RESISTANCE TRAINING-INDUCED ADAPTATIONS IN MUSCLE PROTEIN SYNTHESIS

Abstract

PURPOSE We investigated the effects of a progressive 8 week unilateral leg resistance training program on mixed muscle protein fractional synthesis rate (FSR), at rest and after an acute bout of exercise and on skeletal muscle morphology. METHODS Eight young men performed 4 sets of knee extension (KE) and 4 sets of leg press (LP) at 80% 1 repetition maximum (1RM) per workout (3/wk). Muscle biopsies from the trained (T) leg were taken before and after training and analyzed for fibre composition, cross-sectional area (CSA), and myosin heavy chain (MHC) content. Muscle protein FSR was determined using a primed constant stable isotope infusion of [13C6]-phenylalanine in both the T and untrained (UT) legs. RESULTS Training induced type II fibre hypertrophy (type IIA: +32±8%; p < 0.05 and type IIX: +43±10%; p < 0.05), with no change for type I fibre CSA (+10±11%; p = 0.34). There was no significant change in histochemically determined fibre composition (type IIA: +23±16%; p = 0.64, type IIX; −25±11%; p = 0.21) or MHC content (MHC IIa: +16±3%; p = 0.30. MHC IIx: −15±6%; p = 0.17). After training, 1RM strength of the T leg significantly increased compared to baseline values (KE: +95±20%; p < 0.01, LP: +80±17%; p < 0.01). At rest, muscle protein FSR was significantly elevated in the T versus the UT leg (+76±32%; p < 0.01). Exercise, completed at the same relative intensity (80% 1RM) for the T and UT legs, resulted in a greater synthetic response in the UT versus the T leg (+29±9%; p < 0.01). There was a lower exercise-induced increase in muscle FSR of the T versus the UT leg compared to their respective resting values (T: +28±18%; p = 0.08, UT: +172±52%; p < 0.01). CONCLUSION These data show that resistance training resulted in significant muscle fibre hypertrophy and elevated rate of muscle protein synthesis at rest. In addition, the acute response to resistance exercise was an attenuated rise in muscle protein FSR in the T versus the UT leg. Therefore, resistance training markedly attenuates the acute muscle protein synthetic response even when loads are matched at the same relative intensity. Supported by NSERC.