DIFFERENTIAL RESPIRATORY ACTIVITY OF 4 ABDOMINAL MUSCLES IN HUMANS

Together the abdominal muscles contribute significantly to ventilation under some conditions, but there is little information regarding indi vidual recruitment and timing of activation of the four abdominal musc les in humans. Fine-wire electrodes were inserted under direct vision guided by high-resolution ultrasound into the rectus abdominis (Rectus ), external oblique (Extern), internal oblique (Intern), and transvers us abdominis (Transv) in nine awake healthy subjects. Airflow, end-tid al CO2, and moving-average EMG signals were recorded during 1) supine resting and CO2-stimulated ventilation and 2) resting ventilation in t he standing position. During resting supine breathing, Transv showed s ignificant phasic EMG activity during expiration. As posture changed f rom 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 activat ed first, followed by Intern, then Extern, and finally Rectus. With mo derate CO2 stimulation, Transv and Intern were more active than was Ex tern and Rectus remained least active. EMG activities in the expirator y muscles after cessation of expiratory flow (postexpiratory expirator y activity) and in expiratory muscle activity preceding expiratory flo w were observed consistently during supine stimulated ventilation and standing resting ventilation. These activities before and after expira tory airflow were prominent with stimulated ventilation during a subst antial portion of inspiration, suggesting dual control of inspiratory pump action by both inspiratory and expiratory muscles, which provided 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 active, Intern and Extern are intermediate, and Rec tus is the least active expiratory muscle; and 4) during stimulated ve ntilation, inspiratory and expiratory muscles contribute dually to ins piratory pump action.