VO₂ Max After 40: From 48 to 59 — and Why It Slid Back to 56

In a year, my VO₂ max went from 48 to 59. Then I shifted my priority toward strength training, some aerobic volume disappeared, and the number slid back to 56. Below: what actually drove it up, what pulled it back down, and why the second part doesn’t bother me.

Why VO₂ Max Stops Being a Sports Metric

VO₂ max is, basically, the size of your aerobic engine. How much oxygen the body can push through itself under load.

The inconvenient thing about this metric is that it declines quietly. It doesn’t hurt, it doesn’t beep, it doesn’t show up in a standard check-up. Not until the stairs to the fifth floor become an event.

And it’s surprisingly tightly linked to how long you live. A large study of 122,007 patients published in JAMA Network Open found that low cardiorespiratory fitness carries a risk of death from all causes comparable in strength to smoking or diabetes.

For me, VO₂ max became something like the main gauge on the dashboard. Weight jumps around, the mirror lies in both directions, and how I feel depends on how the day went. VO₂ max is the one number that tells me, fairly honestly, what shape I’m actually in.

More on the metric itself in the pillar article VO₂ Max, Strength and Training Strain.

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Where I Started — and Why the Number Annoyed Me

May 2025. WHOOP showed VO₂ max = 48.

For a 45-year-old man, that’s “above average” by the standard tables. Formally — fine. Emotionally — not so much. I knew that above 50 is “excellent” for men 40–49, and above 56 is “superior.” I was sitting right on the edge of average — and this is a guy running his own longevity project. The cobbler’s children have no shoes. Old combination, still fresh.

Worse, 48 at 45 is a trajectory that, left alone, lands somewhere around 40–42 by age 60. That’s already elevated-risk territory. So I did what I usually do with business metrics: looked at what actually works in the research, and built a plan.

No heroics. Just a decision that in a year the number would be different — and work in that direction every week.

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The Two Levers That Actually Move VO₂ Max

In the literature on training after 40, I kept running into the same pair: Zone 2 and intervals. Everything else is a variation.

Zone 2 is low intensity, 60–70% of max heart rate. The pace where you can still speak in full sentences but singing is off the table. Boring, long, seemingly pointless. In reality, this is the zone that builds the engine itself: mitochondria, capillaries, the heart’s ability to move more blood per beat. Without the engine, intervals are just an attempt to drive fast on a car with a tiny tank.

Intervals are the growth stimulus. The most studied format is the 4×4 Norwegian intervals: four minutes at 90–95% of max heart rate, three minutes of recovery, four rounds, two or three times a week. In the original studies it produced VO₂ max gains of 7–15% over 8–12 weeks, across people of very different fitness levels — from beginners to cardiac rehab patients.

The logic is simple: for VO₂ max to grow, you have to regularly spend time near the ceiling of your current capacity. The cardiovascular system gets no signal to change if it never has to work at its limit.

You can do just one. Only Zone 2 — there will be growth, but slow, with an early plateau. Only intervals — works for the first few weeks, then the body stops recovering between sessions. Hence 80/20: roughly 80% of volume in the low zones, 20% in the high ones. That ratio is used by everyone from elite runners to cardiac rehab, and it works at any level.

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What My Week Actually Looks Like

I train on my balcony and I go to the gym. The gym is strength work and Zone 2 on the treadmill. The balcony, oddly enough, is where my personal intensity records happen. One of the latest — 23 minutes straight in zones 4–5. With my favorite music on. It pulls things out of me that no gym ever could.

No special equipment. A mat on the balcony, a treadmill at the gym, dumbbells. That’s enough.

I track zones through WHOOP — it counts time in each zone by heart rate and sums it up for the week. You can see not just how much I trained, but where exactly the load actually landed. Without that it’s easy to fool yourself: it feels like a lot of Zone 2, and half of it was really Zone 1.

My Week — Built Around Intensities, Not Days

My actual schedule changes almost every week — it adjusts to recovery. So I think about the week not as a calendar, but as a set of intensity doses. A typical week: two moderate strength sessions plus one heavy one, one interval session, three or four Zone 2 sessions at the gym of 30–45 minutes each, and active rest — mobility, stretching.

