The Exact Heart Rate Zone That Improves VO₂ Max (Backed by Science)

If you've ever wondered why some runners seem to get fitter while others plateau despite logging the same miles, the answer usually comes down to one thing: where their heart rate sits during training.

Not how long. Not how far. Where.

After tracking my own VO₂ max and experimenting with different training approaches, I've come to understand that improving cardiovascular fitness isn't about working harder. It's about working smarter within specific heart rate ranges. And the science backs this up convincingly.

What Is VO₂ Max, Really?

Before we talk zones, let's establish what we're actually trying to improve.

VO₂ max measures the maximum amount of oxygen your body can use during intense exercise. It's expressed in milliliters of oxygen per kilogram of body weight per minute (mL/kg/min). A higher number means your cardiovascular system is more efficient at delivering oxygen to working muscles.

For context, an average 40-year-old male has a VO₂ max around 38 mL/kg/min. An elite endurance athlete might hit 70+. The difference isn't just athletic performance. Research from the HUNT Fitness Study, which tested over 4,600 healthy Norwegians aged 20 to 90, shows that higher VO₂ max correlates strongly with lower all-cause mortality. Their data found a 21% reduction in cardiovascular disease mortality for every 5 mL/kg/min increase in VO₂ max.

That's why tracking your biological age matters beyond just running faster.

The Three Zones You Need to Understand

Heart rate training can get complicated with five, six, or even seven zones depending on who you ask. But for improving VO₂ max specifically, you really only need to understand three.

Zone 2: The Aerobic Base (60–70% of Max HR)

Zone 2 feels almost too easy. You can hold a full conversation without gasping. Your breathing is controlled. You might wonder if you're even doing anything productive.

You are.

At this intensity, your body primarily burns fat for fuel. Research published in the Journal of Applied Physiology shows that this zone also stimulates mitochondrial adaptations (the tiny powerhouses inside your cells that process oxygen). More mitochondria means a more efficient aerobic system. Think of Zone 2 as laying the foundation of a house. It's not glamorous, but nothing works without it.

Most recreational athletes don't spend enough time here because it feels unproductive. They push harder, thinking more effort equals more results. This is a mistake.

Zone 3: The Gray Zone (70–80% of Max HR)

Zone 3 is where things get tricky. It's harder than Zone 2 but not hard enough to trigger the specific adaptations that improve VO₂ max. You're working, you're sweating, you feel like you're training. But you're stuck in metabolic no-man's land.

This zone has earned the nickname "junk miles" among coaches. It's too hard to recover from quickly, too easy to stimulate significant cardiovascular improvement. Most people accidentally live here because it feels like proper training.

Here's the uncomfortable truth: if you spend most of your training time in Zone 3, you'll get moderately fit and stay there forever.

Zone 4: The VO₂ Max Zone (85–95% of Max HR)

This is where the magic happens.

At 85–95% of your maximum heart rate, you're pushing your cardiovascular system to its limits. Your heart is working near its maximum stroke volume. Your lungs are extracting as much oxygen as possible. Your muscles are demanding more than your body can comfortably deliver.

This intensity forces adaptation. Your heart gets stronger (literally: the left ventricle wall thickens and the chamber enlarges, allowing it to pump more blood per beat). Your capillary density increases, delivering more oxygen to muscle tissue. Your body becomes more efficient at the thing VO₂ max measures.

But here's the catch: you can't live here. Zone 4 is taxing. It requires recovery. If you try to spend every workout at this intensity, you'll burn out, overtrain, or get injured.

Why 85–95% Is the Sweet Spot (The Science)

The research is remarkably consistent on this point.

A landmark 2007 study published in Medicine & Science in Sports & Exercise compared four different training protocols matched for total work. The researchers assigned 40 healthy men to one of four groups: long slow distance at 70% max HR, lactate threshold training at 85% max HR, and two interval protocols at 90–95% max HR. The result? The high-intensity interval groups (training at 90–95% of max heart rate) showed significantly greater improvements in VO₂ max than the other groups.

More recently, a 2024 systematic review analyzing 14 studies on polarized training found that a training distribution involving 15–20% high-intensity work combined with 75–80% low-intensity training was most beneficial for VO₂ max improvements in endurance athletes.

And here's something fascinating: a study on cardiac rehabilitation patients found that even within the 85–95% zone, exercising at the higher end (above 92% of max HR) produced significantly greater VO₂ max gains than exercising at the lower end. The estimated improvements were 5.2 mL/kg/min for the high group versus 3.1 mL/kg/min for those staying below 88%.

The takeaway? Intensity matters. And the 85–95% range is the sweet spot.

How to Calculate Your Zones

The simplest method uses your maximum heart rate. While laboratory testing is more accurate, the HUNT Fitness Study validated a simple formula based on their data from 3,320 healthy participants:

Max HR = 211 − (0.64 × Age)

For a 35-year-old, that's roughly 189 bpm (compared to 185 with the classic 220-minus-age formula).

From there:

Zone 2: 113–132 bpm (60–70%)
Zone 3: 132–151 bpm (70–80%)
Zone 4: 161–180 bpm (85–95%)

Notice the gap between Zone 3 and Zone 4. That's intentional. You want to be clearly in one or the other, not hovering at the boundary.

If you're a 50-year-old female, your max HR is approximately 179 bpm using this formula, making your Zone 4 range 152–170 bpm.

