INTRODUCTION: The complexity of the spine has made a complete understanding of its mechanical function difficult. As a consequence, biomechanical models have been used to describe the behaviour of the spine and its various components. A comprehensive mathematical model of the muscles of the lumbar spine and trunk is presented to enable computation of the forces and moments experienced by the lumbar intervertebral joints during physiological activities.

METHODS: The model includes the nine major muscles crossing the region and concentrates on improving the estimated line of action for the muscles. The muscles are considered to consist of numerous fascicles, each with its own force producing potential based on size and line of action. The model respects the physical constraints imposed by the skeletal structure by ensuring that muscles maintain their anatomical position in various spinal postures. Validation was performed by comparing model predictions of maximum moments to published data from maximum isometric exertions in male volunteers. To highlight the potential novel uses of the model, three examples of muscle injury caused by surgical procedures were investigated; posterior lumbar surgery, impairment of abdominal muscles from anterior surgery and removal of the psoas major unilaterally during total hip replacement.

RESULTS: The validation indicated that the model predicted forces similar to those measured in normal volunteers. The biomechanical changes resulting from the muscle injuries during the surgical procedures share several common features: decreased spinal compression and production of asymmetric moments during symmetric tasks.

DISCUSSION: The results suggest that interference with muscles crossing or attaching to the lumbar spine can have a significant impact on its function.