Mechanical advantage of the canine triangularis sterni

De Troyer, André, and Alexandre Legrand.Mechanical advantage of the canine triangularis sterni.J. Appl. Physiol. 84(2): 562-568, 1998.Recent studies on the canine parasternal intercostal,sternomastoid, and scalene muscles have shown that the maximal changesin airway opening pressure (Pao) obtained per unit muscle mass(Pao/m) during isolatedcontraction are closely related to the fractional changes in musclelength per unit volume increase of the relaxed chest wall. In thepresent study, we have examined the validity of this relationship for the triangularis sterni, an important expiratory muscle of the rib cagein dogs. Passive inflation above functional residual capacity (FRC)induced a virtually linear increase in muscle length, such that, with a1.0-liter inflation, the muscle lengthened by 17.9 ± 1.6 (SE) % of its FRC length. When the muscle in one interspace wasmaximally stimulated at FRC, Pao increased by 0.84 ± 0.11 cmH₂O. However, in agreement withthe length-tension characteristics of the muscle, when lung volume wasincreased by 1.0 liter before stimulation, the rise in Pao amounted to1.75 ± 0.12 cmH₂O. At thehigher volume, Pao/m thereforeaveraged + 0.53 ± 0.05 cmH₂O/g, such that the coefficientof proportionality between the change in triangularis sterni lengthduring passive inflation and Pao/m was the same as that previously obtained for the parasternalintercostal and neck inspiratory muscles. These observations,therefore, confirm that there is a unique relationship between thefractional changes in length of the respiratory muscles, bothinspiratory and expiratory, during passive inflation and theirPao/m. Consequently, the maximal effect of a particular muscle on the lung can be predicted on the basisof its change in length during passive inflation and its mass. Ageometric analysis of the rib cage also established that thelengthening of the canine triangularis sterni during passive inflationis much greater than the shortening of the parasternal intercostalsbecause, in dogs, the costal cartilages slope downward from thesternum.