Parasternal and external intercostal muscle shortening during eupneic breathing

The interosseous external intercostal (EI) muscles of the upper rib cage are electrically active during inspiration, but the mechanical consequence of their activation is unclear. In 16 anesthetized dogs, we simultaneously measured EI (3rd and 4th interspaces) and parasternal intercostal (PA) (3rd interspace) electromyogram and length. Muscle length was measured by sonomicrometry and expressed as a percentage of resting length (%L(R)). During resting breathing, each muscle was electrically active and shortened to a similar extent. Sequential EI muscle denervation (3rd and 4th interspaces) followed by PA denervation (3rd interspace) demonstrated significant reductions in the degree of inspiratory shortening for each muscle. Mean EI muscle shortening of the third and fourth interspaces decreased from -3.4 ± 0.5 and -3.0 ± 0.4%L(R) (SE) under control conditions to -0.2 ± 0.2 and -0.8 ± 0.3%L(R), respectively, after selective denervation of each of these muscles (P < 0.001 for each). After selective denervation of the PA muscle, its shortening decreased from -3.5 ± 0.3 to +0.6%L(R) (SE) (P < 0.001). PA muscle denervation also caused the EI muscle in the third interspace to change from inspiratory shortening of -0.2% to inspiratory lengthening of +0.2% ± 0.2 (P < 0.05). We conclude that during eupneic breathing 1) the EI muscles of the upper rib cage, like the PA muscles, are inspiratory agonists and actively contribute to rib cage expansion and 2) PA muscle contraction contributes to EI muscle shortening.