Most people have a pretty simplistic view of muscle.

It’s just a bunch of muscle fibres, right?

Well, that’s true.

But it’s also much more than that.

Muscle is made up of different types of fibres—each with distinct properties.

Yes, we lose muscle with ageing.

But certain types of muscle fibres are hit the hardest.

And there’s only one thing that can stop it.

​

Muscle Fibre Types: The Lowdown

Broadly speaking, there are two muscle fibres types:

• Type I (slow-twitch) fibres – smaller, produce less force, but highly resistant to fatigue
• Type II (fast-twitch) fibres – larger, generate more force, and grow more with resistance training

Type II fibres are crucial when force is needed quickly.

Think jumping, sprinting, changing direction, or stepping to avoid a fall.

So these fibres aren't activated unless high levels (or rates) of force are required.

Otherwise, they stay dormant—while the slower type I fibres do the work.

And here’s the problem:

It’s these powerful, fast fibres that deteriorate the most with ageing.

​

The Demise of Type II Fibres With Age

Muscle loss with ageing is striking.

By age 80, the average person has lost 30% of their peak muscle mass.

But here’s what most people don’t realise:

Most of this loss comes from type II fibres.

Why?

Because as we age, we gradually stop doing the things that keep these fibres active.

And when muscle fibres aren’t used, the nerves that control them literally die—taking the fibres with them.

This means:

• Some of these type II fibres disappear completely
• While others convert into slower, weaker fibres

The nervous system tries to compensate by sending out branches from surviving motor nerves to “rescue” these fibres.

But this process isn’t perfect.

By age 75, only 60-70% of muscle fibres remain.

And compared to younger muscles, the quadriceps of a typical 75-year-old male:

• Are 30% smaller
• Have 20-40% fewer fibres
• Show significant muscle fibre grouping (a sign of type II fibre loss)

Which means older muscles:

• Produce less force (strength)
• Generate force with less speed (power)
• Have poorer force control (steadiness)

So we’re not just weaker.

We move slower.

And we have less control over our movement.

It’s easy to see how this impacts daily life.

​

How To Preserve Type II Muscle Fibres

How do we stop the loss of these critical type II fibres?

It’s an important question.

Recent research offers some clues.

​Older men (average age 69) who were recreationally active but not strength training had:

• 32% smaller type II fibres
• 53% more type I (slow) fibres
• 18-fold more grouped type I fibres

Now, let’s look at the other extreme:

Masters strength athletes.

Even when aged in their 70s, the muscles of these athletes can resemble those of people in their 30s.

Compared to inactive older adults, they had:

• 49% more type II fibres
• 66% less grouped type I fibres
• Virtually no atrophic (very small) fibres

And here’s the kicker…

These strength-trained athletes didn’t just retain type II fibres—they had more than even their endurance-trained peers.

Their type II fibre proportions were nearly identical to younger adults and far exceeded those of inactive older adults:

Now, these data are cross-sectional, so genetics could be a factor.

But if anything makes the case for strength training, it’s this.

​

Not All Exercise Is Equal (For Type II Fibres)

The only way to keep type II muscle fibres is to use them.

That’s where exercise comes in.

But here’s the catch: not all movement is enough.

Because of the size principle, your body recruits smaller type I fibres first.

Type II fibres only switch on when things get tough.

That means you need high levels of effort to stimulate them by:

• Moving fast
• Lifting heavy loads
• Or pushing lighter loads close to failure

This explains why even lifelong endurance training doesn’t stop type II fibre loss.

The only way to truly preserve them?

Regular strength training and explosive movements.

​

Are There Sex Differences At Play?

Both men and women succumb to muscle loss with age.

But how muscle is lost seems to differ.

A 2018 study found that in women, type IIx fibres were half the size at age 72 compared to age 25.

In men, fibre size didn’t shrink as much.

Instead, they lost more total muscle fibres over time.

So, while both sexes lost similar amounts of muscle:

• Women kept more fibres, but they got smaller
• Men lost more fibres, but kept their size better

It’s possible changing hormones and inflammation might explain these differences.

Despite these factors, we know both males and females can reverse age-related loss of muscle mass with resistance training.

Which begs the next question:

​

Is It Ever Too Late To Act?

Here’s the harsh truth:

Once a muscle fibre is lost, it’s probably gone for good.

But that doesn’t mean you can’t:

• Increase the size of your remaining muscle fibres
• Take action to preserve what’s left
• Regain lost strength and power

We don’t yet know if lifelong exercise is needed to fully prevent type II fibre loss.

But we do know one thing:

The earlier you start, the better.

You can’t turn back the clock—but you can slow it down.

That’s why strength training isn’t just about staying strong.

It’s the only way to preserve your type II muscle fibres.

​

As always, thanks for being here.

​

Until next week,

Jackson

​

​​​

If you've got a moment, I'd love to hear your thoughts on this edition of The Weekly Exerciser.

Send me a quick message or email - I'll reply to every one!

​

PS: Did someone forward you this email? You can sign up to The Weekly Exerciser here.

​

IMPORTANT:

The information contained herein is of general nature only and does not constitute personal advice. You should not act on any information without considering your personal needs, circumstances, and objectives. Any exercise program may result in injury. We recommend you obtain advice specific to your circumstances from an appropriate health professional before starting any exercise program.