On top of training — almost every day, a brisk walk after lunch and an easy one after dinner. That’s not Zone 2 in my book, just movement. The baseline level of not letting the body sit for eight hours straight. Walks don’t replace structured load, and the reverse is also true — they work on different layers.

Where I Started — 20 Minutes on the Balcony

A year ago I did none of the above. I started with 20-minute sessions — short bodyweight intervals, jumps, basic movements. Compared to my current volume, it’s almost funny. But that’s what moved the first number.

Two or three months in, strength work appeared, then real interval sessions, then regular Zone 2. The progression didn’t follow “a plan” — it followed the feedback from the data. I looked at WHOOP every morning and adjusted the load to whatever recovery was showing.

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What WHOOP Showed Over 12 Months

May–November 2025. After a summer plateau at 48–49, the curve started climbing. The balcony-and-music magic: short intense sessions to a driving track moved VO₂ max where the gym couldn’t.

May 2025: 48 → peak in January–February 2026: 59 → now: 56

The curve was anything but linear — and its first stretch genuinely annoyed me.

For the first four months almost nothing happened. Summer 2025 — a plateau at 48–49. I was training, and the number just sat there. Now I understand why: too many intervals, too little base, and WHOOP showing low recovery for weeks. But in the moment it just looked like “working — and getting nowhere.”

The growth started in autumn. From September the curve moved: 49 → 51 → 54 → 57, and 58 by December. That’s exactly when the balcony sessions to music appeared — short, intense, in zones 4–5. It sounds unserious, but it was that unseriousness that moved the number. By January–February 2026, VO₂ max reached its peak of 59.

Two other metrics moved alongside it, and I consider them no less important than VO₂ max itself: HRV rose from ~62 to 91–94 ms, RHR dropped from ~56 to 42. They moved in sync with VO₂ max — and that synchrony is the sign that the system is genuinely adapting, not just absorbing a shock.

And then, from March, VO₂ max began sliding back — 58, 57, and 56 by May. And that wasn’t a mistake, it was a choice. More on that next.

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The Slide to 56 — a Price, Not a Failure

December 2025 – May 2026. VO₂ max peaked at 59 in January–February, then slid to 56 as strength work took over part of my training time. Even at 56, WHOOP still ranks it as “Superior” — top 5% for my age and biological sex group.

When I shifted my priority toward strength work, there was less aerobic volume — and VO₂ max dropped three points.

The logic here is simple. Total training time is finite. Strength work took hours away from intervals — there were noticeably fewer high-intensity sessions in zones 4–5. And VO₂ max responds most strongly to exactly that kind of work, so the number reacted first.

This is a tradeoff that VO₂ max guides usually don’t mention. Strength and aerobic work after 40 aren’t complementary things — they’re competitors for one resource: time and recovery. Over the same period, muscle mass on the bioimpedance scale grew by about 2 kg, and HRV and RHR held steady. So the VO₂ max slide isn’t system degradation — it’s a reallocation of effort.

Three points of VO₂ max is a fair price for what I’m prioritizing right now. Once the strength base stabilizes, some Zone 2 comes back and the number drifts back toward 58–59. That’s no longer a question of protocol — it’s a question of priorities.

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Recovery — the Part of the Protocol Everyone Skips

Most VO₂ max material is about load. Intervals, zones, volume. Almost no one writes that without recovery, the whole structure starts working against you.

Especially after 40. At twenty-five you can train six days a week and sleep five hours — the body forgives it. At forty-five it forgives one bad week, tops.

Sleep I keep as part of the protocol, not as what’s left over after it. I’m in bed at 21:00, up at 6:00 — around 7.5–8 hours of actual sleep by WHOOP. Adaptation — mitochondrial growth, glycogen restoration — happens mostly during sleep. Less sleep, less adaptation, and intervals turn into accumulated stress with no exit. More on that in the Sleep Protocol.

An RHR of 42 I don’t treat as a trophy. It’s a sensor: if it jumps to 50 tomorrow, something’s off — lack of sleep, stress, a cold setting in. A morning thermometer for the whole system, cheaper and more honest than a blood test.

