Control mechanism of the respiratory muscle during forced expiration

Control mechanism of the respiratory muscle during forced expiration was analyzed. Respiratory flow. EMG of the intercostal muscle (IC), external oblique (EO) and internal oblique (IO) abdominal muscle were analyzed in eleven healthy volunteers. The pattern of EMG activity during forced expiration could be classified according to its shape i.e., taper- or hill-shaped. The taper shape was found in 7 of 9 subjects for IC and in 8 of 11 subjects for EO. The hill shape was found in 4 of 6 subjects for 10. Twelve consecutive trials without intervals did not alter the pattern of flow-volume curve, activity of respiratory muscle and frequency component (i.e., H/L ratio). When the expiratory circuit was obstructed for a short interval during forced expiration, the rise in pleural pressure was slight but that in oral pressure was large, and the activity of respiratory muscle decreased for a short period. The latencies from the onset of expiratory circuit obstruction to the reduction in EMG activity were 61.5-100 msec. In IC, 61.5-106.5 msec in EO. and 58.0-92.3 msec in 10. These values did not differ significantly any given subject. Airway anesthesia with xylocaine did not alter latency. In conclusion, the pattern of respiratory muscle activity for forced expiration is predetermined and is specific for each muscle. Negative feedback from receptors in airway or in respiratory muscle was strongly suggested. Repetitive trials up to twenty times did not cause respiratory muscle fatigue.