The living members of the clade Archosauria, crocodilians and birds, differ markedly in the morphology of their deep dorsal thigh muscles. To investigate whether this diversity is accompanied by differences in motor pattern and muscle function, the hindlimbs of representative archosaurs were studied by electromyography and cineradiography during terrestrial locomotion. In a crocodilian, Alligator, the iliofemoralis and pubo-ischio-femoralis internus part 2 are both active during the swing phase of the stride cycle. This appears to be the primitive motor pattern for archosaurs. There are four avian homologues of these muscles in the helmeted guineafowl, Numida. These are primarily active in the propulsive phase (iliotrochantericus caudalis and iliotrochantericus medius), the swing phase (iliotrochantericus cranialis) and a speed-dependent combination of the propulsive and/or swing phases (iliofemoralis externus). Differences between Alligator and Numida in the number and attachment of deep dorsal muscles are associated with dissimilar motor patterns and functions. Evolutionary modifications of neuromuscular control must be recognized when evaluating avian locomotor history, but are rarely considered by paleontologists. Even within the deep dorsal thigh muscles of Numida, developmentally and anatomically similar muscles are active out-of-phase. Therefore, although the actions of two adjacent muscles appear equivalent, their functions may differ dramatically. The diversity of deep dorsal thigh muscles in modern birds may be a good model for studying the relationship between activity pattern and peripheral morphology.