(a) Old murine hearts displayed reduced levels of MyBP-C phosphorylation at Ser282, which is normalized by SS-31 treatment. (b–c) Aging and SS-31 treatment did not alter phosphorylation of cTnI at Ser23/24 (b) and Ser150 (c) in hearts For panel a-c, n = 5, 6, and five male mice for young control, old control and old SS-31, respectively, analyzed by one-way ANOVA Dunnett’s post hoc analysis for panel a-c. (d) Titin isoform ratio (N2BA/N2B ratio) did not change with SS-31 treatment; n = 8 and 6 female mice were used for old control and old SS-31, respectively, and were compared by unpaired T-test.