A Brief Review of Critical Processes in Exercise-Induced Muscular Hypertrophy
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REVIEW ARTICLE
A Brief Review of Critical Processes in Exercise-Induced Muscular Hypertrophy Stuart M. Phillips
Ó The Author(s) 2014. This article is published with open access at Springerlink.com
Abstract With regular practice, resistance exercise can lead to gains in skeletal muscle mass by means of hypertrophy. The process of skeletal muscle fiber hypertrophy comes about as a result of the confluence of positive muscle protein balance and satellite cell addition to muscle fibers. Positive muscle protein balance is achieved when the rate of new muscle protein synthesis (MPS) exceeds that of muscle protein breakdown (MPB). While resistance exercise and postprandial hyperaminoacidemia both stimulate MPS, it is through the synergistic effects of these two stimuli that a net gain in muscle proteins occurs and muscle fiber hypertrophy takes place. Current evidence favors the post-exercise period as a time when rapid hyperaminoacidemia promotes a marked rise in the rate of MPS. Dietary proteins with a full complement of essential amino acids and high leucine contents that are rapidly digested are more likely to be efficacious in this regard. Various other compounds have been added to complete proteins, including carbohydrate, arginine and glutamine, in an attempt to augment the effectiveness of the protein in stimulating MPS (or suppressing MPB), but none has proved particularly effective. Evidence points to a higher protein intake in combination with resistance exercise as being efficacious in allowing preservation, and on occasion increases, in skeletal muscle mass with dietary energy restriction aimed at the promotion of weight loss. The goal of this review is to examine practices of protein ingestion in combination with resistance exercise that have some evidence for efficacy and to highlight future areas for investigation.
S. M. Phillips (&) Exercise Metabolism Research Group, Department of Kinesiology, McMaster University, 1280 Main Street West, Hamilton, ON L8S 4K1, Canada e-mail: [email protected]
1 Introduction The process of skeletal muscle protein turnover is constant and ongoing. Protein turnover within muscle is the sum of the processes of both muscle protein synthesis (MPS) and muscle protein breakdown (MPB). Beyond childhood growth, chronic imbalances between the processes of MPS and MPB lead to a net gain in protein pool size (hypertrophy: MPS [ MPB) or a net loss (atrophy: MPB [ MPS). Often, athletes seek to maximize a hypertrophic response to exercise with the general acceptance that this may translate into performance gains. Hypertrophy, or the offsetting of atrophy, may also be a goal for athletes in recovery from injury, and so understanding the mechanisms that regulate muscle mass are important. The goal of this review is to provide a brief overview of the factors that regulate hypertrophy and how they can be affected by nutritional factors with a focus on protein.
2 Regulation of Muscle Protein Turnover Resistance exercise provides a loading stimulus to skeletal muscle that results in incre
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