Figure 7. A model depicting recruitment of phrenic motoneurons during progressively more intense diaphragm contraction.

This model was developed by Sieck, Mantilla, and colleagues, and the image presented has been adapted from their work (Fogarty et al., 2018a). The model predicts that a relatively small amount of the total phrenic motoneuron pool (approximately 20%) is required for typical quiet breathing. This number increases during conditions such as intense exercise, but only approaches 100% during high force expulsive maneuvers.