How Do Muscles Grow? The Science Behind Muscle Growth

If you’re working out with weights at the gym then generally you’re there not just to lose fat, but to gain muscle, particularly if you’re a guy.

Although the term ‘muscle’ can refer to organs such as the heart (cardiac muscle) we are of course talking about the skeletal muscles that surround our bodies, protect our internal organs, and help us to achieve functional movements, such as running, lifting and jumping.

In order to grow muscle effectively, and ensure your hard work in the gym isn’t wasted, it’s worth knowing and understanding the science behind muscle growth for both men and women.

So, how do muscles grow?

The Science Behind Muscle Growth

Shirtless guy working out with a green kettlebell

Muscles grow when we continually challenge them to lift, push, or pull higher levels of resistance or weight, and this process is known as ‘muscle hypertrophy’.

When we lift heavier and harder than our muscles our used to, we cause trauma to those muscles creating small tears in the muscle tissue. After our workout, our bodies begin to repair or replace those damaged tissue fibres through a cellular process – fusing muscle fibres together to form new muscle protein strands or ‘myofibrils’. These new myofibrils increase in number and thickness to create what is known as muscle hypertrophy (growth).

This muscle growth occurs when the rate of protein synthesis is greater than the rate of muscle protein breakdown. Growth however doesn’t happen when you’re in the gym lifting weights, rather while you are resting.

So, How Do You Add Muscle To Your Muscles?

When we create damage to our muscles through progressive resistance training we activate satellite cells which act like stem cells, helping to add more nuclei to the muscle cells and contribute directly to their growth. How effectively our bodies are able to activate these satellite cells to repair and grow our muscle tissue may be the difference between what allows some people to grow huge muscles and others to become ‘hard-gainers’ and see little to no progress.

In which case the question then becomes, how do we activate these satellite cells most effectively to increase muscle growth?

The Main Components For Muscle Growth


In order to produce the damage to our muscles required to activate satellite cells and repair and replace muscle tissue bigger and stronger than before, we need to apply a lot of stress which is greater than what our bodies and muscles have previously adapted to.

We do this through lifting progressively, i.e. gradually and continuously increasing the weight we lift so we are always adding additional tension. This added tension will help to promote muscle hypertrophy.


When we create muscle hypertrophy it means we have provided enough stress to our muscles to create damage to the fibres, which then need to be repaired and replaced. Localised muscle damage caused by resistance training leads to the release of inflammatory molecules and immune system cells that activate the satellite cells to spring into action.

If you’ve ever felt sore after a workout then you’ve experienced muscle damage from working out, although it’s important to add here that you don’t need to experience soreness for this to happen.


Certain hormones created by our bodies also contribute to muscle growth and repair, including testosterone, HGH (human growth hormone), and insulin growth factor. They work by:

  • Activating satellite cells
  • Stimulating anabolic hormones to promote muscle growth and protein synthesis
  • Enhancing muscle tissue growth
  • Improving how the body processes proteins
  • Preventing the breakdown of protein

Some people choose to increase the level of these growth hormones within their body, especially if their levels are low, by taking supplements. Generally however, your body should produce enough of these hormones naturally when stimulated through resistance training and a healthy, balanced diet and good nutrition.

Is There A Difference Between Males And Females When It Comes To Muscle Growth?

A woman facing away holding a weighted barbell over her shoulders

The rate at which it is possible to grow muscle depends on a variety of factors, regardless of biological sex, including the levels of testosterone and estrogen in the body, as well as genetics.

Both men and women can have the following body shapes, which require different approaches to building muscle:

  • Mesomorphic – Generally athletic, solid and strong, they can both gain weight and lose weight without much effort and typically build muscle faster than other body types
  • Ectomorphic – Tend to be long and lean with little body fat and little muscle. They usually have a hard time gaining weight.
  • Endomorphic – Typically have lots of body fat, lots of muscle, and gain weight easily. They’re usually of a heavier, rounder body shape.

Saying this, there are several traits that are more pronounced in males than females that support faster muscle growth. These include higher levels of testosterone, tighter joints, and larger muscle mass which is why it is generally easier for men to gain muscle than it is for women.

The Importance Of Rest

Often overlooked, rest plays an essential part in building muscle. If we don’t let our muscles rest then we reduce our ability to repair the damaged tissue. Lack of rest also inhibits our progression in terms of fitness levels and also increase our risk of injury.

Getting sufficient sleep is also important as lack of sleep can decrease protein synthesis, contribute to loss of muscle mass and restrict muscle recovery. Sleep also helps reduce the levels of cortisol in our bodies – a stress hormone which can have a negative impact of muscle growth.

Diet And Muscle-Building

Eating a healthy, balanced diet that is high in protein is key to building muscle mass. Whist the level of protein an individual should consume differs from person to person, it is recommended to consume around 1.2-1.7 grams of protein per kilogram or bodyweight, or 0.5 – 0.8 grams per pound of bodyweight.

Sources of protein include:

  • Poultry
  • Fish
  • Eggs
  • Milk and cheese
  • Beans and lentils
  • Nuts and seeds
  • Soybeans and tofu

If you want more detail around the importance between nutrition and building muscle mass, as well as some of the most protein-dense foods to include in your diet to optimise your results, take a look at my article on The Best Foods to Gain Muscle.

Growing Muscle In Older Age

As we get older, so our risk of reduced mobility and other musculoskeletal problems increases. We also start to lose muscle more easily due to all sorts of environmental and genetic factors including changes to our muscle cells, oxidative damage, decreased hormone levels and typically being more sedentary (sitting still for longer).

That’s not to say that this can’t be reversed and that you can’t continue to build muscle way into your golden years. It is perhaps even more important to strength train as we get older, to both maintain and increase muscle mass. I’ve written more on this subject in a previous article – Building Muscle Mass After 50 – so feel free to check it out.

It’s Easier When You Know How

Graphic listing five main factors in muscle growth

In order to grow muscle we must force them to adapt under greater stress – progressively and consistently over time – through resistance training. This doesn’t just have to be heavier weights, it could also be through increased reps or by constantly changing up our exercises so we continue to push our muscles to fatigue. Add to this adequate rest and a good, balanced, high protein diet and you have all the main elements you need to build muscle.

Do you struggle to build muscle mass, or have you always found it easy to get bigger? What tips or advice would you give to hard-gainers or those just starting out? Comment underneath and let me know, and if you have any questions about what you’ve read, get in touch and I’ll be only too happy to help you out!


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