The synchronization of ventilation and locomotion in horses
It has been observed that some running and hopping animals synchronize their ventilation and locomotion. Horses, dogs and hares all take exactly one breath per stride when they canter and gallop. Hopping wallabies show a similar relationship.
While galloping they seem to increase their speed and respiration by lengthening their stride and increasing their tidal volume. This suggest that a resonant mechanism may be operating.
This investigation considers three possible driving mechanisms: (i) flexion of the back, (ii) loading of the thorax by the forelimbs, and (iii) the visceral piston.
Bending of the back (i) occurs largely at the lumbosacral joint. Flexion of this region and the resulting forward sweep of the pelvis could displace the viscera forwards. The displaced volume of viscera would tend to push up against the diaphragm and, in turn, displace a volume of air from the lungs with each flexion of the back. Loading of the thorax by the forelimbs (ii) may compress it; if this occurs at the appropriate point in the respiratory cycle air would be forced out of the lungs. The viscera may be regarded as a mass suspended by elastic structures within the body wall rather like a piston within a cylinder (iii).
Bony landmarks on the ilium, sacrum and abdomen where used to calculate the back movement. The autors of the study found that flexion of the back probably contributes considerably towards the driving of breathing.
> From: Young et al., Journal of Experimental Biology 166 (1992) 19-31. All rights reserved to Company of Biologists. Click here for the online summary.