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How Do Muscles Grow? The Science of Muscle Growth

By John Leyva / December 31, 2018

If you’re a guy in the gym working with weights, not only are you probably trying to lose some fat, but also gain some muscle.

This article discusses the mechanisms of how muscles grow, plus why most women won’t gain large amounts of muscle when working with weights.

Although there are different types of muscles, such as cardiac muscle (your heart), for our concerns, we will talk exclusively about skeletal muscles. Skeletal muscle is composed of thread-like myofibrils and sarcomeres that form a muscle fiber and are the basic units of contraction.

The 650 skeletal muscles in the human body contract when they receive signals from motor neurons, which are triggered from a part of the cell called the sarcoplasmic reticulum. Motor neurons tell your muscles to contract and the better you become at having those signals tell your muscles to contract, the stronger you can get.

When someone like a powerlifter is able to lift very heavy weight despite not looking very muscular, it’s due to their ability to activate those motor neurons and contract their muscles better. This is why some powerlifters can be relatively smaller compared to bodybuilders, but can lift significantly more weight. Motor Unit recruitment also helps to explain why, after practice, certain movements become easier to perform and most of the initial strength gains will be when you first start to lift weights. Muscle growth tends to occur more steadily after this initial period of strength gain because you are more easily able to activate the muscles.

The Physiology Of Muscle Growth

After you workout, your body repairs or replaces damaged muscle fibers through a cellular process where it fuses muscle fibers together to form new muscle protein strands or myofibrils. These repaired myofibrils increase in thickness and number to create muscle hypertrophy (growth).1 Muscle growth occurs whenever the rate of muscle protein synthesis is greater than the rate of muscle protein breakdown. This adaption, however, does not happen while you actually lift the weights. Instead, it occurs while you rest.

So how do you actually add muscle to your muscle cells? This is where Satellite cells come in and act like stem cells for your muscles. When activated, they help to add more nuclei to the muscle cells and therefore contribute directly to the growth of myofibrils (muscle cells). Activating these satellite cells may be the difference between what allows certain “genetic freaks” to grow massive muscles and what makes other people “hard-gainers.2

In one of the most interesting studies in the past 5 years, researchers showed that those who were “extreme responders” to muscle growth, with an incredible 58% myofiber hypertrophy from an exercise, had 23% activation of their satellite cells. Modest responders, who had a 28% growth, had 19% activation of their satellite cells. What is interesting to note, though, is that some people known as “non-responders” in the study had 0% growth and had a concurrent 0% activation of their satellite cells. Therefore, it seems the more you can activate these satellite cells, the more you’ll be able to grow. So then the question becomes, how do you activate these satellite cells to increase muscle growth?

3 Mechanisms That Make Muscles Grow

Underlying all progression of natural muscle growth is the ability to continually put more stress on the muscles. This stress is a major component involved in the growth of a muscle and disrupts homeostasis within your body. The stress and subsequent disruption in homeostasis causes three main mechanisms that spur on muscle growth.

1. Muscle Tension

In order to produce muscle growth, you have to apply a load of stress greater than what your body or muscles had previously adapted too. How do you do this? The main way is to lift progressively heavier weights. This additional tension on the muscle helps to cause changes in the chemistry of the muscle, allowing for growth factors that include mTOR activation and satellite cell activation.3

Muscular tension also most dramatically effects the connection of the motor units with the muscle cells. Two other factors help to explain why some people can be stronger, but not as big as other people.

2. Muscle Damage

If you’ve ever felt sore after a workout, you have experienced the localized muscle damage from working out. This local muscle damage causes a release of inflammatory molecules and immune system cells that activate satellite cells to jump into action. This doesn’t mean that you have to feel sore in order for this to happen, but instead that the damage from the workout has to be present in your muscle cells. Typically soreness is attenuated over time by other mechanisms.

3. Metabolic Stress

If you’ve ever felt the burn of an exercise or had the “pump” in the gym, then you’ve felt the effects of metabolic stress. Scientists used to question bodybuilders when they said the “pump” caused their muscles to become larger. After more investigation, it seems as though they were onto something.

Metabolic stress causes cell swelling around the muscle, which helps to contribute to muscle growth without necessarily increasing the size of the muscle cells. This is from the addition of muscle glycogen, which helps to swell the muscle along with connective tissue growth. This type of growth is known as sarcoplasmic hypertrophy and is one of the ways that people can get the appearance of larger muscles without increases in strength.

So now that you know the three main mechanisms of muscle growth, the next question is: how do hormones affect muscle growth?

