Muscle Hypertrophy (Growth) Explained To You Like You’re A 5-Year-Old

arms and back

Pixabay / Łukasz Dyłka

It’s common knowledge that lifting weights will build muscle mass – but few lifters understand exactly how it works. To get the most out of your sessions, and trigger as much muscle growth as possible, it’s crucial to understand hypertrophy – the science behind building muscle.

We’ll cover a few complicated concepts, and a few more big words, but don’t worry: I’ll explain it to you like you’re a (weirdly muscular) 5-year old.

1. Muscle is made up of fibers

Muscle is made up of a ton of thick fibers, bound together into tight bundles of tissue. Within each muscle, there are two types of fiber – one designed for endurance, and one designed for strength. The size of your muscles is dependent on the size of these muscle fibers, and by causing growth of the fiber, you’ll see gains in muscle mass.


Slow-twitch fibers aren’t very strong, but they have incredibly high endurance. Surrounded by tons of blood vessels, these muscle fibers have a high supply of oxygen, making them extremely resistant to fatigue.


Fast-twitch fibers are pretty much the opposite – extremely powerful, but incredibly easy to fatigue. Whilst slow-twitch fibers can create a weak movement for a long period of time, fast-twitch fibers will create a powerful movement, but for an extremely short period of time.

2. Exercise stresses the muscle fibers

Exercise causes stress to the body, and triggers it to change these fibers, and increase the size, strength and endurance of your muscles to make future exercise easier. There are 3 ways to trigger changes to your muscle:

Progressive tension overload

‘Tension’ refers to the length of time we use our muscles; and by gradually increasing the weight and duration of our exercises, we trigger the muscles to adapt.

Muscle fiber damage

Exercise can damage the muscle, triggering the body to repair and strengthen the affected muscle fibers.

Cellular fatigue

By pushing a muscle to its limit, we exhaust all of the muscle fibers, and cause muscle adaptation.

3. Different types of stress cause different types of muscle growth

Weightlifting can triggers muscle growth through all 3 of these mechanisms; but your body’s exact response will vary according to the type of exercises you perform.

High-rep, low weight exercise

This type of session is popular with lots of weightlifters, and requires lifting a low-weight (about 50% of your personal best), for sets of 10-20 reps. This type of lifting requires a ton of endurance, but relatively little strength – and uses a small amount of fast-twitch muscle fiber, and a high amount of slow-twitch fiber. This exhausts your muscles, and causes cellular fatigue.  In response, your body triggers a type of muscle growth known as sarcoplasmic hypertrophy, increasing the size of the muscles by flooding them with energy-rich fluid to boost performance.

Low-rep, high weight exercise

Lifting heavier weight (anything above 70% of your personal best), for sets of 1-8 reps, requires high strength and low endurance – causing your body to use your fast-twitch fibers. Instead of triggering cellular fatigue, this type of exercise causes muscle fiber damage, and progressive tension overload. As a result, the type of change your body makes is different: causing myofibrillar hypertrophy. This causes the body to increase the size of your muscles by making your muscle fibers bigger and stronger.

 4. Train for the right type of muscle growth

Both types of training routine cause muscle growth – but by performing lots of high-rep exercises, with relatively light weight, the increase in the size of your muscle isn’t permanent.  Sarcoplasmic hypertrophy is also known as a pump – a temporary increase in muscle size that disappears after a day or two. Whilst this type of training routine will make you look great in the gym, it won’t have much of an impact on actual muscle growth.

True muscle growth is caused by the second type of hypertrophy – myofibrillar. Instead of filling the muscle with fluid for a day or two, this type of growth actually increases the size of your muscle fibers. Higher weight exercises are much better at triggering this type of growth, and whilst you’ll see less of a pump from this style of training, your long-term muscle growth will be much higher.

5. Progressive overload keeps your muscles growing

These changes only occur when the body is challenged by exercise. Curling a 20lb dumbbell for the first time will trigger just enough growth to make curling that same dumbbell easier the next time. If you use the same weight for all of your sessions, the body quickly adapts to it; and because your muscles are now capable of lifting 20lbs, there’s no reason to continue growing.

Continued muscle growth requires progressive overload – a sustained increase in the difficulty of your workouts. By making your exercises harder, at the same rate that your body adapts to each workout, you’ll force your body into continually building muscle. When you find curling a 20lb dumbbell relatively easy, increasing the weight to a 22lb dumbbell will continue to challenge your body, and encourage further muscle growth.

This rate of progression needs to be slow and steady. Muscle growth is a slow process, and your body won’t be able to adapt to an increase in weight every week. Make sure that you’re capable of performing each set with perfect form, before moving on to a higher weight – and even then, an increase of a few pounds will be enough to trigger further muscle growth.

6. Try my hypertrophy workout!

Optimum muscle growth requires heavy weight, relatively low reps, and progressive overload – but what does this look like in practice? To get you started on the optimum muscle building path, I’ve created a sample workout. It follows all the principles of we’ve outlined here, and it’s designed to trigger as much muscle growth as possible. Remember to warm-up properly before each working set, and aim to increase the weight every 2-3 weeks.

Day 1

Barbell Bench Press, 3×5, Heavy

Military Press, 3×8, Moderate

Barbell Rows, 3×5, Heavy

Tricep Pushdowns, 3×8

Dumbbell Curls, 3×8

Day 2

Squats, 3×5, Heavy

Deadlift, 1×5, Heavy

Leg Press, 3×8, Moderate

Calf Raises, 3×8, Moderate

Day 3

Military Press, 3×5, Heavy

Weighted Pull-Ups, 3×8, Moderate

Seated Rows, 3×8, Moderate

Close-Grip Barbell Bench Press, 3×8

Barbell Curls, 3×8