebral fracture. These programs are based on previous models, developed to examine spinal compression loads associated with low back pain in the lumbar spine. One of these previous models, which uses an optimization scheme of minimizing the maximum stress to find the muscle activity, was adapted to examine age-related vertebral fractures. Because these fractures can occur in both the thoracic or lumbar regions, the forces of the rib cage were represented by four forces:​ a longitudinal sternum force, two rib cage half forces, and a horizontal sternum force. These represent the rib cage response to applied moments and were included into the optimization routine through a set of weighting factors.

The weighting factors for the rib cage were found by assuming equal maximum muscle stresses at all levels when subjected to the same applied moment. The model was compared to literature data on electromyographic activity of the erector spinae muscles at levels in the thoracic and lumbar spine. Correlation coefficients of 0.93 at the eighth thoracic level and 0.95 at the third lumbar level were found. This model was then used to examine fracture load and fracture risk for several activities. Compression forces on the spine were found to be as high as 4900 N when lifting a person from a bed.

While more extensive validation and more accurate torso cross-section data are needed, this model does provide reasonable estimates for the compressive forces on the spine for many body positions. This model will enable us to examine the circumstances surrounding vertebral fractures and to explore interventions that can be used for their prevention.