And the main counterintuitive lesson of the year: overtraining slows VO₂ max more than undertraining does. Too many intervals without recovery, and the curve stalls. That’s exactly what I saw on my four-month plateau: I trained more, the result stood still. After 40, “more” almost always loses to “right.”

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What I’d Do Differently If I Started Today

I’d add strength work earlier. I kept it on the back burner for a long time — and that was wrong. It doesn’t just protect against Muscle Loss After 40, it also gives VO₂ max its infrastructure: more functional muscle mass means more tissue that needs oxygen.

I’d do fewer intervals in the first months. The temptation to go to 4×4 three times a week is real, and it leads straight to plateau and burnout. Two sessions are enough, sometimes one.

I’d accept upfront that strength and VO₂ max compete for time. Not “complement each other,” the way the guides put it — they compete. Knowing that, I’d build the year as a series of phases — an aerobic block, a strength block, then aerobic again — instead of trying to maximize both metrics at once.

And one more: I wouldn’t chase someone else’s protocol. Programs in the “+10 to VO₂ max in 8 weeks” genre work for a specific person in a specific state. Other people’s templates almost always turned out to be either too much or not enough for me.

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FAQ

Can you raise VO₂ max with strength training alone, without cardio?

In my experience — barely. Strength work helps indirectly, through muscle mass, but VO₂ max itself grows in response to aerobic load and intervals. My number started moving specifically with balcony cardio, back when there was hardly any strength work at all.

How accurate is WHOOP’s VO₂ max without a lab test?

It’s an algorithmic estimate, not a lab test with a mask on a treadmill. The absolute number can be off. But for tracking the trend it’s enough: if WHOOP shows growth from 48 to 59 over a year, there’s almost certainly real adaptation behind it.

How do you know intervals are intense enough without a heart rate monitor?

The talk test: at 90–95% of max you can squeeze out two or three words between breaths, no more. Full sentences mean you’re not in that zone. The other marker is perceived effort at 8–9 out of 10 — when on the fourth round you want to stop but can still finish.

At what age does VO₂ max stop growing?

Practically — never. The trainable aerobic response holds up past 70, the rate of gain just slows down. The direction of the curve can be reversed even after 60 — the trick is to account for recovery and not train on programs written for thirty-year-olds.

How many weeks until you see the first changes?

For me the first noticeable shift was at week 6–8. The biggest gains come in the first three months, then each additional point gets harder. That’s normal and matches what the research shows.

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What the Research Says

VO₂ max is one of the most studied metrics in training science. The epidemiology — the link to longevity — is covered in the pillar article VO₂ Max, Strength and Training Strain. Here are three sources on the protocol itself.

The original work by Helgerud J et al. (Medicine & Science in Sports & Exercise, 2007) compared several aerobic training formats matched for total work. The protocol of 4×4 minutes at 90–95% of max heart rate outperformed both lactate-threshold work and continuous moderate cardio. This is the study that cemented the “Norwegian 4×4” as the standard protocol for VO₂ max growth.

A meta-analysis by Milanović Z, Sporiš G, Weston M (Sports Medicine, 2015) pooled 28 controlled trials: HIIT and continuous cardio both raise VO₂ max, but HIIT delivers roughly twice the gain for the same time. The authors stress, though, that HIIT doesn’t replace continuous work — it complements it.

And the most relevant one for the 40+ audience — the five-year RCT Generation 100, Stensvold D et al. (BMJ, 2020): 1,567 people aged 70–77, randomized into a control group, moderate training, and 4×4 intervals twice a week. After five years, the interval group showed the largest gain in cardiorespiratory fitness and a trend toward lower all-cause mortality. Aerobic adaptation holds up at any age — you just have to stop treating it as something for the young.

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Disclaimer

Everything above is the observations of one guy who has a WHOOP and too much time to think about metrics. It’s not medical advice and it’s not a training program.

Before starting any interval training after 40 — especially after a long break, or with cardiovascular risk factors — it’s worth talking to a specialist. Ideally, getting an ECG stress test. The cost of one such test is lower than the cost of one unpleasant discovery at an inconvenient moment.