Figure 1.

Classification of diaphragm fibers based on Sr²⁺-activation parameters. “Normal” refers to Ca²⁺- and Sr²⁺-activation profiles where both functions are described by a single exponential function, and is normally observed in skeletal muscle – separation between the force–pCa and force–PSr curves is greater for “fast” twitch fibers, and least for “slow” twitch fibers. “Hybrid” activation profile of the force–pSr curve is typical for fibers from adult diaphragm – the separation between the lower portion of the force–pSr curve and force–pCa curve is typical of that for “slow” twitch fibers, while the separation between the upper part of the force–pSr and force–pCa curves is typical of that for “fast” twitch fibers. “Intermediate” fibers were identified in the developing diaphragm when the force–pSr curve could no longer be fitted by a single exponential; Marquardt curve fitting was used to derive parameters from a double exponential curve, and also to determine the F1 and F2 portions of the complete force–pSr relationship.