How Do Hormones Affect How Muscles Grow?

Hormones are another component largely responsible for muscle growth and repair because of their role in regulating satellite cell activity. Insulin Growth Factor (IGF)-1, in particular Mecho-Growth Factor (MGF) and testosterone are the two most vital mechanisms that promote muscle growth.4

Testosterone is the main hormone that most people think about when working out with weights, and there seems to be some validity to the thought that testosterone increases protein synthesis, inhibits protein breakdown, activates satellite cells, and stimulates other anabolic hormones. Although most testosterone is bound in the body and therefore not available to use (up to 98%), strength training seems to help not only release more testosterone, but also make the receptors of your muscle cells more sensitive to your free testosterone. Testosterone can also stimulate growth hormone responses by increasing the presence of neurotransmitters at the damaged fiber site, which can help to activate tissue growth.

The IGF regulates the amount of muscle mass growth by enhancing protein synthesis, facilitating glucose uptake, repartitioning the uptake of amino acids (the building blocks of protein) into skeletal muscles and once again, activates satellite cells to increase muscle growth.

Why Muscles Need Rest To Grow

If you do not provide your body with adequate rest or nutrition, you can actually reverse the anabolic process and put your body into a catabolic or destructive state. The response of muscle protein metabolism to a resistance exercise bout lasts for 24-48 hours; thus, the interaction between protein metabolism and any meals consumed in this period will determine the impact of the diet on muscle hypertrophy.5 Keep in mind there is a certain limit on how much your muscles can actually grow dependent on gender, age, and genetics. For instance, men have more testosterone than women, which allows them to build bigger and stronger muscles.

Why Rapid Muscle Growth Is Unlikely

Muscle hypertrophy takes time and is relatively slow for the majority of people. People will generally not see visible growth for several weeks or months as most initial changes are due to the ability of your nervous system to activate your muscles.

In addition to that, different people have different genetics, which range from hormonal output, muscle fiber type and number, along with satellite cell activation, that can all limit muscle growth. To ensure you’re doing your best to grow muscle, muscle protein synthesis must exceed muscle protein breakdown. This requires that you take in an adequate source of protein (especially essential amino acids) and carbohydrates to help facilitate the cellular process of rebuilding broken down muscle tissue. Visible muscle growth and evident physical changes in your body’s muscle structure can be highly motivational which is why understanding the science behind how muscles actually grow is important.

How Muscles Grow: Conclusion

For muscle breakdown and growth to occur you must force your muscles to adapt by creating stress that is different than the previous threshold your body has already adapted to. This is can be done by lifting heavier weights, continually changing your exercises so that you can damage more total muscle fibers and pushing your muscles to fatigue while getting a “pump.” After the workout is completed, the most important part begins which is adequate rest and providing ample fuel to your muscles so they can regenerate and grow.

If you want an easy-to-follow program to lose fat and build muscle, check out my 12-Week Body Transformation Program.

Have any questions about how to get muscles to grow? Leave a comment below.

Show 5 References

  1. Young sb Kwon, M. a. (2004). How do muscles grow?
  2. Petrella JK, Kim JS, Mayhew DL, Cross JM, Bamman MM. Potent myofiber hypertrophy during resistance training in humans is associated with satellite cell-mediated myonuclear addition: a cluster analysis. J Appl Physiol. 2008;104(6):1736-42.
  3. Schoenfeld BJ. The mechanisms of muscle hypertrophy and their application to resistance training. J Strength Cond Res. 2010;24(10):2857-72.
  4. Kraemer WJ, Ratamess NA. Hormonal responses and adaptations to resistance exercise and training. Sports Med. 2005;35(4):339-61.
  5. Tipton KD, W. E. (2001). Exercise, protein metabolism, and muscle growth. Int J Sport Nutr Exer Metab, 109-32,.


  • Dr. Jason Ong says:

    Hi John, May i seek your permission to use the pictures for my book on medicated oil. Thank you.

    • Kristin says:

      Hi Dr. Jason,

      We purchased the image on a stock photography website, and we don't have a link to the image. If you like that image, we recommend finding a similar one on a stock photography website and purchasing it for use in your book. Hope that helps!

      -Kristin, BuiltLean Coach & Managing Editor

      • zach redick says:

        OK THAT GOOD

  • zach redick says:

    this is going to help you grow

  • Normand Boccanfuso says:

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    • Kristin Rooke, CPT says:

      Hey Normand,

      Really glad to hear that you and your buddies have been able to apply the lessons, information, and workouts you've learned on BuiltLean! We're purposeful about providing you with the most relevant, scientifically-proven strategies to get lean & strong. Thanks for sharing, and we hope you and your buddies keep reading.

