Cardio that works

For most of my 30s I trained cardio the way most men do. Hard runs when I felt like it, nothing when I didn't, and a vague sense that "doing some cardio" was enough. By 39 my resting heart rate was creeping up, my recovery was getting worse, and my VO2 max (which I'd never measured) was probably collapsing.

Then I learned about Zone 2 and VO2 max as the actual metric that matters, and the way I trained cardio changed completely.

VO2 max is the metric that matters

VO2 max is the maximum amount of oxygen your body can use during exercise. It's measured in millilitres of oxygen per kilogram of bodyweight per minute. It is the single best predictor of all-cause mortality among the standard fitness metrics, and the relationship is well-replicated in the literature. Higher VO2 max, longer life. Not perfectly causal, but the correlation is unusually strong.

VO2 max declines about 10% per decade in untrained adults from your 30s onward. In trained adults the decline is roughly half that. A 50-year-old with the VO2 max of an average 30-year-old has, on most measures, the cardiovascular system of a 30-year-old.

You can build it. You can hold onto it. You cannot do either by accident.

Two zones, two adaptations

Most cardio confusion at 40+ comes from training in the wrong zone for the wrong adaptation. The two that matter most:

Zone 2. Conversational pace. Roughly 60-70% of max heart rate. You can hold a sentence without gasping. If you can sing, you're going too easy. If you can only grunt, you're going too hard. Zone 2 is where you build mitochondrial density, fat oxidation, and the aerobic base that everything else sits on. It's boring. It's also the work that pays the rent.

Zone 5 / VO2 max intervals. All-out, 3-5 minute efforts at the upper limit of what you can sustain. Typically 90-95% of max heart rate, with equal recovery between intervals. This is where you raise the ceiling.

You can ignore zones 3 and 4 for most of your training. The "moderate" middle zone is the one most casual exercisers live in, and it's the least productive for either adaptation. Easy days easy. Hard days hard. The middle is where progress dies.

How much of each

A reasonable cardio prescription for a 40+ man with a strength training base:

• 2-3 Zone 2 sessions per week, 30-60 minutes each
• 1 VO2 max interval session per week, 20-30 minutes total

That's it. Three to four cardio sessions a week, alongside two strength sessions. Total time commitment: roughly 5-7 hours a week of training. For most middle-aged men that's the upper limit of sustainable.

If you can only do less, prioritise this way:

1. Two strength sessions
2. One VO2 max interval session
3. One Zone 2 session
4. A daily walk (10,000 steps is a useful floor, not a magic number)

The Zone 2 prescription

Pick a modality your joints tolerate. For most 40+ men that's not running (high impact, high injury rate). Better options:

• Stationary bike (lowest impact, easiest to control intensity)
• Rowing machine (full body, low impact, cardiovascular and strength carryover)
• Incline walking (treadmill at 12% grade, 5-6km/h, brutal in the best way)
• Swimming (zero impact, technical demand)
• Hiking with a weighted pack (fun, outdoors, sneaky strength work)
• Trail running if your feet, ankles, and knees cooperate

The test for whether you're in Zone 2: can you hold a conversation? If you're alone, can you nasal-breathe? If both answers are yes, you're in the zone.

The mistake most men make on Zone 2 is going too hard. It feels too easy, so they push. Pushing turns Zone 2 into Zone 3, and Zone 3 doesn't build the same adaptations. Stay slow. Build the base.

The VO2 max interval prescription

The simplest evidence-backed protocol is the 4x4 (sometimes called the Norwegian protocol):

• 10-minute warm-up at easy pace
• 4 minutes at 90-95% of max heart rate (hard, sustainable, but you're glad when each interval ends)
• 3 minutes at easy pace (active recovery)
• Repeat 4 times
• 5-minute cool-down

Total session: roughly 35-40 minutes. Done once a week, for 8 weeks, this protocol has produced VO2 max improvements of 10-15% in middle-aged adults across multiple studies. That is a large effect for a single weekly session.

Variations that work:

• 30 seconds on / 30 seconds off, repeated 8-10 times (shorter, more brutal)
• 1 minute on / 1 minute off, repeated 8 times (sweet spot for most)
• Hill repeats on a bike or run, 1-2 minutes each, 6-10 reps

Pick one. Run it for 8 weeks. Test something measurable (mile time, watts on the bike, time to exhaustion at a fixed intensity). Move on if you've plateaued.

The body metaphor

Your aerobic system is a fleet of small engines (your mitochondria). Zone 2 builds more of them. VO2 max intervals make the existing ones run faster and cleaner. You need both. A fleet of slow engines is inefficient. A handful of fast engines burns out under load. The combination is what you actually want.

What about running?

I'm not anti-running. I am anti-running as the only cardio for a 40+ man with no strength base, no Zone 2 base, and shoes from 2019.

If you want to run:

• Build the strength base first (3-6 months of lifting before adding running volume)
• Start with run/walk intervals, not continuous running
• Cap your weekly volume at something modest (20-30km) until your body adapts
• Get fitted for proper shoes
• Run on softer surfaces when you can
• Drop a session if anything starts hurting (don't run through pain)

Running is one option among many. It is not, at 40+, automatically the best one.

Tracking what matters

Useful metrics:

• Resting heart rate. Trend it over months. Falling is good. Rising for a week is a recovery flag.
• Heart rate at a fixed Zone 2 effort. If your easy pace gets faster at the same heart rate, your aerobic base is building.
• VO2 max estimate from a watch. Imperfect, but the trend is more useful than the number.
• Time to recover. How long after intervals does your heart rate drop back to 100bpm? Faster is better.

You don't need a lab test. You need a chest strap and a willingness to look at the data over months, not days.

The aphorism

Slow base. Sharp peak.

Further watching

• How to increase your VO2 max — Peter Attia & Mike Joyner (11:19)
• Building VO2 Max Is Critical for Longevity — Andy Galpin (12:02)