PATTERN OF EXPIRATORY MUSCLE RECRUITMENT DURING INCREMENTAL EXERCISE1637

Abstract

The purpose of this project was to examine the pattern of recruitment in external oblique (EO) and rectus abdominis (RA) muscles during incremental bicycle exercise. Nine human subjects (six male, three female) performed an incremental exercise bout to exhaustion. Hypoxia (11% O2) was administered briefly at each workload. Electromyographic (EMG) activity was recorded using surface electrodes, and quantified as a percentage of the activity elicited during maximal expiration from functional residual capacity(max). Peak phasic EO and RA expiratory EMG activities showed a small initial increase followed by a marked rise at 60% of maximal workload. At the end of exercise, EO EMG was 13.0±3.6% and 14.4±4.5% max during normoxia and hypoxia, respectively, while RA activity was 15.2±5.8% and 20.1±6.2% max during normoxia and hypoxia, respectively. There was a trend of greater activity in both muscles during hypoxia; however, activity was similar when corrected for differences in ventilation. This was likely due to disfacilitation of abdominal motoneurons by hypocapnia, which was prevalent in hypoxia (-3.5 mmHg). In light exercise, the EMG burst began ≈400 and≈600 msec after the beginning of expiration for EO and RA, respectively, and in peak exercise activity began 90±60 msec and 10±40 msec before expiration for EO and RA, respectively. The offset time remained constant at ≈80 msec before inspiration for both muscles. These data indicate that: EO and RA activities increase in parallel as a function of exercise intensity; hypoxia does not influence EO and RA recruitment when normalized for ventilation, possibly due to hypocapnia; EO and RA activities begin earlier in expiration as ventilation increases, and may even precede expiration at high exercise intensities, while the offset of activity does not change.