Human diaphragmatic endurance during different maximal respiratory efforts.

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Abstract
1. This study assessed human diaphragmatic endurance under two conditions: during maximal inspiratory efforts (with minimal elevation of abdominal pressure) and during maximal expulsive efforts (with minimal elevation of pleural pressure). Performance was compared with that of the flexors of the elbow. 2. In series of eighteen sustained maximal efforts begun near functional residual capacity the decline in trans-diaphragmatic pressure was significantly greater during the expulsive than the inspiratory efforts (P < 0.01). Diaphragmatic endurance was greater than that of the flexors of the elbow. 3. Electromyograms recorded during maximal and submaximal expulsive contractions indicated that the relatively rapid decline in abdominal pressure during the series of expulsive contractions reflected fatigue of the diaphragm rather than the abdominal muscles. Supramaximal phrenic nerve stimulation was used to check that complete activation of the phrenic motoneurone pool could be achieved during series of maximal expulsive efforts. 4. It is concluded that the reduced endurance capacity of the diaphragm during expulsive efforts did not reflect peripheral failure of the abdominal muscles or an inability to activate the diaphragm fully during those maximal efforts. 5. The deterioration in diaphragmatic performance when abdominal pressure is elevated may be due to an impairment of muscle perfusion.