How Hypertrophy Works

Someone told Francois he had nice biceps mid-Ironman.

While he was running.

That's not a fluke. That's what happens when you understand what actually drives muscle growth - and build your training around it.

So let's cut through the bro science and talk about what's really going on.

 

What hypertrophy actually is

Not a pump.

Not inflammation.

Not the burning sensation you get chasing a high-rep finisher set.

Hypertrophy is a structural change. You're increasing the amount of contractile proteins - actin and myosin - inside each muscle fibre. Those long, thin strips you see in a steak. You're getting more of them.

The result is a bigger cross-sectional area and greater force production potential.

Real muscle. Not fluid. Not swelling.

The dominant adaptation from resistance training is myofibrillar hypertrophy. More contractile machinery inside each cell. That's what we're chasing.

 

The one thing that drives it

Mechanical tension.

That's it.

When you lift a challenging enough load, the muscle fibres experience tension. That tension activates signalling pathways - most notably mTOR - which increases muscle protein synthesis. Do that consistently and with enough progression, and your muscles grow.

Not soreness. Soreness is frequently induced in research by running downhill on a treadmill. Nobody's quads grew from that.

Not the pump. You can get a massive pump with nowhere near enough tension on the target muscle.

Not acute hormonal spikes from squatting or deadlifting. Those bumps in testosterone and growth hormone are real. The likelihood they have any meaningful effect on your hypertrophy? Extremely low.

Mechanical tension is the cause. Everything else is just noise.

 

What that means for your training

A few practical things follow from this.

You need to recruit the right fibres. High-threshold motor units have the most growth potential. To get them working, your sets need to be challenging enough - think at least a 6-7 RPE. Getting close to failure.

You need enough volume across the week. Six to ten total sets per muscle group per week is a solid starting point for most people.

You need progression over time. Lift the same loads, same reps, same effort every single week - and eventually the stimulus plateaus. Your body's adapted. The stress is no longer sufficient.

Progression doesn't always mean adding weight. It can look like more reps, more sets, better range of motion, improved execution, more tension on the target muscle. There are many ways to skin a cat.

The key question each session is simple: am I increasing the demand on this muscle?

 

Fatigue is not the same as stimulus

This is the one that trips up a lot of athletes - especially those coming from an endurance background.

You can leave a session absolutely cooked and have created almost no hypertrophy signal.

You can leave a session feeling challenged but controlled and have created a far better one.

The goal isn't fatigue. The goal is force production and quality tension to the target muscle.

If your cardiovascular system is the limiting factor before your muscles are — say you're a tall athlete grinding through back squats and you're gassed before your quads are even working hard — your hypertrophy stimulus drops. A leg press or leg extension might serve you better.

This matters even more for hybrid athletes. Endurance fatigue bleeds into the gym and reduces your ability to produce force. Which is why how you sequence your week matters enormously.

 

Recovery is where the growth happens

The stimulus happens in the gym.

The adaptation happens after it.

Mechanical tension elevates muscle protein synthesis for a window following training. If your nutrition and recovery support that window, new contractile proteins are laid down. If they don't, the adaptation is blunted.

This is where nutrition comes in.

At the most basic level: are you in a net state of muscle protein synthesis, or muscle protein breakdown? If you're in breakdown, the muscles aren't growing.

Resistance training pulls you toward synthesis. But you need to supply enough dietary protein to actually get there.

For most people, 1.2 to 2 grams of protein per kilo of bodyweight per day is the right range. Spread across three to five meals, with 20–40 grams per sitting. That's usually enough to clear the leucine threshold that triggers protein synthesis - assuming your diet is reasonably varied.

Timing matters less than people think. The post-workout window is more like a barn door than a keyhole. Total daily intake is what actually moves the needle.

And for endurance athletes especially - chronic under-fuelling kills your hypertrophy potential. Not just because your body can't build tissue without raw material, but because you simply cannot produce the quality sets required when you walk into the gym underfed. Blood sugar low, stomach rumbling, half a session in you before you're even warm.

Fuel properly. Train properly. That's the sequence.

 

Can you actually build muscle as a hybrid athlete?

Yes.

But only if those drivers - tension, progression, sufficient volume, adequate protein and recovery - are present in your training.

Francois is an Ironman athlete with a demanding full-time job involving a lot of travel. At peak, he was training around 15 hours a week across swim, bike, and run. Plus the gym.

A lot of people would look at that schedule and write off muscle gain entirely.

We didn't.

We recognised his training could work in phases - leaning harder into hypertrophy volume when he was out of peak endurance loading, dropping back to maintenance during the heaviest blocks. Two structured strength sessions a week, focused on compound movements that gave maximum bang for his buck. Clear progression targets. Stress consolidated intelligently across the week, so his heaviest lower body work landed closest to his most intense lower body endurance sessions — letting everything recover together rather than spreading fatigue evenly.

In the gym, we didn't chase fatigue. We focused on muscular tension and building it over time.

He added lean mass. He improved his endurance performance. And he got complimented on his arms during a marathon run leg.

Ben was chasing a sub-3-hour marathon. We placed his strength sessions early in the week when he was freshest. Squatting, hinging, pushing, pulling — sets taken close to failure, not to exhaustion. No junk volume. No fluff. Quality over quantity.

He improved his marathon time. He also got heavier doing it.

Matt was after visible muscle alongside a fast half marathon. We kept in at least two upper body stimuli per week, progressively loaded his pressing and pulling movements, worked around a shoulder injury, and managed interference carefully given the demands of his job. The mechanical tension stimulus never disappeared, even when volume had to flex.

The growth came alongside the endurance gains.

 

The dimmer switch

Endurance training does not switch off muscle growth.

It's a dimmer switch, not an on-off.

Yes, adding endurance work might dial things back slightly. But if your programming is sound, your fuelling is right and the hypertrophy drivers are still present — you will still grow.

The question isn't "can I do both?"

The question is: is mechanical tension present in my training each week? Is there progression? Am I recovering and fuelling properly?

If all of that is ticked, you maximise your chances of actually getting both done.

If you want help structuring all of this properly — how to build a hybrid training week, how to periodise strength and endurance together, how to avoid the interference effect and balance your nutrition — you can apply to work with us here. 

Or, if you'd like an introduction to hybrid training - you can download our free hybrid training guide here.