The intercostal muscles expand the rib cage and play an essential role in breathing. The rib cage is a complex mechanism, and the actions and coordination of the actions of the intercostal muscles are poorly understood. In the proposed research, modeling, theory, and experiment will be combined to determine the respiratory action of the intercostal muscles. Mechanical advantage is defined as the change in airway pressure per unit active muscle force. Maxwell's reciprocity theorem is used to show that mechanical advantage can be determined by measuring fractional muscle shortening during expansion of the relaxed chest wall. In previous work, the investigators have demonstrated the validity of this method and measured the distribution of mechanical advantage and pattern of activation of the internal intercostal of the parasternal region of the dog. This method will now be used to determine the respiratory action of the scalenes, sternomastoids, external intercostals, and interosseous internal intercostals of the dog. The following hypotheses will be tested: 1) the scalenes and sternomastoids in the dog have small mechanical advantage; 2) the mechanical advantages of the external intercostals and the interosseous internal intercostals vary in both the dorsoventral and rostrocaudal directions, and the distribution of activity in these muscles during breathing is matched with the distribution of mechanical advantage; and 3) the parasternal and external intercostals drive the rib cage along different trajectories, and a particular coordination of the activation of these two groups is required in order to drive the rib cage along its relaxation trajectory. Mechanical advantage will be determined by measuring muscle shortening during expansion of the relaxed chest wall. The distribution of muscle activation during spontaneous breathing will be measured as a fraction of maximum muscle activation. Finally, the method will be applied to humans. Data on muscle orientation obtained from cadavers and data on rib locations obtained from CT scans of the thorax at two lung volumes will be used to calculate muscle shortening and thereby obtain the mechanical advantages of the human intercostals.
