Differential respiratory activity of four abdominal muscles in humans. Abe, Tadashi, Noriyuki Kusuhara, Naotaka Yoshimura, Tomoyuki Tomita, and Paul A. Easton. Kitasato University, Kanagawa, Japan and Division of Critical Care, University of Calgary, Calgary, Canada
APStracts 3:0001A, 1996.
Together the abdominal muscles contribute significantly to ventilation under some conditions, but there is little information regarding individual recruitment and timing of activation of the four abdominal muscles in humans. Fine wire electrodes were inserted under direct vision guided by high resolution ultrasound into rectus abdominis (RECTUS), external oblique (EXTERN), internal oblique (INTERN) and transversus abdominis (TRANSV) in nine awake healthy subjects. Airflow, end-tidal CO2, and moving average EMG signals were recorded during: 1) supine resting and CO2 stimulated ventilation, and 2) resting ventilation in the standing position. During resting supine breathing, TRANSV showed significant phasic EMG activity during expiration. As posture changed from supine to standing, phasic activity during resting ventilation was greatest in TRANSV, with lesser activity in INTERN and EXTERN, while RECTUS remained inactive. As CO2 began to increase, TRANSV was activated first, followed by INTERN then EXTERN, and finally RECTUS. With moderate CO2 stimulation, TRANSV and INTERN were more active than EXTERN, and RECTUS remained least active. EMG activity in the expiratory muscles after cessation of expiratory flow (postexpiratory expiratory activity: PEEA) and expiratory muscle activity preceding expiratory flow, were observed consistently during supine stimulated ventilation and standing resting ventilation. These activities before and after expiratory airflow were prominent with stimulated ventilation during a substantial portion of inspiration, suggesting dual control of inspiratory pump action by both inspiratory and expiratory muscles, providing acceleration and braking actions, respectively. These results suggest that in awake humans: 1) during resting ventilation, expiration is an active process, 2) abdominal muscles are activated differentially, 3) TRANSV is the most, INTERN and EXTERN are intermediate, and RECTUS is the least, active expiratory muscle, and 4) during stimulated ventilation, inspiratory and expiratory muscles contribute dually to inspiratory pump action. 

Received 5 May 1994; accepted in final form 21 November 1995.
APS Manuscript Number A424-4.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 22 January 96