Previous investigations have shown that, when they run, wallabies and various quadrupedal mammals take exactly one breath per stride. It has been suggested that their breathing movements may be driven by the movements of running, and three mechanisms have been proposed: the lungs may be compressed and expanded by vibrations of a ‘visceral piston’, by bending and extension of the back or by the forces imposed on the rib cage by the forelegs.
The three mechanisms are tested against published observations on wallabies and horses. Simple mathematical models are used to predict the phase relationship between running and breathing for each mechanism, and the amplitudes of breathing that they could cause. The observed breathing movements of hopping wallabies are consistent with a visceral piston mechanism tuned to the stride frequency, but cannot be explained by either of the other mechanisms. Those of cantering and galloping horses are not consistent with a tuned visceral piston mechanism, but could be driven by the bending and extension of the back.