Effects of changes in inspiratory muscle strength on the sensation of respiratory force.

The sensation of respiratory muscle force was compared in seven normal subjects before and after inspiratory muscle strength training. Subjects performed 20 sustained maximal inspiratory maneuvers daily for 6-18 wk. Maximal inspiratory pressures (MIP) increased from 124 +/- 10 to 187 +/- 9 (SE) cmH2O (P less than 0.005). Exponents of the power function relationships between mouth pressure (Pm) and the intensity of the sensation of force, corrected for inspiratory duration, during magnitude scaling of resistive and elastic ventilatory loads were the same before and after training (P greater than 0.05). However, absolute sensation intensity (S) during resistive and elastic loading was reduced significantly after strength training but returned toward baseline levels greater than or equal to 8 wk after the cessation of training when the MIP had fallen to 150 +/- 5 cmH2O. The absolute S at a given Pm during ventilatory loading changed inversely with changes in MIP (P less than 0.001). Furthermore the relationship between absolute S and Pm expressed as a proportion of the MIP (Pm/MIP) was constant over testing periods. These results suggest that the sensation of respiratory muscle force reflects the proportion of the maximum force utilized in breathing and may be based on the level of respiratory motor command signals.