Mechanical advantage of sternomastoid and scalene muscles in dogs.

Theoretical studies have led to the prediction that the maximal effect of a given respiratory muscle on airway opening pressure (Pao) is the product of muscle mass, the maximal active muscle tension per unit cross-sectional area, and the fractional change in muscle length per unit volume increase of the relaxed chest wall. It has previously been shown that the parasternal intercostals behave in agreement with this prediction (A. De Troyer, A. Legrand, and T. A. Wilson. J. Physiol. (Lond) 495: 239-246, 1996; A. Legrand, T. A. Wilson, and A. De Troyer. J. Appl. Physiol. 80: 2097-2101, 1996). In the present study, we have tested the prediction further by measuring the response to passive inflation and the pressure-generating ability of the sternomastoid and scalene muscles in eight anesthetized dogs. With 1-liter passive inflation, the sternomastoids and scalenes shortened by 2.03 +/- 0.17 and 5.98 +/- 0.43%, respectively, of their relaxation length (P < 0.001). During maximal stimulation, the two muscles caused similar falls in Pao. However, the sternomastoids had greater mass such that the change in Pao (delta Pao) per unit muscle mass was -0.19 +/- 0.02 cmH2O/g for the scalenes and only -0.07 +/- 0.01 cmH2O/g for the sternomastoids (P < 0.001). After extension of the neck, there was a reduction in both the muscle shortening during passive inflation and the fall in Pao during stimulation. The delta Pao per unit muscle mass was thus closely related to the change in length; the slope of the relationship was 3.1. These observations further support the concept that the fractional changes in length of the respiratory muscles during passive inflation can be used to predict their pressure-generating ability.