Fig. 2.

Des^−/− bundles from young and adult muscle were stiffer compared with wt. A: passive Cauchy stress-sarcomere length quadratic curve fits for young wt (n = 10) and des^−/− (n = 12) bundles. Aa: neonatal mice. Ab: young mice. Ac: adult mice. Bundle curves were more nonlinear than fiber curves (compare with Fig. 1A) with increased nonlinearity in the young des^−/− curve relative to young wt. Young and adult des^−/− bundles had significantly higher stress than wt at sarcomere lengths greater than 4.1 μm and 3.7 μm, respectively, as determined by ANOVA with repeated measures. B: tangent moduli calculated from curve fits at a sarcomere length of 3.9 μm for des^−/− and wt bundles in three age groups. Young and adult des^−/− bundles were significantly stiffer compared wt. However, there is no difference in neonatal bundle stiffness with genotype. Additionally, there was a significant genotype-age interaction as determined by two-way ANOVA due to continually increasing bundle stiffness with age in des^−/− bundles that was absent in wt bundles. *P < 0.05.