Mechanical advantage of the human parasternal intercostal and triangularissterni muscles

1. Previous studies in dogs have demonstrated that the maximum change in ai rway pressure (Delta P-ao) produced by a particular respiratory muscle is t he product of three factors, namely the mass of the muscle, the maximal act ive muscle tension per unit cross-sectional area (similar to 3.0 kg cm(-2)) , and the fractional change in muscle length per unit volume increase of th e relaxed chest wall (i.e. the muscle's mechanical advantage). In the prese nt studies, we have used this principle to infer the Delta P-ao values gene rated by the parasternal intercostal and triangularis sterni muscles in man . 2. The mass of the muscles and the direction of the muscle fibres relative to the sternum were first assessed in six cadavers. Seven healthy individua ls were then placed in a computed tomographic scanner to determine the orie ntation of the costal cartilages relative to the sternum and their rotation during passive inflation to total lung capacity. The fractional changes in length of the muscles during inflation, their mechanical advantages, and t heir Delta P-ao values were then calculated. 3. Passive inflation induced shortening of the parasternal intercostals in all interspaces and lengthening of the triangularis sterni. The fractional shortening of the parasternal intercostals decreased gradually from 7.7% in the second interspace to 2.0% in the fifth, whereas the fractional lengthe ning of the triangularis sterni increased progressively from 5.9 to 13.8%. These rostrocaudal gradients were well accounted for by the more caudal ori entation of the cartilages of the lower ribs. 4. Since these fractional changes in length corresponded to a maximal infla tion, the inspiratory mechanical advantage of the parasternal intercostals was only 2.2-0.6% l(-1), and the expiratory mechanical advantage of the tri angularis sterni was only 1.6-3.8% l(-1). In addition, whatever the intersp ace, parasternal and triangularis muscle mass was 3-5 and 1-3 g, respective ly. As a result, the magnitude of the Delta P-ao values generated by a maxi mal contraction of the parasternal intercostals or triangularis sterni in a ll interspaces would be only 1-3 cmH(2)O. 5. These studies therefore confirm that the parasternal intercostals in man have an inspiratory action on the lung whereas the triangularis sterni has an expiratory action. However, these studies also establish the important fact that the pressure-generating ability of both muscles is substantially smaller than in the dog.