The Muscle You Don't Notice Losing (Until You Do)

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I want to tell you about something that's been happening to you for years, quietly, without your permission.

If you're over 40, you've probably been losing muscle. Not dramatically. Not in a way you'd notice week to week. But steadily, in the background, while you were busy logging miles and living your life.

Researchers call it sarcopenia — age-related muscle loss — and the general pattern in the literature looks something like this: starting sometime in our 30s or 40s, most adults begin losing muscle mass at a rate often estimated around 1% per year. After 60, that rate tends to accelerate. And here's the part that got my attention when I first dug into this: the loss isn't evenly distributed. The research suggests we lose fast-twitch muscle fibers — the ones responsible for power, speed, and quick reactions — disproportionately faster than slow-twitch fibers.

If you've ever wondered why your 5K pace faded before your ability to run long did, that's likely a big part of the answer.

This is the question I asked myself for years, and I suspect many of you have too. We're runners. We're active. Surely all those miles are keeping our muscles young?

Here's the uncomfortable truth I've had to accept: running is wonderful for a lot of things — cardiovascular health, endurance, mental health, and simply feeling like yourself — but the research consistently suggests it's not enough, on its own, to prevent age-related muscle loss.

The reason makes sense once you see it. Distance running is primarily a slow-twitch, endurance activity. It's repetitive, relatively low-load movement. Your muscles adapt to exactly what you ask of them — and what most of us ask of them is to do the same moderate-effort thing, thousands of times in a row. What we rarely ask of them is to produce high force. And high force — meaningful resistance — appears to be the signal that tells muscle to stick around.

Think of it this way: your body is ruthlessly efficient. Muscle is metabolically expensive to maintain. If you never demonstrate that you need strength and power, your body concludes you don't — and quietly lets it go.

I'm not a doctor or a physical therapist, and none of this is medical advice — it's what I've pieced together from reading the research and living in a 50-something runner's body. But here's my plain-English summary of what the science generally points to as we age:

Muscle fibers decline — especially the fast ones. We don't just lose muscle size; studies suggest we lose motor units (the nerve-muscle connections that fire our fibers), and the fast-twitch ones seem to go first. That's why power fades faster than endurance, and why a curb you used to bounce up now requires a bit more deliberate effort.

Tendons stiffen and lose capacity. If you've dealt with a cranky Achilles or plantar fascia in the last decade — and in this community, who hasn't? — this will sound familiar. Tendons appear to adapt more slowly than muscles and tolerate less abuse as we age. Interestingly, some of the better-supported approaches for tendon health involve heavy, slow resistance work — the very thing most runners avoid.

Bone density declines. Running provides some impact loading, which helps, but research suggests it may not be sufficient by itself, particularly for women after menopause. Progressive resistance training is one of the most commonly recommended tools for supporting bone health as we age.

Hormones shift. For men, testosterone tends to decline gradually. For women, menopause brings a steeper drop in estrogen, which affects muscle, bone, and tendon. I won't pretend to be an expert on hormonal health — that's a conversation for your doctor — but the takeaway I've drawn is that the older we get, the more deliberately we have to work for adaptations that used to come free.

Recovery slows. You already know this one. The workout that used to cost you a day now costs two. That's not weakness or laziness. It's biology, and it's manageable — but it does mean the "just run more" solution to every problem stops working somewhere in our 40s.

For most of my running life, I treated strength work the way I treat flossing: something I knew was good for me, did in short guilty bursts, and abandoned the moment training got busy.

What changed my mind wasn't a single study. It was connecting the dots between the research and my own body. The nagging injuries that took longer to clear. The hills that felt steeper. The finishing kick that had quietly retired without telling me.

Here's the reframe: after 40, strength training isn't cross-training. It's the foundation that makes running sustainable.

When we're 25, we can get away with treating strength work as optional, because we're drawing on a deep reserve of muscle and tissue resilience. After 40, that reserve is shrinking — unless we actively maintain it. Every year we don't push back, the baseline drops a little.

And the encouraging news — the genuinely encouraging news — is that the research suggests muscle remains remarkably trainable at every age. Studies on resistance training have shown meaningful strength improvements in people in their 60s, 70s, 80s, and beyond. The adaptation machinery doesn't shut off. It just stops running automatically. You have to turn the key yourself.

Before you picture yourself grinding through hour-long gym sessions five days a week: that's not where this is going, and it's not what the evidence seems to require.

It doesn't mean becoming a bodybuilder. It doesn't mean sacrificing your running. It doesn't necessarily mean joining a gym at all. From what I've read and experienced, relatively modest amounts of consistent, appropriately challenging resistance work — for many people, a couple of focused sessions a week — may be enough to meaningfully push back against these age-related changes.

It also doesn't mean panic. If you're reading this at 55 having never lifted anything heavier than a grocery bag, you have not missed the boat. The research on later-life strength training is honestly one of the most hopeful bodies of evidence in all of exercise science.

