Muscle Growth: What the Science Says

If you’re looking to gain lean muscle, “get big,” or add some definition to your physique — you’re going to want to keep reading. Today’s article is a little bit different than the usual side of Kail. It’s loaded with lots of good stuff though so keep reading!

And if you find yourself confused or just looking for the basic message, look for sentences in blue — they’ll be simplified!

The science is clear: muscle growth and strength don’t necessarily go hand-in-hand. Meaning, you can be the biggest but that doesn’t mean you’re the strongest.

If you’re focus is building a leaner, harder physique, then you need to hone in on certain areas of your training. Before digging in, let’s get some basic definitions out of the way.

  • Anabolic: Related to building muscle.
  • Hypertrophy: The enlargement of an organ or tissue from an increase in size of its cells. Or, simpler yet, when protein synthesis exceeds protein breakdown.
  • Mechanical tension: The amount of tension developed by muscle fibers in response to a stimulus.
  • Muscle damage: Microtears in your muscle. For this use case we’re referring to tears caused by strength training.
  • Metabolic stress: A physiological process that occurs during exercise in response to low energy that leads to metabolite accumulation in muscle cells.
  • Metabolite: A substance made or used when the body breaks down muscle tissue (or food, drugs, and chemicals).

The Big 3:

Brad Schoenfeld, training genius and PhD, dove deep into research and came up with 3 major contributing factors to body composition and muscle hypertrophy: tension, damage, and stress. Here’s what he found.

Mechanical Tension

When muscles are put under tension, the body responds by calling for backup: growth hormones, repair cells, extra hydration (fluid), etc., leading to muscle growth [1].

Tension doesn’t necessarily mean lifting the heaviest weights possible. It means using enough weight to disturb the integrity of the muscular tissue (generally 80%+ of your 1RM) [2]. Learning how to properly execute tension takes years of practice and honing in on proper form.

This is where intensity comes into play, or a percentage of your 1RM you’re able to use for a given number of reps. It can be further broken down into low (1-5), moderate (6-12), and high (15+).

It’s widely believed that a moderate rep-range (6-12) is your best bet for achieving optimal muscle hypertrophy [3,4]. This has a lot to do with the metabolic stress your body is under when using this level of intensity. The metabolites your body builds up (blood lactate, intramuscular lactate, glucose and glucose-6-phosphate) aid significantly in anabolic processes.

Takeaway: sticking to a moderate rep-range places your muscles under the optimal amount of tension with the optimal load and fires our anabolic responses at the chemical level. Equaling muscle growth. For optimal tension, lift heavy weights and take them through a full range of motion.

Tip: If you’re able to do 12+ reps easily with a weight, it’s probably too light for you. Overload is key to mechanical tension. You should always be pushing yourself and increasing your weights as you get stronger.

Muscle Damage

When we lift weights, disturbing the integrity of our muscles (tension), we’re creating microtears and damage to our muscle fibers. This damage generates an inflammatory response that’s been shown to start hypertrophic processes, such as the release of growth factors and hormones [1].

Basically, our body releases immune cells to repair the muscle and prepare it to better handle future damage. This is where the money is – this is where the growth happens.

Given that muscle damage is brought about primarily by eccentric exercise, this reinforces the need to emphasize the eccentric portion of each lift.

This phase is when a muscle contracts while lengthening. For example, the lowering portion of a biceps curl and lowering the bar to your chest during bench-press. During this portion of the lift you want to move slow and controlled.

Takeaway: Lift enough weight enough times to create muscle damage, and then allow your body to do its magic and heal up.

Tip: DOMS (Delayed Onset Muscle Soreness) is caused by those tiny microtears. It’s a general rule that you shouldn’t work a muscle again for around 48 hours or until you’re no longer sore. Extreme soreness indicates the muscle hasn’t repaired yet, meaning it hasn’t had the time to completely heal and grow.

Metabolic Stress

Metabolic stress, “a pump,” happens when we keep constant tension on our muscles and our cells swell with fluid [1]. Because of this swelling, protein synthesis increases and leads to greater growth. In layman’s terms, cells fill with fluid, expand as a defense mechanism, and grow.

Studies show that training to failure on some lifts is the ultimate way to achieve peak metabolic stress [4]. This means pushing your muscles just one more rep when you feel like giving in.

Takeaway: Because they must adapt beyond homeostasis, muscles grow when we put them under stress.

Tip: Utilize drop sets, pause sets, and low rest periods (60-90 seconds) to achieve optimal stress.

Other findings

  • Keep volume high. It’s consistently been shown to achieve better muscle growth results than single sets [4]. For this reason, split-routines (as opposed to full-body) may be better suited for muscle growth and packing on size.
  • Move in many different ways. Our muscles have multiple attachments points and often connect to different joints. Meaning in order to achieve optimal results and activate entire muscles, we need to switch things up. Think varying angles of pull, different grips, differing positions (sitting, standing, kneeling, etc.).
  • Hormones matter. Testosterone, produced in the testes, ovaries, and adrenals, increases muscle mass by increasing muscle protein synthesis [5]. IGF-1, a peptide hormone similar to insulin increases the rate of protein synthesis and stays elevated for up to 72 hours post exercise. It’s believed to “kick start” muscle hypertrophy [6].
  • Nutrition, gender, hormones, age, sleep, and experience all come into play as well.

If your goal is muscle hypertrophy, use heavy weights, utilize volume, take short rest periods, move in a variety of ways, and eat well.


  1. Schoenfeld, BJ. The mechanisms of muscle hypertrophy and their application to resistance training. Journal of Strength and Conditioning, 2010; 24(10): 2857–2872
  2. Fry A. The role of resistance exercise intensity on muscle fibre adaptations. Sports Medicine,2004; 34(10): 663-679.
  3. Willardson, JM. The application of training to failure in periodized multiple-set resistance exercise programs. J Strength Cond Res
    21: 628–631, 2007.
  4. Hansen, S., Kvorning, T., Kjaer, M. et al. The effect of short-term strength training on human skeletal muscle: the importance of physiologically elevated hormone levels. Scandinavian Journal of Medicine & Science in Sports, December 2001; 11(6): 347-354
  5. Krieger, JW. Single vs. multiple sets of resistance exercise for muscle hypertrophy: A meta-analysis. Journal of Strength Cond, 2010; 24: 1150–
  6. Griggs RC, Kingston W, Jozefowicz RF, Herr Be, et al. Effect of testosterone on muscle mass and muscle protein synthesis. American Physiological Society, 1985 January; 66(1): 498-503
  7. McKay B., O’Reilly C., Phillips S., Tarnopolsky M., et al. Co-expression of IGF-1 family members with myogenic regulatory factors following acute damaging muscle-lengthening contractions in humans. The Journal of Physiology, November 2008; 586(Pt 22): 5549-5560

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