ABSTRACT: We used old (~96-102 weeks of age) and young (~28-34 weeks of age) rats from HCR and LCR generations 29 and 32, respectively. The study included eight groups; HCR-Old-Exhausted (H-O-E, n=6), HCR-Old-Rest (H-O-R, n=6), HCR-Young-Exhausted (H-Y-E, n=6), HCR- Young -Rest (H-Y-R, n=6), LCR-Old-Exhausted (L-O-E, n=6), LCR-Old-Rest (L-O-R, n=6), LCR-Young-Exhausted (L-Y-E, n=6), and LCR- Young -Rest (L-Y-R, n=6). For the exhausted rats, dissections were performed within 10 min after the maximal running distance was reached. We extracted skeletal muscle RNA from a total of 48 female animals (n=6 in each of the 8 group). Skeletal muscle tissue was obtained from the Extensor digitorum longus (EDL). All rats were dissected immediately after sacrificing, and all tissue samples were immediately weighed and snap frozen in liquid nitrogen, and stored at -80 ºC. Total RNA was extracted from frozen tissue with Trizol reagent (Invitrogen, Carlsbad, CA), treated with DNase-free (Invitrogen, Carlsbad, CA) and cleaned up with RNeasy columns (Qiagen, Hilden, Germany)