1. Muscle units in the right sternocostal region of the cat diaphragm (DIA) were isolated in situ by dissecting filaments of the C5 ventral root. Isometric contractile and fatigue properties of DIA units were then measured. Contractile properties included: twitch contraction time (CT), peak twitch tension (P(t)), maximum tetanic tension (P0), and the frequency dependence of tension production. Muscle-unit fatigue resistance was estimated using a 2-min fatigue test. 2. DIA muscle units were classified as fast (F) or slow (S) based on the presence or absence of sag in their infused tetanic force responses. Muscle-unit fatigue indices (FI) were used to further classify DIA units as slow-twitch fatigue-resistant (S), fast-twitch fatigue-resistant (FR) fast-twitch fatigue-intermediate (FInt), or fast-twitch fatigable (FF) types. 3. Based on a total of 47 completely characterized DIA muscle units, 21% were classified as S, 4% as FR, 28% as FInt, and 47% as FF. In contrast to the distribution of unit types in other mixed appendicular muscles, the DIA was composed of a very low proportion of FR units and a relatively high proportion of FInt units. An interval of FIs between 0.50 and 0.75 separated units into fatigue-resistant and fatigable groups. The distribution of FIs for FF and most FInt units was continuous, indicating that they formed a single fatigable group. Relatively few FF units in the DIA had FIs < 0.10. 4. A wide range of contractile properties was observed for DIA muscle units. Type S units had longer CTs and lower P(t) and P(o) values than type F units. The mean P(t) and P0 of FF and FInt units were comparable, whereas the mean P(t) and P0 of the two FR units were lower. Type S units produced a greater proportion of their P0 at lower frequencies of activation than type F units. The lower P0s produced by type F units in the DIA indicated that they were smaller than similar units in appendicular muscles. It was concluded that in meeting most normal ventilatory requirements, adequate force could be generated by the recruitment of only type S and FR units. The recruitment of the more fatigable FF and FInt units may occur only during more forceful respiratory and nonrespiratory behaviors of the DIA.
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