The maximal voluntary pressure generated by the diaphragm (transdiaphragmatic pressure, Pdi) is about 50% greater during maximal expulsive efforts than during maximal inspiratory efforts against a closed airway. However, these pressures cannot be increased by interpolated phrenic stimuli in trained subjects. This suggests that variable neural drive is not responsible for the difference in voluntary pressure. To investigate whether dynamic changes in diaphragm length during inspiratory and expulsive efforts could account for this difference, we used digital sequential radiography at 6 frames per second. 2. During the development of peak Pdi in inspiratory efforts, total diaphragm length decreased by about 20% in the antero-posterior and lateral projections. During maximal expulsive efforts (with glottis open), the diaphragm shortened slightly in the early stage of pressure development but then lengthened due to contraction of abdominal muscles before peak pressure was achieved. 3. Given that force increases when a contracting muscle is lengthened (expulsive effort) and decreases during shortening (inspiratory effort), this study provides a definitive explanation for the difference in maximal voluntary pressure between pure inspiratory and expulsive efforts.
