Running, breathing and visceral motion in the domestic rabbit (Oryctolagus cuniculus): testing visceral displacement hypotheses.

The relative motion of the visceral mass may be important to ventilation during running. A visceral piston hypothesis predicts that, during galloping, cranial motion of the liver during expiration and caudal motion of the liver during inspiration may characterize efficient quadrupedal mammalian locomotion. Although a theoretical model based on vibration mechanics casts doubt on this prediction, only limited direct measurements of visceral mass motion during galloping have been reported. In the present study, mechanical interactions between running, breathing and liver oscillations in the domestic rabbit are recorded using synchronized videographic, cineradiographic and pneumotachographic techniques. The analysis focuses on the variation in locomotor-respiratory coupling (LRC) and on the relative position of the liver. Results from running rabbits show (1) variation in phase angle between the locomotor and respiratory periods that is inconsistent with the 1:1 LRC ratio that has been reported for other galloping mammals; (2) a tendency towards a 1:1 LRC ratio at higher speeds and stride frequencies; and (3) that the relative motion of the liver is caudal during expiration and cranial during inspiration, which is inconsistent with the visceral piston hypothesis. The data presented here are generally consistent with the theoretical vibration mechanics model for liver motion and with a pneumatic stabilization hypothesis that the lungs serve an important role in the stabilization of the thorax during locomotion.