Resistance Training-Induced Changes in Human Muscle Protein Synthesis and Fibre Morphology

Abstract

Muscle proteins are in a continuous state of recycling. This process involves a balance between synthesis and breakdown. These opposing processes dictate muscle protein gains and losses. Muscle hypertrophy occurs when synthesis exceeds breakdown. In order for the accretion of new muscle proteins, a chronic state of net positive muscle protein balance (synthesis> breakdown) is required. Resistance exercise is a potent stimulus of protein turnover and the combined effects of exercise and feeding have shown to be necessary for net protein anabolism. Resistance training has been reported to increase muscle strength and induce changes in skeletal muscle morphology. These positive strength adaptations include muscle fibre hypertrophy and a shift in fibre type from IJX to IIA. Previous investigations of resistance training-induced changes in muscle protein synthesis and fibre morphology have utilized cross-sectional or longitudinal, bilateral training designs. Thus, the purpose of this study was to investigate the effects of a progressive eight week unilateral leg resistance training program on skeletal muscle morphology, and resting and exercise-stimulated mixed muscle protein fractional synthesis rate (FSR). Eight young men performed two training sessions each week, and each session consisted of four sets of knee extension (KE) and four sets of leg press (LP) at 80% 1 repetition maximum (1 RM). Needle biopsies from the vastus lateralis muscle of 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. Training induced type IIX and IIA fibre hypertrophy (P <0.05) with no change for 1ype I fibre CSA. There was no significant change in histochemically determined fibre composition or MHC content. After training, 1RM strength of the T leg significantly increased compared to baseline values (P < 0.01). At rest, FSR was significantly elevated in the T versus the UT leg (P < 0.01). Following an acute bout of resistance exercise, which was performed at the same relative intensity (80% 1 RM) for the T and UT legs, FSR was greater in the UT versus the T leg (P < 0.01). There was a lower exercise-induced increase in muscle FSR in the T versus the UT leg compared to their respective resting values <T: P = 0.08, UT: P < 0.01). 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 characterized by an attenuated rise in muscle protein FSR in the T versus the UT leg. We conclude that resistance training markedly attenuates the acute muscle protein synthetic response following resistance exercise, even when loads are matched at the same relative intensity.