How Do Muscles Grow? The Science of Muscle Growth

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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 nuclie 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.

Muscle Growth Mechanism #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.

Muscle Growth Mechanism #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.

Muscle Growth Mechanism #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.

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

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,.
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17 Comments on “How Do Muscles Grow? The Science of Muscle Growth

  1. Rajesh
    September 17, 2013 #

    What an article. I am sure to not find a similar one in the whole internet.

    I understand, muscle cells breaks down during work out and they rebuild later, ensuring muscle growth. My question is – Still, only when we (at least for me) work out, the working muscles seems to be in its best shape and bigger.,
    for eg. on doing push ups, one can immediately feel some expansion on the chest.
    My biceps looks big and toned, only when I am working out for biceps, which is same for all my body parts.
    Why this, if the muscles are only breaking down at this point, while working out?

    forgive me, I am not a native English speaker, if I havent put this question across, very well.

    1. September 18, 2013 #

      @Rajesh – The reason the muscle looks bigger during the workout is because of the increased blood flow to the muscle while you’re working out. This is the “pump” factor that I discussed in the article can definitely make the muscle look and feel noticeably larger during and immediately following a workout.

      1. Rajesh
        September 19, 2013 #

        Thank you. got it.

  2. Ann Gales
    September 17, 2013 #

    Excellent article which summarises it all well. Now just got to go and build that muscle…

  3. Matt
    September 18, 2013 #

    Could you be more explicit about this ‘pump’ that occurs? I really have no idea what this is referring to.

  4. Justin
    September 19, 2013 #

    John, great article! Will combining weight lifting in the morning and cardio exercises in the afternoon prevent me from getting the necessary rest? In addition, will working out through slight soreness help alleviate and expidite my recovery or hinder it? Thanks.

    1. September 20, 2013 #

      @uncandego – The first question that really needs to be addressed is if you’re taking in enough protein in order to repair the muscle tissue that’s being broken down through your workouts. In addition to that, the second question is how’s your sleep and stress levels. Third is really are you taking in enough calories overall. If you’re not taking in enough protein, your body will have a hard time recovering from the workouts, whereas if you’re stress is high or sleep is poor, you can have a hard time utilizing the protein’s benefits for your muscles. If you’re not taking in enough calories, and more carbs will help here if you’re not primarily working on losing weight, then again, your body may not be utilizing the protein effectively to help you recover. The epsom salt baths should help as it allows for an increase in magnesium which has been shown many people are deficient in.

      Sometimes though a week off can help the body’s response to exercise as it not only rests your muscles but nervous system. If all of those things are in line, then you might want to consider taking some BCAA’s before and during your workout. (You can find more info here: http://www.builtlean.com/2013/06/12/bcaa-supplement/ ) If that doesn’t help, I would add a multi-vitamin with enough of B-complex as some people have a hard time utilizing BCAA’s effectively if they are deficient in some of the B-Vitamins. Hope that helps.

      1. September 20, 2013 #

        Great answer, John.

    2. September 20, 2013 #

      @Justin – Glad you liked the article. Breaking up the weight lifting in the morning and cardio in the afternoon should be sufficient time for you to rest and recover. This is really dependent though on how hard you’re pushing the weights and how intense the cardio is. If both are intense multiple times per week, you may have a hard time recovering consistently and might need to take an “easier” week every once in a while.

      As for working out through slight soreness, you should be able to workout at a moderate level and that will help to speed up the recovery process, but too intense and it might hinder it.

      This though is not a black and white answer as there are other factors that come into play that include the intensity of the workouts, your workout experience, your nutrition, genetics and overall goals. For example, if you’ve been working out hard for years and are taking in enough protein/calories, and have trained yourself to work the same body part multiple times per week, you might be able to recover faster and without soreness after a few weeks. An example would be someone working construction might be sore the first day or few weeks, but after a while their body can adjust to the work load. On the other hand, if you’re not taking in enough calories/protein and are a relative novice, there might not be much benefit to the extra work. If you have any other questions, let me know. Thanks.

  5. uncadonego
    September 20, 2013 #

    Once every few months, I just can’t shake the muscle soreness. It can actually get to the point where it’s hard to fall asleep. Three or four sessions a week is fine most of the time, but I just finished taking seven straight days off of weight lifting and feel much better now, and I’m ready to start again. Epsom salt baths help a lot.

    Is this normal?

    1. September 20, 2013 #

      That’s actually not normal. It could be from a number of things including dehydration to some type of vitamin/mineral deficiency. John should be able to hop in to the conversation and give you some more ideas.

    2. September 20, 2013 #

      See John’s response to your question above.

  6. uncadonego
    September 20, 2013 #

    Thanks for the response John. :D

  7. Zach
    September 21, 2013 #

    Great article, never found before such detail and concise text. Thank you. I am curious to know how it happens when we stop the workouts. I recently had to stop them for 20 days due to a liver issue, which is now solved. I noticed that after three days of lighter workout than I was used to, I was again in shape to perform the same weight and workout routine as before. How this would work for longer periods, will the muscle gain by muscle damage revert completely if we stop excercising?

    1. September 29, 2013 #

      @Zack – This question really varies from individual and is based on a number of factors that can influence how quickly you lose strength, including muscle fiber type and recruitment, your absolute strength levels, your overall experience, if you were over-training before the layoff, etc.
      In addition to that, strength levels do tend to decline rather quickly, but research has shown that you can stave off most decreases in strength by performing at least one workout per week, where you work up to at least 80% of your 1 RM for as many reps as possible. This helps to maintain strength levels (or 90%) for up to a month. If you’re not going to be doing anything, then those other factors really come into play. If you’re good at accessing the muscle fibers, then you will be better at maintaining your strength levels and that’s usually associated with how long you’ve been working out. If you have average or slightly above average strength levels, then you should be able to maintain most of your strength for 2-3 weeks without too much of an issue after a couple of workouts. A longer time period than that and your strength levels will tend to drop more significantly.

      Muscle loss though is based on different factors and is usually associated with how much muscle you have (if you have a lot, it might be harder to maintain it), your diet while not working out (the higher the protein and carbs, usually the less you’ll lose), your baseline hormonal levels and overall stress on your body. I know people that have taken years off and bounce back within 2-3 months to where they were years ago, whereas other people take 2 months off and it takes them 6 months to get them back to where they were previously.

      1. Zach
        September 30, 2013 #

        Then it depends on each person. Just to share an interesting fact: I almost never feel muscle pains. That is, I feel that normal ache during the sets with freeweights and when reaching failure I feel the muscles burn. I usually do all the sets until failure, but while a friend next day cannot move his worked out muscles, I feel absolutely nothing. It is as if I had not done the workout the day before. I think this has to do with metabolism. John, thank you for the insight. This is among the best websites on the subject. Really good technical stuff.

  8. Deepak
    September 23, 2013 #

    Excellent article John..Thanks for sharing your inputs…Great job Marc with BuiltLean, Keep up the good work !

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