Activation of the human diaphragm during maximal static efforts.

Voluntary activation of the human diaphragm was assessed in four subjects by interpolation of supramaximal stimuli (one to four shocks, interstimulus interval 20 ms) to one phrenic nerve during graded static inspiratory and expulsive efforts at functional residual capacity. There was an inverse relationship between the size of the voluntarily generated pressure and the size of the electrically evoked change in pressure. Each subject activated the diaphragm fully during brief (2-5 s) maximal inspiratory and expulsive efforts, as judged by the failure of supramaximal stimuli to augment the voluntarily maintained pressure. During prolonged inspiratory efforts or following fatigue of the diaphragm produced by a sustained contraction each subject was able to activate the diaphragm maximally but did not do so on all occasions.