Hypertrophy – Do You Actually Know What It Is?
No? Well don’t worry, you would be surprised how many gymgoers don’t. For such a popular and important word in fitness, it’s very under-explained online. Don’t worry, by the time you’ve finished reading this, you’ll know how it all works, and learn some big words to look like the Einstein of your gym.
What Hypertrophy Actually Is, And What It Isn’t
Your muscles do not grow because of hypertrophy, hypertrophy is the sciency word for muscle growth. If you broke it down to its simplest explanation, hypertrophy is an increase in the cross sectional area (CSA) of your existing muscle fibres – it makes them bigger and stronger.
Now some technicalities to help explain things: Just one singular muscle fibre (myocyte) contains thousands and thousands of, essentially, microscopic rods called myofibrils. The myofibrils themselves are made up of sarcomeres, which are lined end to end to make up these myofibrils. Sarcomeres are the fundamental bedrock of your muscles as they are a contractile unit of striated muscle tissue, they contain the primary contractile proteins of actin (the thin filaments) and myosin (the thick filaments) which allow a muscle to contract and generate a force.
Hypertrophy can be split into two different types:
Myofibrillar Hypertrophy
Sarcoplasmic Hypertrophy
This increases the size and number of the myofibrils, which adds more actin-myosin bridges which directly increases the muscles maximum force production – simply put it makes you stronger.
This expands the non-contractile parts of the sarcoplasm, namely the surrounding fluid, glycogen stores, sarcoplasmic reticulum and the organelles that surround the myofibrils. This will increase the overall volume of the muscles, but does not contribute to force production – it makes your muscles bigger, not stronger.
This is where your training programme becomes vital, as myofibrillar and sarcoplasmic hypertrophy are caused by two completely different types of weight training.
Myofibrillar Hypertrophy
Sarcoplasmic Hypertrophy
Training Stimulus: Heavy loads (greater than 80–85% of your 1-rep max, typically in the 1–6 rep range) with longer rest periods (3–5 minutes) to maximize force on every set.
The Effect: High force deforms mechanical sensors on the muscle cell membrane (costameres) and inside the sarcomere (titin). This physical strain triggers robust mTORC1 signaling, signaling the cell nucleus to transcribe more actin and myosin RNA.
The Outcome: New sarcomeres are synthesized and layered side-by-side within myofibrils. This increases the density of cross-bridges that can form during a contraction, maximizing muscle density and peak strength.
Training Stimulus: High volume with moderate-to-light loads (60–75% of 1-rep max, typically 8–20+ reps) and shorter rest intervals (30–90 seconds). .
The Effect: Sustained muscle contractions compress local capillary beds, restricting oxygen (hypoxia) and preventing the removal of metabolic byproducts like lactate, inorganic phosphate and hydrogen ions. .
The Outcomes: Osmotic Cell Swelling – The accumulation of metabolites draws extracellular water directly into the myocyte. The cell senses this internal fluid pressure as a structural threat and adapts by expanding its volume. Glycogen Supercompensation – Depleting local glycogen forces the cell to adapt by expanding its glycogen storage capacity. Because 1 gram of glycogen binds roughly 3 to 4 grams of water inside the cell, storing more glycogen causes significant sarcoplasmic swelling. Organelle Expansion – To handle high-volume calcium recycling and rapid anaerobic energy production, the cell expands its sarcoplasmic reticulum and glycolytic enzyme concentration.
| Myofibrillar Hypertrophy | Sarcoplasmic Hypertrophy | |
| Primary Driver | Peak Mechanical Tension | Metabolic Stress & Glycogen Depletion |
| Cellular Expansion | Actin & Myosin filaments | Glycogen, water, SR, enzymes |
| Strength Correlation | High (Direct increase in force production) | Low (Increases volume/endurance more than peak force) |
| Training Style | Heavy load, low reps (1–6 reps) | Moderate/light load, higher reps (8–20+ reps) |
The Best Way To Train
I have seen a lot of people promoting either low weight, high reps or high weight, low reps training styles, but as you will now understand, it is neither one or the other that is the best way to train, but a mixed approach.
It is actually a medium weight, medium reps training style that is most efficient, as it provides a good mix of both myofibrillar and sarcoplasmic hypertrophy without taking too long per set (from high rep sarcoplasmic hypertrophy training) and without lifting too heavy, putting unnecessary strain on joints (from heavy weight myofibrillar hypertrophy training). Occasional inclusion of high rep, low weight training and low rep, high weight training incorporated into a training plan focusing on 6-12 repetitions per set will yield the best training results.
You probably already knew that, as that is what the majority of online sources will recommend, but now you understand why, and hopefully you will understand the benefits of occasionally mixing in sets specifically aimed at myofibrillar and sarcoplasmic hypertrophy. If you are more endurance based as an athlete – think Hyrox or other endurance events – or you just want visually larger muscles, then mixing in mainly, or even mainly just focusing on sarcoplasmic hypertrophy may be the way forward. Similarly, if you are mainly a strength based athlete or you just want to be stronger, then factoring in more myofibrillar training could be the best for you.
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