EXERCISE-INDUCED HIGH FREQUENCY DIAPHRAGM FATIGUE 528

Abstract

The purpose of this study was to determine if high frequency diaphragmatic fatigue was present after intense whole body endurance exercise. We have adapted the paired stimuli technique (Yan et al., Eur. Respir J. 6: 211-218, 1993), to determine changes in the transdiaphragmatic pressure (Pdi) response to low and high frequency supramaximal bilateral phrenic nerve stimulation(BPNS) before and after intense whole body endurance exercise. Subjects (N=6) exercised for 10.6 ±0.5 min until volitional fatigue at 93.3±2.3% of VO2max. BPNS Pdi was measured at FRC using single twitch, tetanic 10 Hz (4 stimuli in 400 ms) and paired stimuli at 10, 20, 50, 70 and 100 Hz frequencies before exercise, immediately after exercise, 30 min and 60 min post-exercise. The paired stimuli data were analyzed by subtracting the ensemble averaged Pdi twitch response from each of the paired supramaximal stimulation. The residual Pdi was the response to the second high frequency stimulus (T2). Low frequency fatigue was shown by the mean percent decrease in the supramaximal BPNS Pdi (twitch and tetanic 10 Hz) which was-20.4 ±5.1%, immediately after exercise. It did not recover until > 60 min post exercise. The paired stimuli Pdi (at 50, 70, 100 Hz) were decreased from pre-exercise control by an average of 24.2 ±2.1% immediately following exercise. Four of six subjects T2 amplitude recovered to pre-exercise levels by 30 min post-exercise. These data suggest that the diaphragm experienced high frequency fatigue following intense whole body endurance exercise. Intense whole body endurance exercise not only caused peripheral fatigue in the diaphragm but also may cause nerve transmission disruptions at the muscle membrane.