      -Kristin, BuiltLean Coach & Managing Editor

  • Olufeeky says:

    Anytime I wake up, I usually notice too much muscle on my legs and hands, do I have to reduce my exercise which I normally do then? Please, is there any side effect and if there is, what is the way out? Thanks

    • Kristin Rooke, CPT says:

      Hi Olufeeky, I'm not sure I understand what you have going on. Do your legs and hands look bigger in the morning, and then smaller later in the day? If that's the case, it sounds like you might be experiencing edema. Edema is swelling caused by excess fluid trapped in your body's tissues, usually due to injury or inflammation. However, I'm not a medical professional, and as such, am not allowed to diagnose symptoms or suggest treatments. I would recommend seeing your primary care doctor to discuss what you're experiencing, and to determine the cause and any potential negative side effects. I hope that helps! If you have more questions, feel free to reach out to support@builtlean.com.

      -Kristin, BuiltLean Coach & Managing Editor

  • Dhananjay says:

    How much time does it take to see noticeable muscle growth from exercise?

    • Kristin Rooke, CPT says:

      That's a great question. Honestly, it varies from person-to-person, and depends on factors such as your genetics, your current fitness level, your workout program, and your nutrition. Because of genetics, some people put on muscle more easily than others. And if you're new to exercise, you can expect to see muscle growth and development within 4 weeks of starting a strength training program.

      All that said, if you follow an effective muscle building workout program while eating slightly more calories than you burn every day, you should be able to build muscle. Under optimal conditions, you can gain up to around 1-2 lbs of muscle per month.

      -Kristin, BuiltLean Coach & Managing Editor

  • Gayle Ellenbee says:

    I love the efforts you have put in this, appreciate it for all the great blog posts.

  • luwy says:

    great post for guys aiming to build sweet muscles.

    • Kristin Rooke, CPT says:

      Glad you found it helpful!
      -Kristin, BuiltLean Coach & Managing Editor

  • Rachel Shorland says:

    I want to build muscle and tone but because Iv had Fibromyalgia, (this is not a question about curing FM, I'm in that faze) ....I find if I try any resistance exercise, specifically strength training, weights no weights, Pilates, yoga etc all cause a flare up and malaise.
    Iv built up a running programme which I react well to (endorphins mainly) I think! The very 'stress' you have to put on a muscle to make it 'grow, triggers my body into FM! I can't find ANY info on what I CAN do or if it's actually possible to 'grow' them! I wonder if you have any experience in this or theories?
    Thank you

    • Kristin says:

      Hi Rachel - your story is amazing, and it's awesome that you're starting to find exercises that work for you. I would recommend chatting with your primary care doctor, or a physical therapist to determine the best way for you to start building strength. A physical therapist (especially one who has experience with FM) should be able to help you design a progressive workout program to build your body's tolerance of strength training.

      Also, you might want to start with less. If a whole strength training session causes a flare up, then do less. For example, do 1 set each of 10 push ups, 10 bodyweight squats, and 10 cross body crunches. If you don't experience a flare up, stick with that bare-bones program for about 4 weeks. Then add on, increasing it to 2 sets, etc.

      Again, consult with your primary care doctor or a physical therapist before starting any new workout program. That's incredibly important. That said, good luck & keep us posted on your progress!

      -Kristin, BuiltLean Coach & Managing Editor

  • Rachel Shorland says:

    THANKYOU for your reply! Iv been trying to find a strength trainer who's experienced with FM but no joy! Iv worked it all out myself so far (Iv got a good supportive Doc in the back ground) hence the running and I surf!! I think your right, I'm going to have to break it down and start with just a few reps and build up mega slow! I guess I just want to know if it will actually do anything! I.e build muscle tone .. eventually! Or at all! Or if it's just not! FM advice is so vague there's such a huge gap I can't find anything concrete! Maybe it's because there isn't any!
    I'm a sports therapist so have a real anatomy interest in this!
    I'll keep searching!
    Thank you ?

  • Patrick says:

    Will working out for longer work instead of heavier and heavier weights?

    • Marc Perry, CSCS, CPT says:

      Possibly, but to maximize your results, both exercise volume and progression (lifting heavier weights or using more challenging exercises) are important. You can choose harder exercises which add more stress to the muscle instead of using heavier weights.