Fitness Is a Reserve You Spend Later

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What the number actually measures

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VO2 max is the maximum volume of oxygen your body can take in and use per minute at full effort. It isn’t a lung score or a heart score. It’s a single figure for an entire chain — lungs pulling oxygen in, heart pumping it, vessels delivering it, muscles receiving it, mitochondria burning it. Every link has to work for the number to be high, which is why it functions as a composite readout of physical condition rather than one system’s performance.

Think of it as engine horsepower. A bigger number means more capacity available when you need it.

The mortality data is unusually stark

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Mandsager et al. (2018, JAMA Network Open) analysed 122,007 patients referred for exercise treadmill testing at the Cleveland Clinic between 1991 and 2014. Two findings stand out.

First, the size of the effect. Comparing elite performers against the lowest-fitness group produced roughly an 80% reduction in mortality risk. The authors were explicit that the risk associated with reduced cardiorespiratory fitness was comparable to, and in some comparisons greater than, established clinical risk factors like coronary artery disease, diabetes, and smoking.

Second, and more unusual: no observed upper limit of benefit. Even people classified as “high” fitness still had higher long-term mortality than those classified “elite”. Most health variables have a plateau, or a J-curve where more becomes harmful. This one kept going.

One honest caveat about the population: these were patients referred for stress testing, not a random population sample, which limits how cleanly it generalises. And fitness is partly heritable — some of the gap isn’t earned. But the direction and magnitude are hard to dismiss.

Why it matters more with each decade

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From roughly your thirties, VO2 max declines about 10% per decade without deliberate intervention. That slow drain is what eventually shows up as struggling with stairs, finding shopping heavy, or being unable to get down on the floor and back up to play with a grandchild.

Raising your capacity doesn’t stop the decline. It starts the decline from a higher place. A fit 60-year-old can carry the aerobic engine of an unfit 30-year-old — same slope, different intercept, and the intercept is what determines when you cross the threshold into losing function.

The real argument is reserve

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Performance is the least interesting reason to build aerobic capacity. Reserve is the real one.

Illness, injury, surgery and hospitalisation happen to nearly everyone eventually. What differs enormously is what you have available to spend when they do. Someone who enters a serious illness with substantial capacity emerges having dropped a few rungs. Someone already near the floor can be knocked off the ladder entirely — and the frailty that follows a hospital admission in an already-deconditioned person is often permanent.

You’re not training for a race. You’re training for the two weeks in hospital you haven’t scheduled yet.

How to raise it, and the trap in doing so

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Interval training raises VO2 max faster than steady moderate work. Meta-analytic comparisons put pooled improvement from HIIT at around 4.9 mL/kg/min against 1.9 for moderate continuous training — a substantial gap. Worth flagging the nuance: at least one meta-analysis in women found no statistical difference between interval and continuous training for VO2 max, so the advantage may not be uniform across populations.

Two protocols with good evidence, run about three times weekly:

• 4×4 — four minutes at 90–95% of max heart rate, three minutes easy recovery around 70%, repeated four times. Long-interval, higher-volume protocols like this show the largest VO2 max effects.
• 15/15 — fifteen seconds at 90–95%, fifteen seconds easy at 60–70%, repeated 40–60 times.

Estimate max heart rate as 220 minus your age. It’s crude but adequate to start, and any decent smartwatch will refine it and show live zones.

Now the part that gets skipped. The dominant risk of interval training isn’t cardiac, it’s musculoskeletal. Stress fractures, tendon injuries, and the long layoffs that follow are common, and in your fifties or sixties a serious injury can end your training life permanently. The injury takes more than the bone with it — it takes the habit, the confidence, the mood, and the fitness you spent years accumulating.

That risk is why “just do HIIT” is bad advice despite the training data favouring it.

The balanced weekly shape

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• Steady aerobic base, 2–3 sessions. Conversational pace — brisk walking, easy jogging, cycling, swimming. This does raise VO2 max, just more slowly, and it builds the tissue durability that lets you tolerate hard sessions.
• Resistance training, 2+ sessions, covering all major muscle groups.
• HIIT, 1–2 sessions. Enough to sharpen the ceiling, not enough to accumulate damage.
• Daily movement. Break up sitting regardless of what training you did that morning.

The base builds durability; the intervals raise the ceiling. Doing only the second maximises the training signal and the injury risk simultaneously.

Summary

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Cardiorespiratory fitness predicts long-term mortality about as powerfully as smoking or diabetes, with roughly 80% lower risk at elite versus lowest fitness across 122,007 people — and no ceiling where benefit stopped. Capacity falls around 10% per decade from your thirties, so building it raises the starting point of an inevitable decline. The deepest argument isn’t performance but reserve: what you have available when illness or injury arrives. Interval training raises VO2 max faster than steady work, but carries the injury risk most likely to end training altogether, which is why a mixed week beats a pure one.

What to actually do:

1. Establish a steady aerobic base first — 2–3 conversational-pace sessions weekly — before adding hard intervals.
2. Add one or two interval sessions weekly, not four. Use 4×4 or 15/15.
3. Estimate max heart rate as 220 minus age and train the hard intervals at 90–95% of it.
4. Keep resistance training in the week. Muscle and connective tissue are what let you tolerate the intervals.
5. Treat injury avoidance as part of the programme, not an inconvenience — a two-year layoff costs more than a slower protocol ever would.
6. Break up sitting daily, regardless of whether you trained that morning.

Sources & further reading

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• Mandsager K. et al., Association of cardiorespiratory fitness with long-term mortality among adults undergoing exercise treadmill testing, JAMA Network Open (2018) — JAMA Network Open / PubMed
• Effects of different protocols of high intensity interval training for VO2max improvements in adults: a meta-analysis of randomised controlled trials (2019) — PubMed
• Moderate to vigorous-intensity continuous training versus high-intensity interval training for improving VO2max in women: a systematic review and meta-analysis (2023) — PubMed
• Effectiveness of high-intensity interval training and continuous endurance training for VO2max improvements: a systematic review and meta-analysis of controlled trials — ResearchGate
• Cleveland Clinic Consult QD, No ceiling to mortality benefits of cardiorespiratory fitness in patients undergoing stress testing — Cleveland Clinic
• Effects of high-intensity interval vs. moderate-intensity continuous training on cardiac rehabilitation in patients with cardiovascular disease — PMC