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The dolphin upstroke is powered primarily by the m. multifidus and m. longissimus—robust muscles that insert serially along the vertebral column. Intramuscular and tendon morphology coupled with kinematic data are used to hypothesize regionally specific functions of these muscles. Both muscles develop equivalent forces (approximately 2kN) in the region of the thoraco-lumbar spine, but transmit those forces to different regions of the vertebral column. The m. multifidus transmits the majority of its force locally to the thoraco-lumar spine, a region of the body that undergoes no measurable bending. The action of the m. multifidus appears to be to stiffen its deep tendon of insertion, forming a temporary skeletal element for the m. longissimus. The m. longissimus transmits the majority of its force to the caudal spine, by way of a novel interaction between its insertional tendons and the subdermal connective tissue sheath. This insertional pattern confers torsional stiffness on the caudal peduncle, and allows the m. longissimus to do relatively more work (118 J) than if it had a typical mammalian insertional pattern. The caudal extension of the m. multifidus does 12 J of work on the caudal spine during an upstroke. The caudal extension of the m. longissimus transmits its force to the vertebrae in the caudal flukes and is the only epaxial muscle that acts to control the angle of attack of the fluke blade.