Temporal characteristics of exercise-induced diaphragmatic fatigue

Abstract

There is evidence suggesting diaphragmatic fatigue (DF) occurs relatively early during high-intensity exercise; however, studies investigating the temporal characteristics of exercise-induced DF are limited by incongruent methodology. Eight healthy adult males (25 ± 5 yr) 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. Two additional constant-load exercise tests were performed at the same intensity on days 3 and 4 in a random order to either 50 or 75% TTE. DF was assessed on days 2, 3, and 4 by measuring transdiaphragmatic twitch pressure (P_di,tw) in response to cervical magnetic stimulation. DF was present after 75 and 100% TTE (≥20% decrease in P_di,tw). The magnitude of fatigue was 15.5 ± 5.7%, 23.6 ± 6.4%, and 35.0 ± 12.1% at 50, 75, and 100% TTE, respectively. Significant differences were found between 100 to 75 and 50% TTE (both P < 0.01), and 75 to 50% TTE ( P < 0.01). There was a significant relationship between the magnitude of fatigue and cumulative diaphragm force output ( r = 0.785; P < 0.001). Ventilation, the mechanical work of breathing (WOB), and pressure-time products were not different between trials ( P > 0.05). Our data indicate that exercise-induced DF presents a relatively late onset and is proportional to the cumulative WOB; thus the ability of the diaphragm to generate pressure progressively declines throughout exercise. NEW & NOTEWORTHY The notion that diaphragmatic fatigue (DF) occurs relatively early during exercise is equivocal. Our results indicate that DF occurs during high-intensity endurance exercise in healthy men and its magnitude is strongly related to the amount of pressure and work generated by respiratory muscles. Thus we conclude that the work of breathing is the major determinant of exercise-induced DF.

Keywords: diaphragm fatigue; dyspnea; metaboreflex; work of breathing.

Fig. 1.

Relative transdiaphragmatic twitch pressure (P_di,tw) responses to increasing cervical magnetic stimulation intensity. Gray lines represent individual values and the thick black line represents group mean ± SD *P < 0.01 vs. 100% stimulation intensity.

Fig. 2.

One subject representative raw data traces from within-day [100% time-to-exhaustion (TTE) trial] (A), and between-days (50, 75, and 100% TTE trials) (B) of transdiaphragmatic twitch pressures (P_di,tw) and right hemidiaphragm (RHD) electromyography (EMG). Note that on A, there are 4 EMG traces superimposed for each time point; however, on B, there is only one representative EMG trace in light gray from baseline.

Fig. 3.

Transdiaphragmatic twitch pressure (P_di,tw) responses during recovery after 50, 75, and 100% time-to-exhaustion (TTE) trials of exercise. Values are means ± SD. *P < 0.01 vs. 50% TTE trial; †P < 0.01 vs. 75% TTE trial.

Fig. 4.

A: individual relationship between total diaphragmatic force output and the decrease of transdiaphragmatic twitch pressure (P_di,tw). Each participant represents one symbol linked three times, which corresponds to 50, 75, and 100% of time-to-exhaustion (TTE). B: correlation of this relationship from 50 (open circles), 75 (half-open circles), and 100% (closed circles) TTE trials. The dashed horizontal lines in both panels represents the criterion used to consider diaphragmatic fatigue present (≥20% reduction in P_di,tw).

Fig. 5.

A: accumulated mechanical work of breathing (WOB) after 50, 75, and 100% time-to-exhaustion (TTE) trials. Individual values are linked through gray lines along trials. Group means ± SD are also presented for each trial. B: cumulative WOB throughout exercise trials. Values are means ± SD. C: relationship between WOB and minute ventilation (V̇e) during 50, 75, and 100% TTE trials (mean values of WOB and V̇e were calculated in 10% increments throughout exercise). *P < 0.05 vs. 50% TTE trial; †P < 0.05 vs. 75% TTE trial.

Fig. 6.

Ratio between diaphragmatic (PTP_di) and esophageal (PTP_es) pressure-time products during 100% time-to-exhaustion (TTE) (A), 75% TTE (B), and 50% TTE exercise trials (C). Values are means ± SD. D: all-trials correlations between PTP_di/PTP_es and TTE.