The sensation of respiratory muscle force

The sensation of respiratory muscle force was examined in 8 normal subjects using the direct scaling method of magnitude production. At the end of a normal expiration, at functional residual capacity (FRC), subjects generated an inspiratory effort against a closed airway to produce a force that correspond to specific numerical values for sensation magnitude. Thereafter, at lung volumes of 2 L above FRC and 1 L below FRC, subjects attempted to reproduce the same force as that previously generated at FRC. Diaphragm electromyographic activity, recorded with an esophageal electrode, was used as a measure of central respiratory motor output. The difference between the mouth pressure during relaxation against a closed shutter and the peak airway pressure during the static inspiratory maneuver (Pmus) was employed as an index of muscle tension. The exponent for the magnitude production of force during maneuvers at FRC, i.e., the slope of the line relating log airway pressure and log sensation magnitude was 1.60 +/- SE 0.15. The exponent for force scaling was 1.47 +/- SE 0.17 after correcting for differences in inspiratory duration during maneuvers of different force. During inspiratory maneuvers at FRC plus 2 L, diaphragm EMG was 441 +/- SE 70%, but Pmus was 111 +/- SE 11% of the respective values during subjectively equal efforts at FRC. During inspiratory maneuvers at FRC minus 1 L, diaphragm EMG was 54 +/- SE 5%, but Pmus was 94 +/- SE 4% of the respective values during efforts at FRC perceived to be of equal force. These results suggest that the sensation of respiratory muscle force depends primarily on signals related to the tension generated by the contracting muscles rather than on the sensation of innervation associated with the central nervous system motor command.