Response of individual abdominal muscles to nasal CPAP in supine and standing positions in humans

The respiratory function of individual abdominal muscles in humans is not well known. Some workers have reported that abdominal muscles act as a single unit during breathing, whereas others have reported differential activation. In the present study, fine wire electrodes were inserted into the rectus abdominis (RA), external oblique (EO), internal oblique (IO), and transversus abdominis (TA) muscles in six healthy awake subjects under direct vision provided by high-resolution ultrasonography. Electromyographic (EMG) signals were then recorded in the supine and standing positions during resting ventilation without nasal continuous positive airway pressure (nCPAP), then on nCPAP at 5, 10, 15, and 20 cmH2O. Peak values of integrated EMG during expiration were measured. For each abdominal muscle, the number of subjects showing expiratory EMG activity increased with increasing nCPAP or with a change in posture from supine to standing or both. The number of subjects showing expiratory abdominal muscle activity under any condition followed the order: RA < or = EO < or = IO < or = TA. Peak expiratory EMG activity increased significantly with increasing nCPAP in the standing position for EO (p < 0.05), and in both positions for IO and TA (both p < 0.01). These results suggest that: 1) both the standing position and nCPAP imposed an expiratory load, 2) TA and IO are most sensitive to expiratory loading, and are the primary expiratory muscles, and 3) RA is least sensitive to expiratory loading, and is a secondary expiratory muscle. We conclude that individual abdominal muscles respond differently to expiratory loading.