Abstract
Introduction: The purpose of this study was to characterize the rectus abdominis (RA), external oblique (EO), and internal oblique (IO)muscle activity level in healthy adults under expiratory resistance to expose the limitations of conventional expiratory muscle training.
Methods: A total of 37 healthy adult subjects were randomly assigned to one of three groups: 30% of maximal expiratory intraoral pressure (30% PE max) group, 20% PE max group, and 10% PE max group. The RA, EO, and IO muscle activity level were measured during maximum voluntary contraction and during performed forced expiration under the assigned condition afrer measurement of maximal expiratory intraoral pressure (100% PE max) and muscle activity level during 100% PE max for all subjects.
Results: At 100% PE max, the EO (P < .01) and IO muscle groups (P < .01) showed significantly elevated activities compared to the RA muscle. Furthermore, at 20% and 30% PE max intensities, the EO muscle (20% PE max: P < .05; 30% PE max: P < .01) and the IO muscle (20% PE max: P < .05; 30% PE max: P < .01) had significantly elevated activities compared to the RA muscle. There was no significant activity difference between 10% PE max and 20% PE max, but the activity during 30% PE max was significantly greater than that of 20% PE max (EO muscle: P < .05; IO muscle: P < .01).
Conclusion: The two abdominal oblique muscles are the most active during forced expiration. Moreover, 30% PE max is the minimum intensity required to achieve significant muscle activity when expiratory resistance is applied, but the muscle activity obtained at 30% PE max is very small.
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