Contractile characteristics and operating lengths of canine neck inspiratory muscles.

The neck inspiratory muscles are recruited to support breathing under numerous conditions. To gain insight into their synergistic actions we examined the isometric contractile properties of bundles from canine scalene and sternomastoid muscles. In addition, we also related the length of the neck muscles, measured sonomicrometrically in vivo at different lung volumes and body positions, to their optimal force-producing length (Lo) determined in vitro. We found that the speed of the sternomastoid is somewhat faster than that of the scalene owing to a shorter relaxation rate; the sternomastoid generates higher forces at submaximal stimulation frequencies than the scalene; the maximal tetanic force corrected for cross-sectional area is the same for both neck muscles; the neck muscles are significantly faster than the canine costal diaphragm; at supine functional residual capacity (FRC), the scalene is operating at a length corresponding to 85% Lo, whereas the sternomastoid is significantly shorter at 75% Lo; increasing lung volume shortens both muscles slightly, the length at supine total lung capacity being approximately 5% shorter than at FRC; and in the upright posture, both neck muscles lengthen toward their Lo, with the sternomastoid lengthening more than the scalene. We conclude that the scalene is a more effective force generator than the sternomastoid with the animal lying supine; the neck muscles appear to maintain their force-generating potential regardless of the lung volume; and the force-generating potential of the neck muscles is greatly enhanced with the animal in the upright vs. the supine position. This may contribute to the augmented rib cage motion characteristic of breathing in the upright posture.