Function of the intercostal muscles in trotting dogs: ventilation or locomotion?

Although the intercostal muscles play an important role in lung ventilation, observations from fishes and ectothermic tetrapods suggest that their primary function may be locomotion. To provide a broader understanding of the role these muscles play in locomotion, I measured ventilatory airflow at the mouth and activity of the fourth and ninth intercostal muscles in four dogs trotting on a treadmill. During rest and thermoregulatory panting, activity of the intercostal muscles was associated with inspiratory and expiratory airflow. However, during trotting, activity of the interosseous portions of the intercostal muscles was correlated with locomotion. When ventilation and stride cycles were not synchronized, activity of the interosseous intercostal muscles stayed locked to the locomotor events and drifted in time relative to ventilation. In contrast, activity of the parasternal portion of the internal intercostal muscles was always associated with inspiratory airflow. These observations suggest that, in dogs, locomotion is the dominant function of the interosseous portions of the intercostal muscles. However, the parasternal intercostal muscles are primarily inspiratory in function.