FIGURE 1.

Different DIAm motor unit types are distinguished by their intrinsic, mechanical, and fatigue properties

A: different DIAm motor unit types are distinguished by their intrinsic, mechanical, and fatigue properties, and are classified as type S, FR, FInt, and FF. Within a particular motor unit, all muscle fibers are homogeneous, as evidenced by myosin heavy chain (MyHC) expression. B: force (normalized to percent maximum tetanic force) generated by cat diaphragm motor units at different frequencies of stimulation. Results are for individual motor units classified by their contractile and fatigue properties. The steepest portion of the force-frequency curve occurs between 10 and 30 Hz for all types of motor units in the diaphragm muscle (52), consistent with onset and peak discharge frequencies of ~8 and ~25 Hz, respectively, for type S and FR units (top arrows) and ~15 and ~60 Hz, respectively, for type FInt and FF units (bottom arrows), reported in Ref. 198. C: diaphragm motor units are recruited to accomplish a range of motor behaviors. Ventilation (eupnea, hypercapnia, and hypoxia) is accomplished by recruitment type S and FR motor units, whereas higher-force airway clearance behaviors require recruitment of more fatigable DIAm motor units.