The 80/20 Rule That Actually Works

Elite endurance coaches have converged on a remarkably simple prescription: spend 80% of your training time in Zone 2 and 20% in Zone 4. Skip Zone 3 almost entirely.

This polarized approach sounds counterintuitive. Shouldn't you gradually build up? Work in the middle before going hard?

The research says no. A meta-analysis in the Journal of Strength and Conditioning Research found that polarized training resulted in significantly greater improvement in time-trial performance compared to threshold training. Another study from Frontiers in Physiology showed that of four training concepts tested on well-trained athletes, polarized training produced the greatest increases in VO₂ peak.

Here's why it works:

Zone 2 (80% of training) builds your aerobic engine without accumulating fatigue. You can do a lot of it. You recover quickly. It improves your base and enhances your body's ability to utilize fat as fuel.

Zone 4 (20% of training) provides the stimulus for cardiovascular adaptation. It's the signal that tells your body to improve. Because you're well-rested from all that easy Zone 2 work, you can actually hit Zone 4 intensities properly.

Zone 3 does neither effectively. It's hard enough to create fatigue but not hard enough to trigger adaptation. It's the worst of both worlds.

What About Zone 2 for Mitochondria?

You might have heard that Zone 2 is the optimal intensity for building mitochondria. This claim has exploded in popularity through podcasts and social media. But the science is more nuanced.

A 2025 narrative review published in Sports Medicine critically examined this claim and concluded that current evidence does not support Zone 2 training as the optimal intensity for improving mitochondrial capacity. The researchers found that higher-intensity exercise actually activates mitochondrial signaling pathways more strongly and produces larger improvements in enzyme activity.

That said, Zone 2 isn't useless. It allows you to accumulate training volume without excessive fatigue. It improves fat oxidation in beginners. And when combined with high-intensity work in a polarized model, it creates a solid foundation.

The key insight: Zone 2 is valuable as part of a training program, not as the entire program.

Practical Application: A Sample Week

Let's say you train five times per week. Following the 80/20 rule:

Monday: 45 minutes Zone 2 (easy run/bike/swim)
Tuesday: Zone 4 intervals (e.g., 4 × 4 minutes at 90% max HR with 3-minute recoveries)
Wednesday: Rest or light Zone 2
Thursday: 60 minutes Zone 2
Friday: Rest
Saturday: Zone 4 intervals or a tempo session that touches Zone 4
Sunday: Long Zone 2 session (60–90 minutes)

That's roughly 4 hours of training with 3+ hours in Zone 2 and 40–50 minutes of accumulated Zone 4 work.

The 4×4 protocol (four 4-minute intervals at 90–95% max HR separated by 3-minute recovery periods) is particularly well-studied. Research published in PLOS ONE found that participants spent approximately 13 of the 16 working minutes between 85 and 95% of max HR using this protocol. It's effective and time-efficient.

How to Know It's Working

Track your VO₂ max over time. If you have an Apple Watch, it estimates this automatically based on your outdoor walks and runs. You can find it in Apple Health under Cardio Fitness.

Expect gradual improvement. A gain of 1–2 mL/kg/min over several months is meaningful. Compare your progress against benchmarks for your age and gender to understand where you stand.

More subjectively, you'll notice that paces that once felt hard start feeling easier. Your recovery heart rate improves. You can sustain conversations at speeds that used to leave you breathless.

The Mistake Most People Make

The most common error isn't training too little. It's training too much in the wrong zone.

People go out for an "easy" run but let their ego push the pace. Their heart rate drifts into Zone 3. It feels like work. They're sweating. They assume they're improving.

They're not. At least, not efficiently.

Meanwhile, their Zone 4 sessions suffer because they're fatigued from all that Zone 3 work. They can't hit the intensities that actually drive adaptation.

The fix is simple but requires discipline: slow down your easy days. Like, really slow. Embarrassingly slow. So slow that you wonder if it counts as exercise.

Then, when it's time for Zone 4 work, actually go there. Push into that 85–95% range. Make it count.

The Bottom Line

Improving VO₂ max comes down to spending enough time at the right intensity. Zone 4 (85–95% of your maximum heart rate) is where cardiovascular adaptation happens. Zone 2 supports this by building your aerobic base without creating excessive fatigue. Zone 3 mostly wastes your time.

Follow the 80/20 polarized model. Go easy most of the time. Go hard when it matters. Skip the middle.

Your VO₂ max benchmarks will thank you.

References:

Helgerud J, et al. (2007). Aerobic high-intensity intervals improve VO₂max more than moderate training. Medicine & Science in Sports & Exercise. PubMed
Stöggl TL, Sperlich B. (2014). Polarized training has greater impact on key endurance variables than threshold, high intensity, or high volume training. Frontiers in Physiology. PMC
Moholdt T, et al. (2014). The higher the better? Interval training intensity in coronary heart disease. Journal of Science and Medicine in Sport. ScienceDirect
Nes BM, et al. (2013). Age-predicted maximal heart rate in healthy subjects: The HUNT Fitness Study. Scandinavian Journal of Medicine & Science in Sports. Wiley
NTNU CERG. Fitness Calculator and HUNT Fitness Study data. NTNU
Storoschuk KL, et al. (2025). Much Ado About Zone 2: A Narrative Review. Sports Medicine. PubMed

Want to track your VO₂ max progress automatically? VO2 Max Pro syncs with Apple Health to calculate your biological age and show you exactly where you stand compared to others your age.