Mechanisms for diaphragmatic fatigue following high-intensity leg exercise.

Diaphragmatic fatigue can occur following high-intensity leg exercise to exhaustion. Exercise-induced diaphragmatic fatigue may be due to changes in the milieu to which the diaphragm is exposed (i.e., acidosis, etc.) and/or to increases in diaphragmatic activity during exercise. The purpose of this study was to determine whether changes in milieu are responsible for exercise-induced diaphragmatic fatigue. Fourteen subjects exercised at 70 to 75% of their maximal work capacity until volitional exhaustion. Transdiaphragmatic twitch pressure (Pdi) was measured during bilateral transcutaneous supramaximal phrenic nerve stimulation and adductor pollicis twitch force was measured during transcutaneous supramaximal ulnar nerve stimulation before and 10 to 15, 30, and 60 min after exercise. On a separate day, arterialized venous blood samples were obtained for lactate measurements at baseline and 5, 15, 30, and 60 min postexercise. Nine subjects demonstrated a > or = 10% decrease in twitch Pdi postexercise, which was considered indicative of diaphragmatic fatigue (fatigue group). In these subjects, twitch Pdi fell from 24.3 +/- 1.6 at baseline to 19.7 +/- 1.6 cm H2O at 10 min postexercise (p < 0.0001, ANOVA). Adductor pollicis twitch force was not significantly different from baseline at any time postexercise in either the fatiguers or nonfatiguers. In both groups, a significant lactic acidosis developed during exercise and the magnitude of this acidosis was similar for the two groups. The adductor pollicis muscle (a nonexercising muscle during cycle exercise) is exposed to the same milieu as the diaphragm. Because adductor pollicis twitch force was unchanged postexercise while twitch Pdi fell, changes in milieu cannot be solely responsible for exercise-induced diaphragmatic fatigue.