Figure 1:

Different diaphragm muscle motor unit types are distinguished by their intrinsic, mechanical, and fatigue properties, and are classified as type S, FR, FInt, and FF. Within an individual motor unit, all constituent muscle fibers exhibit homogeneous myosin heavy chain (MyHC) expression. In the diaphragm muscle of most species, type I and IIa muscle fibers have smaller cross-sectional areas than those of type IIx and/or IIb fibers. Forces produced by type I fibers are less than forces produced by type IIa fibers that are less than forces produced by IIx and/or IIb fibers. Recruitment of diaphragm muscle motor units is in an orderly fashion, necessary to accomplish a range of motor behaviors. Ventilation is accomplished by recruitment type S and FR motor units, whereas higher-force airway clearance behaviors and straining/expulsive manoeuvres require recruitment of more fatigueable type FInt and FF motor units.