With 1 figure in the text) The principal bones, muscles and tendons of the legs have been measured in a selection of running birds, ranging in size from 0.1 kg quail to 40 kg ostrich. Maximum stride frequencies of the same species have been determined from films. Allometric equations have been derived. Most of the exponents agree well with McMahon's (1973, 1975a) theory of elastic similarity, which is discussed.

With 1 figure in the text) Ostriches have been filmed running fast in their natural habitat. A female ostrich has been dissected and the principal bones, muscles and tendons in a leg have been measured. It is calculated that stresses up to 240 kN m-2 and 40 MN m-2, respectively, act in the digital flexor muscles and their tendons during running. Tensile and compressive stresses up to about 70 MN m-2 and 110 MN m-2 act in the tibiotarsus. A large proportion of the energy which would otherwise be required for running is probably saved by elastic storage in tendons. Comparisons are made with the legs of flying birds and of antelopes.