Evidence is provided to show that: 1) the prepubic process, diagnostic of an ornithischian, did not provide the main support for the abdomen; 2) the abdomen was large and extended ventral to the pelvic girdle with a strong M. rectus abdominis that was not functionally replaced by the lateral abdominal muscles; 3) either the M. pubo-tibialis or the ventral part of the M. puboischio-femoralis internus originated on the lateral surface of the prepubic process. An important femoral protractor, the anterior part of the M. pubo-ischio-femoralis externus, had to be functionally replaced before the pubis changed position to lie close to the ischium in ornithischians and birds. This was accomplished by the development of a long anterior process to the ilium with the differentiation of a large M. ilio-tibialis 1. The lateral curvature of the anterior process of the ilium enabled it to clear the adjacent ribs and improved the mechanical position of part of the M. dorsalis trunci, the M. ilio-tibialis 1 and the dorsal part of the M. pubo-ischio-femoralis internus. A third dorsal muscle to the femur, the M. ilio-trochantericus, was differentiated in Hypsilophodon. The well-developed depression at the base of the fourth trochanter was the main insertion area of the M. caudifemoralis longus. The large size of the fourth trochanter of the ornithopod femur lengthened the moment arm of the M. caudifemoralis brevis during the initial part of femoral retraction; its pendant form resulted from stresses imposed by part of the M. gastrocnemius which originated on the tendon connecting the trochanter to the fibula.