Exercise-induced activation of the mitochondrial biogenesis pathway in mice skeletal muscle during aging. Western blotting analysis to detect (A) Peroxisome Proliferator-Activated Receptor-γ Coactivator 1α (PGC-1α) and (B) cytochrome C at different survival time points. Representative blots are shown. For the densitometric analysis of the results, values are shown as mean (± SD). The content of α-actin, a housekeeping protein marker in skeletal muscle, was determined in all the experimental groups. (*) indicates P <0.05, (**) indicates P <0.01 versus the sedentary group at different ages. Values were normalized to those observed in the samples obtained from the three-month-old group, which was assigned a value of 100%. Continuous lines show statistically significant differences between the sedentary and the wheel-running groups. The discontinuous lines show statistically significant differences between the periods studied in the sedentary group.