Changes in Muscle Protein Synthetic Rate and Ultrastructure Following Resistance Exercise

Abstract

The purpose of this study was to correlate the extent of myofibrillar disruption with muscle protein synthetic rate (MPS) following an isolated bout of concentric or eccentric elbow flexor resistance exercise. Six strength-trained males performed 8 unilateral sets of 8 repetitions at 80% concentric 1RM. The absolute amount of work performed by each arm was controlled by having both the concentrically-exercised (CON) and eccentrically-exercised (ECC) arms lift or lower the same weight through the same range and magnitude. Biopsies from biceps brachii of each arm, extracted ~21 h post -exercise, were analyzed electron microscopically to quantify myofibrillar disruption. The severity of disruption was classified as focal (FOC), moderate (MOD), or extreme (EXT). MPS of both arms was calculated from the increment in L- [1,2-¹³C₂] leucine abundance in biopsy samples relative to the mean plasma [1, 2-¹³C₂] -α-KIC enrichment at isotopic plateau using the primed-constant infusion technique over ~10 h so that the midpoint of the assessment period was ~24 h post-exercise. The severity of disruption was significantly (P < 0.001) greater in both the FOC (11.2%) and MOD (12.2%) compared to the EXT (1.6%) rating. Absolute disruption of fibers was significantly greater (P = 0.007) in the ECC (44.7%) as compared to baseline (BASE) samples (3.9%), obtained following 5 d where no arm training had occurred. In addition, ECC samples showed ~40% greater total disruption than CON samples (44.7% vs. 26.7%). Despite this, a positive correlation (r = 0.89) was found between individual values for MPS and the percentage of disrupted fibers in tissue from the ECC but not CON arm. These findings indicate that, in strength-trained males, residual myofibrillar disruption from a previous training session is essentially repaired within 5 d, but that resistance exercise induced muscle damage did not appear to provide the activating signal for elevating MPS between -21-29 h post-exercise.