Selective activation of parasternal muscle fibers according to breathing rate

The activity of single parasternal intercostal muscle fibers of rabbits was recorded during control breathing and during heat tachypnea at near-constant tidal volume. In order to identify with high probability the type of fibers on the basis of available knowledge, differences in firing pattern were enhanced by occluding the airways during an inspiration or adding dead space. Under both conditions the percentage of fibers with relatively high values of peak firing rate (fp) and rising rate of firing frequency (delta f/delta t) was greater during tachypnea. Fibers with relatively low values of fp and delta f/delta t during control breathing showed little or no increase of these parameters during tachypnea. Fibers with low fp and delta f/delta t should be slow, and others fast. Hence, the greater percentage of fibers with high fp and delta f/delta t during tachypnea indicates a recruitment of fast fibers. these findings support our previous hypothesis of selective activation of inspiratory muscle fibers according to the speed of movements.