What it does mean is that "I run, so I'm covered" is a belief worth retiring — gently, without guilt, the way we retire an old pair of shoes that served us well but can't do the job anymore.

I'll be writing much more in the coming weeks about the practical side: what kinds of strength work seem to matter most for masters runners, how little you can get away with, and how to fit it around your running without wrecking your legs.

For now, if this has you thinking about adding strength training to your routine, my strongest suggestion is the least exciting one: start by talking to your doctor or a qualified professional — especially if you have existing injuries, health conditions, joint issues, or you're new to resistance training. Everyone's body, history, and situation are different, and a physical therapist or certified trainer who can actually see you move is worth far more than any newsletter, including this one.

If you get the green light, start smaller than your ego wants. The runners I know who've made strength training stick — myself included — didn't start with ambitious programs. They started with something almost embarrassingly modest and let consistency do the work.

Because that's the real lesson buried in all this research: aging isn't a cliff. It's a slow slope, and the direction you drift depends heavily on what you ask of your body. For decades, we asked it to endure. Now it's time to also ask it to be strong.

Your future running self — the one who's still out there at 70, moving well, doing this on their own terms — is counting on the decision you make this year.

Run strong, Ageless Runner

This newsletter is for general informational and educational purposes only and is not medical, health, or fitness advice. The information here is based on publicly available research and my personal experience as a runner, and may not be appropriate for your individual circumstances. Always consult your physician or a qualified healthcare provider before beginning any new exercise or strength training program, particularly if you have pre-existing health conditions or injuries. Never disregard professional medical advice or delay seeking it because of something you read here. Any reliance you place on this information is strictly at your own risk.

USA/Canada/UK & Europe/Rest of World

August 1–2

• adidas Runners City Night 5K & 10K (Berlin, Germany)
https://www.berlin-citynight.de/en/

• Derby Running Festival 5K & 10K (Derby, England)
https://www.letsdothis.com/gb/running-events/10k/august

August 8–9

• RunThrough Tatton 5K & 10K (Knutsford, England)
https://www.letsdothis.com/gb/running-events/10k

• Solihull 5K (Solihull, England)
https://www.runthrough.co.uk/events-timeline

August 15–16

• Battersea Park 5K & 10K (London, England)
https://www.runthrough.co.uk

• Lee Valley Velo Park 5K & 10K (London, England)
https://www.letsdothis.com/gb/running-events/10k

August 22–23

• Bellahouston Park 5K & 10K (Glasgow, Scotland)
https://www.letsdothis.com/gb/running-events/10k/august

• The Lucky Clover Run 5K (Bury St Edmunds, England)
https://www.letsdothis.com/gb/running-events/10k/august

August 29–30

• Summer 10K & 5K Series Events (UK)
https://findarace.com/10k-runs/august

• European local 5K & 10K events continue across Germany, France, Spain, Italy, Ireland, and Scandinavia
https://www.ahotu.com/calendar/running/10-km/august/europe

USA/Canada/UK & Europe/Rest of World

August 1–2

• Ardbeg Islay Half Marathon (Isle of Islay, Scotland)
https://www.islayhalfmarathon.com

• Isle of Mull Half Marathon (Isle of Mull, Scotland)
https://www.isleofmullhalfmarathon.com

• Hella Marathon Night – Marathon & Half Marathon (Rostock, Germany)
https://www.rostocker-marathon-nacht.de

August 8–9

• Perth Running Festival – Half Marathon (Perth, Scotland)
https://www.perthrunningfestival.co.uk

• Thames Meander Summer Half Marathon (Richmond, England)
https://www.runnation.co.uk

• Solihull Half Marathon (Solihull, England)
https://www.runthrough.co.uk

August 15–16

• Reykjavik Marathon & Half Marathon (Reykjavik, Iceland)
https://www.rmi.is/marathon

• Leeds Running Festival Half Marathon (Leeds, England)
https://www.runthrough.co.uk

August 22–23

• Antrim Coast Half Marathon (Larne, Northern Ireland)
https://antrimcoasthalfmarathon.com

• Helsinki Marathon (Marathon & Half Marathon) (Helsinki, Finland)
https://helsinkimarathon.fi

August 29–30

• Run Dorney Lake Half Marathon (Windsor, England)
https://www.runthrough.co.uk

• Richmond Park Half Marathon (London, England)
https://www.runthrough.co.uk

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This article is for general informational purposes only and isn’t medical advice. Ageless Runner isn’t a substitute for a doctor or physical therapist. Check with a qualified healthcare professional before starting or changing any exercise program, especially if you have a health condition or injury. Exercise carries inherent risk — if you feel pain, dizziness, or discomfort, stop and seek medical attention. See our full Disclaimer for details.

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