Contractile properties of intercostal muscles and their functional significance

To have some insight into the functional coupling between the parasternal intercostals (PS) and the diaphragm (DPM), we have examined the isometric contractile properties of bundles from canine PS and DPM muscles. Bundles of external (EXT) and internal (INT) interosseous intercostals were studied for comparison. In addition we have related sonometrically measured length of the intercostals in vivo at supine functional residual capacity (FRC) to in vitro optimal force-producing length (L(o)). We found that 1) intercostal twitch speed is significantly faster than DPM, thus displacing their relative force-frequency curve to the right of that of the DPM; 2) the ascending limb of the active length-tension curve of all intercostals lies below the DPM curve; i.e., at 85% L(o), PS force is 46% of maximal force (P(o)), whereas DPM force is still 87% P(o); 3) for any given length change beyond L(o), all intercostals generate greater passive tension than the DPM; 4) P(o) is greater for the intercoastals than the DPM; and 5) at supine FRC, both EXT and INT in dogs are nearly operating at L(o), whereas the PS are operating at a length greater than L(o). We conclude that 1) PS produce less force than DPM during breathing efforts involving low- (10-20 Hz) stimulation frequencies, but they generate more force than DPM when high- (> 50 Hz) stimulation frequencies are required; and 2) the pressure-generating ability of the PS is better preserved than that of the DPM with increases in lung volume. These differences between the parasternals and diaphragm thus would enable the integrated inspiratory musculature to generate substantial force over a broader range of stimulation frequencies and lung volumes than any one muscle group alone. This may have important implications to the pattern of inspiratory muscle use in conditions like exercise and acute hyperinflation.