When you lift weights, you do it to build muscle strength but also to increase the size of your muscles, a process called muscle hypertrophy. Although you might not want to become muscle-bound, you still must work your muscles against resistance. Why? You fight a battle with losing muscle mass after the age of 30. Therefore, training your muscles against resistance is important for reducing age-related muscle loss and the functional limitations that go along with it. Studies even correlate muscle strength with greater longevity. A study found that men and women with less muscle strength were 50% more likely to die early relative to those with greater muscle strength.
When you work your muscles against resistance, muscle fibers increase in size and this leads to a boost in muscle size. When you train muscles against resistance, one factor that stimulates muscle growth is mechanical tension. When you hold weight and do an exercise through its full range-of-motion, it subjects your muscles to active and passive mechanical tension. This stimulates mechanosensors on the muscles and that signals pathways that initiate muscle protein synthesis that leads to muscle growth.
Most experts believe that mechanical tension is the strongest stimulus for muscle hypertrophy. That’s why weight training is so important, it generates tension within the muscle and serves as the initial stimulus for muscle growth. However, there’s also evidence that metabolic stress contributes to muscle hypertrophy too. You might wonder what metabolic stress is and what role it might play in building muscle mass.
What is Metabolic Stress?
When you weight train, metabolites build up in muscle tissue that is generated by the metabolic pathways muscles use to produce ATP. One that you’re probably familiar with is lactate, but with repeated muscle contraction, phosphate and hydrogen ions accumulate too and lower the PH. Metabolites like these build up mainly when your body uses anaerobic means of energy production. The metabolites are a product of anaerobic energy pathways.
Your muscles build up more metabolites like lactate when you lift at a moderate intensity and do a moderate number of repetitions. You get less lactate accumulation when you weight train with light weights and do high reps or lift very heavy weights for only a few repetitions. The “sweet spot” for metabolite build-up and metabolic stress is a repetition range of between 8 and 12 reps with full muscle fatigue by the final repetition.
Blood Flow Restriction to Maximize Metabolic Stress
Another way to increase metabolic stress is to restrict blood flow during weight training. Some physical therapists use blood flow restriction in a rehab setting for patients who are injured or who have just undergone surgery and are unable to exercise as intensely but still need to maintain muscle strength and mass. To give them an advantage, the therapist places a cuff around a leg or arm to partially block blood flow when they train that muscle. With restricted blood flow, metabolites build-up, creating metabolic stress, and offers an additional stimulus for muscle growth. The theory is that with restricted blood flow, patients can lift lighter weights and still get muscle growth due to metabolic stress.
In a study, researchers asked healthy guys of college-age to walk regularly for three weeks while blood flow to their thigh muscles was restricted. At the end of the three weeks, the subjects experienced a significant increase in thigh cross-section area and the only exercise they did was walking with restricted blood flow to their legs. Researchers believe that the increased metabolic stress created by restricted blood flow explains the muscle grains.
Metabolic Stress and Muscle Hypertrophy
How important is metabolic stress for muscle hypertrophy? Based on the fact that restricting blood flow during training allows patients to preserve or build muscle lifting lighter loads, there is strong evidence that it’s a factor for muscle hypertrophy. When you create metabolic stress, muscle cells swell and that stimulates the release of growth factors called myokines that boost muscle growth and anabolic hormones that boost muscle hypertrophy.
With metabolic stress, you don’t need to lift heavy, you only have to train in a way that triggers the build-up of metabolites, like lactate, hydrogen ions, phosphate. You do this by lifting in a range that fatigues the muscle after 8 to 12 repetitions. The prolonged tension you place on the muscle and the onset of fatigue causes these metabolites to accumulate. Without metabolic stress, you’d have to depend on heavy force production to generate muscle growth.
The question is whether mechanical forces and metabolic stress are additive, whether you can build more muscle if you have both, or do you only need to generate a certain threshold of mechanical tension. It’s hard to separate the two since they often occur together. The strongest evidence of the importance of metabolic stress comes from restricted blood flow training.
The Bottom Line
Metabolic stress appears to play a strong role in muscle hypertrophy and may explain why people can build muscle lifting lighter weights if they lift to muscle fatigue and also by restricting blood flow to the muscle. Now you know how to structure your training to maximize metabolic stress – moderate repetitions and moderate repetitions in the 8 to 12 rep range while fatiguing the muscles. Keep your rest periods short to maintain the build-up of the metabolites that create metabolic stress and boost muscle growth.
References:
- Journal of Applied Physiology. Stimuli and sensors that initiate skeletal muscle hypertrophy following resistance exercise. Henning Wackerhage, Brad J. Schoenfeld, D. Lee Hamilton, Maarit Lehti, and Juha J. Hulmi. 09 JAN 2019https://doi.org/10.1152/japplphysiol.00685.2018.
- The Mystery of Skeletal Muscle Hypertrophy. Richard Joshua Hernandez, B.S. and Len Kravitz, Ph.D.
- Science and Development of Muscle Hypertrophy. Brad Schoenfeld. Human Kinetics. 2016.
- World J Methodol. 2017 Jun 26; 7(2): 46–54.Published online 2017 Jun 26. doi: 10.5662/wjm.v7.i2.46
- Dev Dyn. 2009 Jun;238(6):1526-34. doi: 10.1002/dvdy.21972.
- com. “Training for Maximum Muscle Growth Explained”
- Appl Physiol Nutr Metab. 2019 Jul;44(7):759-764. doi: 10.1139/apnm-2018-0321. Epub 2018 Dec 19.
- co.UK. “High Muscle Strength Could Help You Live Longer, Study Finds”
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