Characteristics of diaphragmatic fatigue during exhaustive exercise until task failure.

The diaphragm was postulated to fatigue relatively early during exhaustive whole body exercise without further loss in contractility as exercise proceeds towards task failure. Diaphragmatic contractility was investigated prior/during/after exhaustive whole body exercise until task failure by using lung volume corrected twitch transdiaphragmatic pressure (TwPdi(c)) during magnetic phrenic nerve stimulation (every 45s). Eleven cyclists exercised to exhaustion (workloads ≥85% maximal oxygen uptake; 20.7±9.8min). Individual post hoc calculation of TwPdi(c) was conducted (diaphragmatic contractility versus lung volume). Diaphragmatic fatigue (i.e. TwPdi reduction baseline/recovery ≥10%) occurred in 9/11 subjects (82% "fatiguers"; baseline/recovery TwPdi(c) -16±13%, p<0.01). Fatiguers TwPdi(c) was: baseline: 2.99±0.40kPa, exercise-onset: 2.98±0.41kPa, initial third: 2.80±0.67kPa, second third: 2.54±0.55kPa, final third-task failure: 2.51±0.44kPa, recovery: 2.50±0.52kPa. Diaphragmatic contractility and lung volume (rest) were strongly related (r(2)=0.98, mean TwPdi(c) gradient 0.78kPa/l). To conclude, diaphragmatic contractility (lung volume corrected) decreases relatively early (initial two thirds) during exhaustive exercise and remains preserved towards task failure. This confirms previous assumptions postulating that respiratory performance is sustained without further fatigue of the primary inspiratory muscle.