Does Metabolic Stress Induce Muscle Mass?
Muscle fatigue is a governing factor related to muscle growth.
Current recommendations by the American College of Sports Medicine to increase muscle mass is to train with a weight corresponding to at least 70 percent of an individual’s one repetition maximum. In layman’s terms, if your maximal bench press for one repetition is 100 pounds, then to increase muscle mass, your workouts should consist of 70 pounds for multiple repetitions. Blood flow-restricted exercise, which consists of using weights at 30 percent of a one repetition maximum, has also been shown to promote similar increases in muscle size to that of traditional heavy weight training. This is contrary to the recommendations of the governing organization’s recommendations to increase muscle mass. The notion that blood flow restriction training protocols produce similar muscle hypertrophy has led many scientist and researchers to hypothesize that it is not just muscle tension solely responsible for muscle growth. Blood flow restriction, which is high metabolic stress but low muscle tension, suggests alternative mechanisms (i.e., metabolites) to induce muscle growth.
Of the metabolites produced during resistance exercise, lactate appears to have the most support as a potential anabolic molecule for muscle hypertrophy. For example, one study reported that 30 minutes of running on the treadmill for four weeks produced greater increases (relative to an exercising control group) in the muscle weight of rats after oral lactate (100 mg/kg) and caffeine (36 mg/kg) consumption. The authors speculated that lactate might be anabolic via alterations in proteins expressed in the exercised rat muscles, which included increased myogenin (which regulates differentiation of satellite cells), decreased myostatin (a negative regulator of muscle growth), and increased follistatin (an inhibitor of myostatin). This suggests that metabolic stress may be enhancing the anabolic actions of resistance exercise.
Researchers recently published an interesting theory on metabolic stress-induced muscle hypertrophy in the European Journal of Physiology. The authors wrote that the production of metabolites (i.e., lactate, hydrogen ions, etc.) during resistance exercise does not have anabolic properties per se, but instead promotes muscle hypertrophy indirectly via inducing muscle fatigue and enhancing muscle activation. One of the exciting findings of blood flow occlusion studies is the high level of muscle fatigue reported in conjunction with increased activation of high threshold type II fibers which are predominately recruited with heavy weightlifting.
It has recently been discovered that muscle fatigue is a governing factor related to muscle growth. New research has found that both low and high intensity exercise, when taken to complete muscle fatigue, results in similar increases in muscle growth. Although fatigue is likely multifactorial and cannot be solely attributed to metabolites, fatigue has been shown to stimulate the recruitment of additional higher threshold motor units. High threshold fibers (type II fibers) are more prone to muscle growth than slow twitch (type I) fibers. Therefore, it seems more likely that it’s the recruitment of higher threshold motor units. Thus, the activation of a large proportion of muscle fibers that is of primary importance for muscle growth. If increases in metabolic byproducts were the critical factor for muscle growth than the application of blood flow restriction with heavier weights would further increase muscle growth; however, this is not supported by science.
One study found that blood flow restriction does not enhance muscle growth occurring during high load resistance exercise despite the enhanced metabolic stress. It can be speculated that the fibers are maximally stressed, and further metabolic stress will not enhance muscle growth. Based on the available evidence, the anabolic role of exercise-induced metabolites lies in their ability to assist with augmenting muscle activation. Metabolic stress is not a direct factor for increasing muscle growth, but it may be considered an indirect contributing factor.
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Dankel SJ, Buckner SL, Jessee MB et al (2017a) Can blood flow restriction augment muscle activation during high-load training? Clin Physiol Funct Imaging. doi:10.1111/cpf.12414
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Oishi Y, Tsukamoto H, Yokokawa T et al (2015) Mixed lactate and caffeine compound increases satellite cell activity and anabolic signals for muscle hypertrophy. J Appl Physiol Bethesda Md 1985 118:742–749. doi:10.1152/japplphysiol.00054.2014
Dankel, S. J., Mattocks, K. T., Jessee, M. B., Buckner, S. L., Mouser, J. G., & Loenneke, J. P. (2017). Do metabolites that are produced during resistance exercise enhance muscle hypertrophy? European Journal of Applied Physiology.”