The Effect Of Exercise Intensity On The Development Of Diaphragm And Expiratory Abdominal Muscle Fatigue

Abstract

Exhaustive high-intensity exercise (85-95% of V̇O2peak) elicits respiratory muscle fatigue, likely due to a high work of breathing (WoB) combined with a competition for available cardiac output (Q̇). However, the WoB associated with submaximal heavy-intensity exercise (≤75% V̇O2peak) makes insufficient demands for V̇O2 and Q̇ to engender a substantial competition for available blood flow. Whether the respiratory muscles fatigue in response to exhaustive heavy-intensity exercise remains unclear. PURPOSE: To investigate the effect of exercise intensity on the presence and severity of exercise-induced diaphragm and expiratory muscle fatigue in healthy humans. METHODS: Ten healthy adults (25 ± 3 y, 3 females) performed a ‘ramp sprint’ test to determine critical power (CP), peak ramp power (Ppeak) and V̇O2peak (54 ± 9 ml/kg/min). The subjects then performed two constant-power cycling tests to exhaustion: 1) 5% < CP (173 ± 50 W, heavy intensity); 2) ~25% of the difference between CP and Ppeak (215 ± 53 W, severe intensity). Diaphragm and expiratory muscle fatigue were quantified as the pre- to post-exercise reduction in the transdiaphragmatic (Pditw) and gastric (Pgatw) twitch pressure response to magnetic stimulation of the cervical and thoracic nerves, respectively. RESULTS: Exercise time was longer for heavy vs. severe exercise (36 ± 6 vs. 10 ± 3 min, P < 0.05). Final min V̇O2 was lower during heavy (3.12 ± 0.74 L/min; 82 ± 5% of V̇O2peak) vs. severe exercise (3.60 ± 0.83 L/min; 95 ± 4% of V̇O2peak) (P < 0.05). Both heavy and severe exercise elicited a significant reduction in Pditw (−13 ± 11% and -19 ± 13%) and Pgatw (−23 ± 20% vs. -24 ± 17%) (both P < 0.05); however, the magnitude of exercise-induced respiratory muscle fatigue was not different between trials (P > 0.05). The cumulative diaphragm and gastric pressure-time products were greater for heavy vs. severe exercise (PTPdi: 16790 ± 6727 vs. 5945 ± 1956 cmH2O/s; PTPga: 7818 ± 3368 vs. 2595 ± 1233 cmH2O/s; both P < 0.05). CONCLUSION: The diaphragm and expiratory muscles fatigue in response to exhaustive heavy- and severe-intensity exercise in healthy humans. The magnitude of exercise-induced respiratory muscle fatigue between trials was not different despite a substantially greater cumulative PTPdi and PTPga for heavy- vs. severe-intensity exercise.