Divergent myofibrillar protein synthesis and signalling in skeletal muscle in response to light and heavy resistance exercise loading Lars Holm, Gerrit van Hall, Adam J Rose, Simon Doessing, Erik A Richter and Michael Kjaer ABSTRACT Objective. By standardizing exercise volume (intensity times repetitions), we investigated the isolated effect of concentric muscle contraction intensity on the anabolic response in the working skeletal muscle. One leg performed a light resistance exercise (LE) load at 15.5% of 1 repetition maximum (1RM) and the other leg a heavy resistance exercise (HE) load at 70% of 1RM. Methods. In the overnight fasted state a primed, continuous infusion of 1-13C-leucine was initiated. Before exercise intervention with LE and HE intensities resting biopsies were obtained from vastus lateralis (VL) muscle. At 0:30, 3:00, and 5:30h post exercise biopsies were taken bilaterally from VL. Results. LE resulted in a moderate elevation in FSR (0.11±0.01%/h, P<.05) in the early post exercise phase (0:30-3h), and decreased thereafter towards basal values. The HE resulted in a marked increase in FSR only in the late phase (3–5:30h) after exercise (0.13±0.02%/h) compared to rest (P<.01), early HE (P<.05), and late LE (P<.10). The s6kinase(t421/s424) was markedly hyper-phosphorylated (h-phos) in HE at 0:30h compared with rest and LE (P<.001). The 4E-BP1(t37/46) was h-phos weakly in LE at 0:30h but markedly (P<.01) h-phos following HE at 3:00h compared with rest and 0:30h. Discussion. The temporal changes in myofibrillar FSR following LE and HE seem to be more related to the h-phos of 4E-BP1(t37/46) than to the s6kinase(t421/s424).