Rib cage motion and muscle use in high tetraplegics.

In an attempt to assess the action of the sternocleidomastoid muscles on the human rib cage, we studied the pattern of rib cage motion in 2 patients with complete transection of the upper cervical cord. Measurements of rib cage motion were obtained with magnetometers and chest roentgenograms, and concentric needle electrodes were used to record the electromyograms (EMG) of various respiratory muscles. Spontaneous quiet breathing elicited a large amount of phasic inspiratory EMG activity not only in the sternocleidomastoids, but also in the trapezii, platysma, mylohyoid, and sternohyoid muscles. This pattern of muscle use was associated with substantial rib cage deformation. During spontaneous inspiration both patients showed a clear-cut decrease in lower rib cage transverse diameter, and they both had a cranial displacement of the sternum and an increase in upper rib cage anteroposterior diameter that was disproportionately larger than the increase in lower rib cage anteroposterior diameter. Radiographic measurements confirmed these deformations but also demonstrated that the upper rib cage transverse diameter increased rather than decreased with inspiration. These results indicate that: (1) Patients with high tetraplegia use many neck muscles in addition to the sternocleidomastoids to breathe; (2) The synchronous contraction of these muscles acts to pull the sternum cranially, expands the upper rib cage, and causes paradoxical inward displacement of the lateral walls of the lower rib cage; (3) As in dogs, the motion of the upper rib cage in humans is more tightly linked to the sternum than that of the lower rib cage.(ABSTRACT TRUNCATED AT 250 WORDS)