Background: There is much interest in the capacity of resistance exercise to prevent the age-related loss of skeletal muscle mass and function, known as sarcopenia. This study investigates the molecular basis underlying the benefits of resistance exercise in aging C57BL/6J mice of both sexes. Results: This study is the first to demonstrate that long-term (34weeks) voluntary resistance wheel exercise (RWE) initiated at middle age, from 15months, prevents sarcopenia in selected hindlimb muscles and causes hypertrophy in soleus, by 23months of age in both male and female C57BL/6J mice. Compared with 23-month-old sedentary (SED) controls, RWE (0-6g of resistance) increased intramuscular mitochondrial density and oxidative capacity (measured by citrate synthase and NADH-TR) and increased LC3II/I ratios (a marker of autophagy) in exercised mice of both sexes. RWE also reduced mRNA expression of Gadd45α (males only) and Runx1 (females only) but had no effect on other markers of denervation including Chrng, Chrnd, Musk, and Myog. RWE increased heart mass in all mice, with a more pronounced increase in females. Significant sex differences were also noted among SED mice, with Murf1 mRNA levels increasing in male, but decreasing in old female mice between 15 and 23months. Conclusions: Overall, long-term RWE initiated from 15month of age significantly improved some markers of the mitochondrial and autophagosomal pathways and prevented age-related muscle wasting.