Hypaxial muscle activity during running and breathing in dogs.

The axial muscles of terrestrial vertebrates serve two potentially conflicting functions, locomotion and lung ventilation. To differentiate the locomotor and ventilatory functions of the hypaxial muscles in mammals, we examined the locomotor and ventilatory activity of the trunk muscles of trotting dogs under two conditions: when the ventilatory cycle and the locomotor cycle were coupled and when they were uncoupled. Patterns of muscle-activity entrainment with locomotor and ventilatory events revealed (i) that the internal and external abdominal oblique muscles performed primarily locomotor functions during running yet their activity was entrained to expiration when the dogs were standing, (ii) that the internal and external intercostal, external oblique thoracic and transversus abdominis muscles performed both locomotor and respiratory functions simultaneously, (iii) that the parasternal internal intercostal muscle performed a primarily respiratory function (inspiration) and (iv) that the deep pectoralis and longissimus dorsi muscles performed only locomotor functions and were not active while the dogs were standing still. We conclude that the dual function of many hypaxial muscles may produce functional conflicts during running. The redundancy and complexity of the respiratory musculature as well as the particular pattern of respiratory-locomotor coupling in quadrupedal mammals may circumvent these conflicts or minimize their impact on respiration.