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Fig. 1. Model geometry. (A) A 4-bar linkage consisting of a stiff hindleg (red, length L_hind), back, foreleg (blue, length L_fore) and the ground between the hind and front feet. The path of the body centre of mass (COM) and its rotations are calculated from purely geometric considerations as the hindleg vaults over the vertical (B). The stride begins at the instant of hindfoot placement. Both hind- and foreleg angles are assumed to sweep an equal angle before and after vertical. The initial foreleg angle _fore^* is determined from the phase. The hindlimb angle is incremented, changing COM position (green dots) and body angle, and the appropriate time step for each increment throughout the stride is calculated such that the total mechanical energy is constant (although see the `powering strategies' section). Interchanges between mechanical energy components (potential, kinetic and rotational kinetic energies), velocities and accelerations were thus calculated from the 4-bar linkage geometry as a function of time. Total mechanical energy was adjusted so that the model and observed mean horizontal velocities corresponded.

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