Force–frequency characteristics of unfatigued diaphragm muscle bundles. A, Symbols are mean ± SEM for CTRL (closed circles), NAC (open circles), NACA (squares), and ERGO (triangles). Lines are best fit of mean data using Hill equation: CTRL (solid gray line), NAC (solid black line), NACA (long-dashed line), and ERGO (dotted line). B, Stimulus frequency that elicited 50% maximal tetanic force. Results obtained from curve fitting of individual muscle responses using Hill equation. * P < 0.05 versus control.