Exercise-induced Expiratory Muscle Fatigue in Healthy Humans

Abstract

The diaphragm fatigues in response to sustained whole-body exercise, as shown using phrenic nerve stimulation. The expiratory muscles have been shown to fatigue in response to voluntary isocapnic hyperpnea using direct nerve stimulation techniques. Whether whole-body exercise elicits expiratory muscle fatigue, as assessed using nerve stimulation, is unknown. PURPOSE: To investigate whether the expiratory muscles fatigue in response to high-intensity, whole-body exercise. METHODS: Eleven male subjects (mean ± S.E.M. peak oxygen uptake [V.O2peak] = 50.0 ± 1.9 ml kg−1 min−1, range 44.6 – 62.6) cycled at >90% V.O2peakto exhaustion (14.2 ± 4.2 min). Expiratory muscle function was assessed before and up to 30 min after exercise by magnetically stimulating the nerve roots supplying the expiratory muscles at 1 through 25 Hz and measuring the change in gastric pressure (Pga). Oneway repeated measures ANOVA was used to detect changes in expiratory muscle function after exercise. RESULTS: Immediately after exercise there was a decrease in Pga at all stimulation frequencies (mean −25 ± 4%; P <0.001) that persisted up to at least 30 min post-exercise (−12 ± 4%; P = 0.011). Concurrent declines in the Pga response to maximal voluntary expiratory efforts also occurred after exercise (158 ± 13 before vs. 145 ± 10 cmH2O after exercise, P = 0.005). The reductions in stimulated Pga were unlikely due to changes in membrane excitability because amplitude, duration and area of the rectus abdominis M-wave were unaffected. The reductions in voluntary Pga were not mediated centrally because voluntary activation, assessed using twitch interpolation, did not change (67 ± 6 before vs. 64 ± 2% after exercise, P = 0.20) and electromyographic activity of the rectus abdominis increased during the volitional maneuvers. CONCLUSIONS: Expiratory muscle fatigue is present after sustained, high-intensity exercise in normal subjects, and this fatigue is primarily due to peripheral mechanisms.