Exercise-Induced Diaphragmatic Fatigue is Determined by the Work of Breathing in Healthy Men

It has been purported that diaphragmatic fatigue (DF) may occur relatively early during high-intensity exercise. However, studies examining the temporal characteristics of exercise-induced DF are limited by incongruent methodology. PURPOSE: To determine the time-course of exercise-induced DF during high-intensity exercise. METHODS: Eight healthy adult males (25 ± 5 yrs, 182 ± 8 cm, 74.5 ± 5.8 kg) performed a maximal incremental exercise test on a cycle ergometer on Day 1. A constant load time-to-exhaustion (TTE) exercise test was conducted on Day 2 at 60% delta between the calculated gas exchange threshold and peak work rate (85-90% of maximal oxygen consumption). On Days 3 and 4, constant load exercise tests were performed at the same intensity to either 50 or 75% TTE in random order. Assessment of DF was made by measurements of transdiaphragmatic twitch pressure (Pdi,tw) using cervical magnetic stimulation. RESULTS: DF (≥ 20% decrease in Pdi,tw) was present in 2/8 subjects after 50% TTE, 6/8 subjects after 75% TTE and all subjects after 100% TTE. The magnitude of fatigue at 100% TTE (35.0 ± 12.1%) was significantly greater than 75 (23.6 ± 6.4%) and 50% TTE (15.5 ± 5.7%) (both P < 0.01), and 75 to 50% TTE (P < 0.01). Ventilation, the mechanical work of breathing (WOB), and pressure-time products responded similarly between trials (P > 0.05). The accumulated WOB was significantly greater after 100 (3,420 ± 1,093 J) compared to 75 (1,792 ± 611 J) and 50% (877 ± 427 J) TTE (both P < 0.01), and after 75 compared to 50% TTE (P < 0.01). A significant relationship was found between the magnitude of DF and cumulative diaphragmatic force output (r = 0.785; P < 0.001). CONCLUSION: Our data indicate that exercise-induced DF is proportional to the cumulative WOB; thus, the ability of the diaphragm to generate pressure progressively declines throughout exercise. Supported by NSERC and FAPESP (grants no. 2016/08999-5 and no. 2014/10145-9).