Abdominal muscle length during respiratory defensive reflexes.

The purpose of the present study was to assess the mechanical behavior of an expiratory muscle during defensive reflexes. Transversus abdominis muscle length changes were measured using sonomicrometry in anesthetized dogs. The abdominal muscle lengthened during the inspiratory phase and shortened at a rapid velocity during the expiratory portion of coughs and sneezes. The mean extent of muscle shortening was not different during coughing compared to breathing (P greater than 0.20) but was approximately double in magnitude during sneezing compared to breathing (P less than 0.005). On the other hand, the peak velocity of muscle shortening was approximately 5-fold greater during coughing (P less than 0.002) and 10-fold greater during sneezing (P less than 0.05) than during breathing. During the largest coughs and sneezes in each animal, peak velocity of muscle shortening averaged 77 +/- 9 and 179 +/- 65% of end-inspiratory length per sec, respectively. Muscle end-inspiratory length during coughs and sneezes differed from values during breathing (range +/- 8%), although for the group of animals the mean changes were small (+/- 1%). Despite these changes in end-inspiratory length, the abdominal muscle continued to operate at lengths both above and below its resting length. These results suggest that during defensive reflexes, greater increases occur in the velocity than in the extent of transversus abdominis muscle shortening relative to during breathing. In addition the transversus abdominis muscle appears to play an active respiratory role during defensive reflexes.