Mechanisms of muscle growth The growth of muscles such that they become bigger in size is referred to as “muscle hypertrophy”. Muscle hypertrophy is largely induced by strength training (i.e. weight training) such that we become stronger, whereas endurance training causes our muscles to make more mitochondria so that our aerobic performance is improved. The size of our muscles is under the control of two processes that are in competition with one another: muscle protein synthesis (MPS) versus muscle protein breakdown (MPB). Over a 24 h period, MPS is usually equal to MPB such that our muscle mass remains constant. However, for hypertrophy to occur, MPS must exceed MPB over the course of a training program. In contrast, for muscles to get smaller (i.e. muscle atrophy), MPB must exceed MPS. Whereas hypertrophy occurs with a well formulated strength training program and nutritional plan, muscle atrophy occurs during times of inactivity and sub-optimal energy intake, such as that which occurs during times of injury or prolonged periods of no training. The strength training plan Fundamentally, the initial stimulus to grow muscle is to actually stress the muscle to a level that it is not accustomed to. Whereas endurance training represents a low load performed for a prolonged period of time, strength training consists of a high load performed for a short period of time. Recent studies have suggested that multiple sets of resistance training performed to the point of the failure provides a sufficient stimulus for muscles to grow(1,2). Additionally, each repetition is best performed in a slower more controlled approach as opposed to a fast explosive movement(3). In this way, the “time under tension” increases, all of our muscles fibres are recruited (by performing each set to fatigue) and the training stress then causes our muscles to adapt in the recovery period such that over time, they can become bigger and stronger. Strength training can cause muscle damage and soreness in the hours and days after each session and for this reason(4), it is crucial that muscles are provided with the correct nutrients and rest to recover. The recovery period During the actual training session itself, MPB increases and subsequent muscle damage occurs. If we did not consume any nutrients (especially protein) in the recovery period, then net muscle protein balance would become negative. This is often referred to as being in a “catabolic state”. In order to induce a positive net protein balance and provide the conditions to grow muscle, it is crucial to ingest protein within 30 minutes of finishing your training. In this way, amino acids are delivered to the muscle thereby providing the building blocks to help our muscles rebuild, grow and recover. At this time, muscle is said to be in an “anabolic state”. With the correct training and nutrition plan, MPS will exceed MPB and hence muscles accrue protein so that they get bigger and stronger. In termsterms of protein feeding, feeding protein that contains the amino acid leucine is especially important(5). Leucine acts as a trigger to instruct our muscles to actually begin the anabolic process. We then need the additional essential amino acids to subsequently provide the building blocks of new muscle protein to be made(6). Protein intake considerations Ensuring that you have an adequate daily protein intake and take on protein at the right times (within 30 minutes of finishing exercise) will help you “switch on” and maintain muscle protein synthesis. Follow these guidelines: Choose a protein source that is high in branched chain amino acids (BCAAs) and leucine like whey protein(5, 7) Consume 20-30g of protein every 3-4 hours throughout the day to help maintain muscle protein synthesis(8) Aim to take on between 1.4 – 1.8 grams of protein per kilo of your body mass, per day(7) Immediately post exercise (within 30 minutes), the muscle is most responsive to the intake of nutrients. Consume 0.3 grams of protein per kilo of body mass immediately post exercise(9). This will usually equate to 20-40 g of protein(10) References Burd, N. A., Holwerda, A. M., Selby, K. C., West, D. W., Staples, A. W., Cain, N. E., & Phillips, S. M. (2010). Resistance exercise volume affects myofibrillar protein synthesis and anabolic signalling molecule phosphorylation in young men. The Journal of Physiology, 588(16), 3119-3130. Mitchell, C. J., Churchward-Venne, T. A., West, D. W., Burd, N. A., Breen, L., Baker, S. K., & Phillips, S. M. (2012). Resistance exercise load does not determine training-mediated hypertrophic gains in young men. Journal of Applied Physiology, 113(1), 71-77. Burd, N. A., Andrews, R. J., West, D. W., Little, J. P., Cochran, A. J., Hector, A. J., & Phillips, S. M. (2012). Muscle time under tension during resistance exercise stimulates differential muscle protein sub‐fractional synthetic responses in men. The Journal of Physiology, 590(2), 351-362. Damas, F., Phillips, S. M., Libardi, C. A., Vechin, F. C., Lixandrão, M. E., Jannig, P. R., & Tricoli, V. (2016). Resistance training‐induced changes in integrated myofibrillar protein synthesis are related to hypertrophy only after attenuation of muscle damage. The Journal of Physiology, 594(18), 5209-5222. Tang, J. E., Moore, D. R., Kujbida, G. W., Tarnopolsky, M. A., & Phillips, S. M. (2009). Ingestion of whey hydrolysate, casein, or soy protein isolate: effects on mixed muscle protein synthesis at rest and following resistance exercise in young men. Journal of Applied physiology, 107(3), 987-992. Churchward‐Venne, T. A., Burd, N. A., Mitchell, C. J., West, D. W., Philp, A., Marcotte, G. R., & Phillips, S. M. (2012). Supplementation of a suboptimal protein dose with leucine or essential amino acids: effects on myofibrillar protein synthesis at rest and following resistance exercise in men. The Journal of physiology, 590(11), 2751-2765. Phillips, S. M., & Van Loon, L. J. (2011). Dietary protein for athletes: from requirements to optimum adaptation. Journal of Sports Sciences, 29(1), 29-38. Areta, J. L., Burke, L. M., Ross, M. L., Camera, D. M., West, D. W., Broad, E. M., & Hawley, J. A. (2013). Timing and distribution of protein ingestion during prolonged recovery from resistance exercise alters myofibrillar protein synthesis. The Journal of Physiology, 591(9), 2319-2331. Thomas, D. T., Erdman, K. A., & Burke, L. M. (2016). Position of the academy of nutrition and dietetics, dietitians of Canada, and the American college of sports medicine: Nutrition and athletic performance. Journal of the Academy of Nutrition and Dietetics, 116(3), 501-528. Macnaughton, L. S., Wardle, S. L., Witard, O. C., McGlory, C., Hamilton, D. L., Jeromson, S., & Tipton, K. D. (2016). The response of muscle protein synthesis following whole‐body resistance exercise is greater following 40 g than 20 g of ingested whey protein. Physiological Reports, 4(15), e12893. Written By Ted Munson (Performance Nutritionist) Ted is a Performance Nutritionist here at Science in Sport.