Mechanics of parasternals and triangularis sterni in upright vs. supine dogs.

The parasternal intercostals in supine dogs are activated and shorten during inspiration, whereas the triangularis terni is activated and shortens during expiration (J. Appl. Physiol. 61: 539-544, 1986). How the two muscles respond to posture, however, is not known. Thirteen vagotomized, phrenicotomized, spontaneously breathing animals were thus studied during multiple postural changes from supine to 80 degrees head up and 20 degrees head down. Head-up tilting elicited a gradual increase in the electrical activation of both the triangularis sterni and the parasternals. Recruitment of the triangularis sterni promoted an increase in the amount of expiratory muscle shortening, but recruitment of the parasternals was invariably associated with a considerable reduction in the amount of inspiratory muscle shortening. This reduction was abolished after sectioning of the abdominal wall. We conclude that 1) the contribution of the canine triangularis sterni to rib movement increases with the assumption of the upright posture, whereas the contribution of the parasternals decreases and 2) this decrease results primarily from the load imposed on the rib cage by gravitational forces. Thus assuming the upright posture adversely affects the rib cage inspiratory muscles as well as the diaphragm.