Axial-Appendicular Dynamics and the Integration of Breathing and Gait in Mammal

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Abstract
SYNOPSIS. This paper proposes a biomechanical model for locomotor-respiratory coupling (LRC) in galloping mammals in which gait and breathing cycles are phase-locked on a 1:1 basis. It also explores some of the physiological and neuromotor implications of LRC. The mechanical coupling of locomotor and respiratory cycles depends upon the coordinated, reciprocal oscillations of the cranio-cervical and lumbo-pelvic components of the axial system and their attendant actions on the intervening thorax via muscular linkages. Concurrently, accelerational and decelerational forces imparted to the axial system by the limbs help to drive lung ventilation by inducing inertial displacements of a “visceral piston’ connected to the diaphragm. Several lines of evidence (including cineradiographic data) suggest that an important function of the crural diaphragm is to control the displacement of the visceral piston. The kinematics of LRC indicate that the interosseous intercostal muscles must simultaneously operate to assure thoracic stability against locomotor stresses as well as to promote breathing. The former may be their more essential role, however. The characteristic design of the rib cage in cursorial mammals (=deep and narrow) appears to maximize the leverage of certain “accessory respiratory muscles” (i.e., sternocleidomastoid, scalenes) while minimizing torsional loading of the thorax during forelimb support. Physiological implications of LRC include the prediction that large mammals will breathe relatively faster and with relatively smaller lung volumes when galloping than small species. An additional prediction, that running mammals could automatically gear lung ventilation to speed by simply linking breathing rate to stride frequency and depth of breath (=tidal volume) to stride length, appears to be supported by experimental data from horses. Finally, the neuromotor basis of LRC probably depends upon the direct interaction of central pattern generators for locomotion and respiration. This interaction might be modulated, however, by afferent input from thoracic mechanoreceptors, particularly the intercostal stretch receptors.

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