(*A*) Passive tension and (*B*) passive (relaxed) to rigor stiffness ratio plotted against sarcomere length. Glycerinated (*solid triangles*) and flash-frozen (*open triangles*) porcine data are plotted with standard errors, whereas flash-frozen human myocardium data (*inverted open triangles*) are plotted as individual data points. For *A*, the similarity between glycerinated porcine and flash-frozen human myocardium passive tension dependence on sarcomere length is apparent. Lines are fits of *as*^{k}, where *s* is sarcomere length; *k* = 5.15 ± 0.62 and *a* = 0.13 ± 0.08 mN *μ*m^{−k} for porcine myocardium (*solid line*); and *k* = 4.91 ± 0.45 and *a* = 0.15 ± 0.07 mN *μ*m^{−k} for human myocardium (*dashed line*). In *B*; although human myocardium data points are shifted ∼0.1 mm to the right of glycerinated porcine, flash-frozen porcine myocardium (*open triangles*) points are shifted to the left, so the position of the curves cannot be associated with cryopreservation. (*Left inset*) Length change as a percentage of fiber length, *L*_{0} (*upper trace*), and force responses in activating solution (pCa 4.6), rigor (*middle traces*), and relaxing solution (*lower trace*). In general, rigor tension was close to that of maximum activation, but decreased after any mechanical disturbance. The rigor response here is the slightly larger of the middle two traces. (*Right inset*) Force-extension plots for the stretch phase of the length change, where rigor is the steepest line and relaxed is the shallowest. Force immediately before the stretch has been subtracted. The slope of the line was obtained as a linear regression of force upon length (*dashed lines*) and was taken as the dynamic stiffness. Flash-frozen human myocardium, diameter 400–450*μ*m.

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