Dalbo, VJ, Roberts, MD, Hassell, SE, Brown, RD, and Kerksick, CM. Effects of age on serum hormone concentrations and intramuscular proteolytic signaling before and after a single bout of resistance training. J Strength Cond Res 25(1): 1-9, 2011-This study examined mRNA expression patterns for atrogin-1 and muscle ring finger-1 (MuRF-1) before and 24 hours after a resistance training bout. Furthermore, basal, 5-minute and 24-hour postexercise serum concentrations of cortisol and insulin like growth factor-1 (IGF-1) and the relationships between these hormones and the genetic expression patterns of atrogin-1 and MuRF-1 were examined. Younger and older men completed a resistance exercise bout consisting of 3 × 10 repetitions at 80% of their predetermined 1 repetition maximum for Smith squat, leg press and leg extension. Muscle biopsies from the vastus lateralis were obtained before and 24 hours after exercise. Basal and postexercise gene expression differences between age groups were analyzed using the Mann-Whitney U test, whereas separate 2 × 3 repeated measures analyses of variance were performed to analyze changes in hormone concentrations. Spearman's correlations were performed to examine relationships between gene expression patterns and hormone concentrations. Serum cortisol was significantly greater in younger men before and 24 hours after exercise (p < 0.05), whereas serum IGF-1 was significantly greater in younger men at all time points (p < 0.001). Exercise significantly increased cortisol 5 minutes after exercise in both groups (p < 0.05), whereas older men experienced significant elevations in IGF-1 24 hours postexercise (p < 0.05). At baseline, MuRF-1 gene expression was significantly greater in older men (p = 0.03), whereas no age-related differences were found for atrogin-1 (p = 0.24). Fold change in atrogin-1 and MuRF-1 24 hours postexercise revealed no significant differences between younger and older men. Differential baseline expression of MuRF-1 may suggest a regulatory attempt by the aging transcriptome to accommodate changes necessary for homeostatic maintenance. An enhanced understanding of molecular and genetic level adaptations can aid researchers in developing optimal therapeutic and exercise interventions to mitigate decrements in force, power, and loss of muscle mass seen in aging and many clinical populations.