Fig. 4: Myofibrillar ultrastructural disorganization in relaxed mdx EDL fibers is a predictor of reduced contractile performance and reduced Ca²⁺ sensitivity of the contractile apparatus.

a SHG images of single EDL fibers from a wt mouse (14 weeks) and an mdx mouse (79 weeks) in the relaxed state (pCa 9) with indicated sarcomere lengths (SL), maximum diameters, VD and CAS. Steady-state force at a given pCa normalized to the maximum force with indicated pCa₅₀ and Hill parameters for the shown fibers. Single mdx EDL fibers showed vast biomechanical deficits in active force production: significantly reduced max. absolute (b) and specific force (c) at a pCa of 4.92–5.67 and significantly reduced pCa₅₀ values indicative of myofibrillar Ca²⁺ desensitization (d) with otherwise similar Hill parameters (e). This correlates well with myofibrillar structural deficits, as shown by the markedly increased VD (f) and decreased CAS (g) in mdx mice over wt mice. At similar sarcomere lengths (h), fiber diameters were larger in mdx EDL fibers compared than in wt fibers (i). Box plots with box (25 to 75 percentiles), median (line), whiskers (5–95 percentiles), minimum and maximum (x), mean (rectangle) and significances from one-way ANOVA with post hoc Bonferroni test (equal variance) or post hoc Tukey test (no equal variance) indicated as p < 0.05 (*) and p < 0.01 (**)