Effects of time of day on resistance exercise-induced anabolic signaling in skeletal muscle

Abstract

This study examined the effect of morning versus afternoon exercise on acute responses in phosphorylation of proteins regulating muscle size and metabolism. Twenty-two untrained men, divided into the morning (n = 11) or afternoon (n = 11) group, performed maximal isometric leg extensions before and after resistance loading at 07:30–08:30 h and 16:00–17:00 h, respectively. Muscle pre- and postloading biopsies were analyzed for phosphorylated Akt, p70S6K, rpS6, p38 mitogen-activated protein kinase (MAPK), Erk1/2, and eukaryotic elongation factor (eEF) 2. Muscle force declined after exercise in both groups (p < 0.001). p70S6K Thr389 (p < 0.05) and Thr421/Ser424 and rpS6 (all p < 0.001) increased after exercise in both groups. The afternoon but not morning group showed postloading decrease (p < 0.05) and increase (p < 0.01) in eEF2 and p38MAPK, respectively. Akt and Erk1/2 were statistically unchanged. In conclusion, the time of day did not have an overall effect on protein synthesis signaling, but morning phosphorylated eEF2 and p38MAPK showed significantly larger between-subject variability in the exercise response compared to the afternoon.