OBJECTIVE: To determine the morphology of the latissimus dorsi in order to assess its actions on the shoulder, the lumbar spine and the sacroiliac joint. DESIGN: A dissection study accompanied by an analysis of the force vectors of the muscle and its parts. BACKGROUND: Although recognised as a muscle of the shoulder, latissimus dorsi has been accorded a role as an extensor of the lumbar spine, and is said to brace the sacroiliac joint. Consideration of the anatomy of the latissimus dorsi suggests that the magnitude of these actions has been overstated. METHODS: The fascicular anatomy of the latissimus dorsi was determined by dissection in five adult cadavers. The size, attachments, and orientation of each fascicle were determined. By applying a force coefficient the maximum force of each fascicle was estimated from its physiological cross-sectional area. By summing the forces and moments of each fascicle the maximum force exerted by latissimus dorsi was calculated for its actions on the shoulder, the lumbar spine, and the sacroiliac joint. RESULTS: The latissimus dorsi was found to consist of a series of fascicles with segmental attachments to the lower six thoracic spinous processes, the L1 and L2 spinous processes, the lateral raphe of the thoracolumbar fascia, the iliac crest and the lower three ribs. These fascicles were uniform in size across a given muscle but varied from specimen to specimen. The maximum total force exerted by the latissimus dorsi on the shoulder was estimated to range between 162 and 529 N, but in view of the attachments of the muscle, only a portion of that force can be exerted on the lumbar spine. The maximum extensor moment exerted on the lumbar spine was calculated to be 6.3 N m. The maximum force exerted across the sacroiliac joint was calculated to be 30 N. CONCLUSIONS: The latissimus dorsi is designed to move the upper limb or to raise the entire trunk in brachiation. Its possible contribution to extension of the lumbar spine is trivial as is its capacity to brace the sacroiliac joint. RELEVANCE: Despite assertions and concerns to the contrary, the latissimus dorsi is of little mechanical importance in the lumbosacral region.