Changes in abdominal muscle length during breathing in supine dogs

To assess the mechanical function of the abdominal muscles during eupnea in the dog, we have measured the electrical activity and the respiratory changes in length of the rectus abdominis, external oblique, and transversus abdominis muscles in eight supine, lightly anesthetized, spontaneously breathing animals. Seven animals had phasic expiratory electromyographic (EMG) activity in the transversus and showed expiratory shortening of the muscle below its in situ relaxation length (Lr). In contrast, only three animals had expiratory EMG activity and expiratory shortening of the external oblique, and no animal had expiratory EMG activity in the rectus. Seven animals, however, showed shortening of the rectus muscle during expiration. The amount of transversus expiratory shortening in the eight animals averaged (mean +/- SE) - 7.61 +/- 1.72% Lr and was significantly larger (P less than 0.005) than the amount of external oblique (-0.11 +/- 0.10% Lr) or rectus (-0.90 +/- 0.39% Lr) expiratory shortening. Hyperoxic hypercapnia amplified these differences. These data thus indicate that in supine anesthetized dogs (1) the transversus is, in real mechanical terms, the primary abdominal muscle of expiration; and (2) the abdominal compartment of the chest wall during eupnea moves both below and above its neutral position, and not exclusively above it.