Diaphragm fatigue in self-paced running exercise of different durations

Abstract

Introduction: Diaphragm fatigue (DF) was shown to develop during high intensity constant-load exercise (CLE) above 85% of maximal oxygen consumption (VO_2max). CLE does, however, not appropriately reflect field trial conditions where exercise intensity is regulated by complex feedforward and feedback mechanisms, possibly aiming to reduce/prevent DF. Therefore, the development of DF was assessed in 11 well-trained athletes (age=31±4 yrs, VO_2max=66.7±4.6 mlmin^-1kg^-1) in 15 and 30min running time trials (TT). We hypothesized that DF would be larger in the 30TT where more time is spent above 85%VO_2max.

Methods: Before and 4min after completion of the TTs, esophageal and gastric pressures were assessed to calculate transdiaphragmatic twitch pressures (P_di,tw) during cervical magnetic stimulation.

Results: All subjects developed DF (P_di,tw reduction, ΔP_di,tw ≥10%) in the 15TT and 7/11 subjects in the 30TT. On average, ΔP_di,tw was 23.0±6.5 (15TT) and 18.5±12.1% (30TT; p=0.13). Mean exercise intensities were 89.4±3.5 (15TT) and 86.0±3.7%VO_2max (30TT; p=0.07) with similar mean durations above 85%VO_2max (11.9±2.0min; range: 7-14min; 15TT) and (16.2±9.7min; 1-28min; 30TT; p=0.19). The individual between-TT difference in ΔP_di,tw did not correlate with the difference in time spent above 85%VO_2max (R²=0.20) but correlated with the difference in the average exercise intensity above 85%VO_2max (R²=0.49; p=0.02).

Conclusion: In TT-conditions, the degree of DF is not related to the duration spent above 85%VO_2max but it is related to the exercise intensity when exercising above the 85%VO_2max-threshold.