Why Your VO₂ Max Isn't Improving (Even Though You Run a Lot)

I used to think more running meant better fitness. Three to four runs a week, 30 to 45 minutes each, at a pace that felt "challenging but sustainable." My legs were getting stronger. My resting heart rate dropped a little. But my VO₂ max stayed completely flat for months.

Then I realized something that changed everything: I was training in no man's land.

The uncomfortable truth about "moderate" running

Here's what I discovered after digging into the research: the pace that feels like "a good workout" is probably the worst intensity for improving your VO₂ max.

Sports scientists call it the "black hole" of training. It's Zone 3, roughly 80 to 87% of your max heart rate. Hard enough to feel like you're doing something. Not hard enough to trigger the adaptations that actually improve your aerobic ceiling.

Dr. Stephen Seiler, who has spent decades studying elite endurance athletes, found something surprising: the best athletes in the world spend almost no time in this middle zone. His research on Olympic-level cross-country skiers, rowers, and cyclists consistently shows about 75 to 80% of their training happens at low intensity, with 15 to 20% at high intensity. The middle zone? Around 5%.

Meanwhile, recreational runners do the opposite. We gravitate toward moderate effort on almost every run because it feels productive. It's not.

Why Zone 3 is a dead zone for VO₂ max

Your body adapts to specific demands. Zone 3 training sits in an awkward middle ground where you're:

Too hard to build a massive aerobic base (that's what easy Zone 2 running does)
Too easy to push your cardiovascular system to grow stronger (that requires Zone 4 and 5)

A landmark 2007 study by Norwegian researchers Helgerud and colleagues compared four training protocols over eight weeks. All groups did the same total work. The results?

Running at 70% of max heart rate (easy pace): VO₂ max improved by 0.5 ml/kg/min.

Running at 85% of max heart rate (lactate threshold, Zone 3): VO₂ max improved by 2.4 ml/kg/min.

4x4 minute intervals at 90 to 95% max heart rate: VO₂ max improved by 7.2 ml/kg/min.

The high-intensity group improved their VO₂ max nearly three times more than the "threshold" group, despite doing the same amount of work. This wasn't just about working harder. It was about working at the right intensity.

What "easy" actually means

This is where it gets uncomfortable. True easy running should feel almost too slow. You should be able to hold a full conversation without gasping. Your breathing stays relaxed. You might feel like you're not really working out.

For most people, easy running happens below 75% of max heart rate. For a 35-year-old male, that might mean keeping your heart rate under 140 bpm on recovery days. A 40-year-old female might aim for under 135 bpm.

I started tracking my runs more carefully and discovered my "easy" runs were actually in Zone 3. My heart rate would drift to 155 to 165 bpm because that pace felt comfortable to my ego. But it was too hard to recover from properly and too easy to stimulate real cardiovascular adaptation.

The Norwegian 4x4 protocol

The same research team that discovered the Zone 3 problem also developed one of the most effective protocols for improving VO₂ max. It's called the Norwegian 4x4, and it's brutally simple:

Warm up for 10 minutes
Run 4 minutes at 85 to 95% of your max heart rate
Recover for 3 minutes at 60 to 70% (active walking or light jogging)
Repeat 4 times
Cool down

That's it. Sixteen minutes of hard work. Studies show this protocol can improve VO₂ max by 10 to 11% in just six to eight weeks, even in people who are already moderately trained.

The key is hitting the right intensity during those four-minute blocks. You should be breathing hard enough that speaking in complete sentences becomes difficult. If you can chat comfortably, you're not going hard enough.

A practical example from the treadmill

Let's say you normally run at 6.0 mph (10 min/mile pace) and your heart rate sits around 160 bpm. That might feel like a solid effort. But if your max heart rate is 185, you're running at about 86% of max. Classic Zone 3.

Instead, try this structure for one run per week:

Easy runs (3 to 4 times per week): Drop to 5.0 mph or whatever keeps your heart rate below 140 bpm. Yes, it will feel slow. That's the point.

One interval session: After warming up, increase to 7.5 to 8.0 mph for 4 minutes. Your heart rate should climb to 165 to 175 bpm by minute two or three. Recover at 4.0 mph for 3 minutes. Repeat four times.

The contrast should feel dramatic. Your easy days feel almost lazy. Your hard day feels legitimately challenging. No more "moderate effort" runs that accomplish nothing.

What the Apple Watch is actually measuring

Your Apple Watch estimates VO₂ max during outdoor walks, runs, and hikes by analyzing your heart rate relative to your pace. It needs GPS data and a workout lasting at least 20 minutes. The data is there, but Apple buries it deep in the Health app.

But here's the catch: the watch measures your current fitness. It doesn't tell you whether your training is working.

I've seen my own Apple Watch VO₂ max reading bounce around by 1 to 2 points from week to week based on sleep, hydration, and how I felt that day. The trend over months matters more than any single reading.

If your VO₂ max has been stuck in the same range for three months or longer despite consistent running, that's a signal. Your training needs to change. More miles at the same moderate effort won't help.

Common mistakes I made (and you might be making too)

Running every run at the same effort. This was my biggest problem. I would start each run thinking "I'll take it easy today" and somehow end up at 75 to 85% effort by the end. No easy days meant no recovery. No hard days meant no stimulus.

Ignoring heart rate data. I used to run by feel and pace. But feel is unreliable. On hot days, your heart rate runs higher at the same pace. When you're tired or stressed, the same effort feels harder. Heart rate doesn't lie.

Chasing weekly mileage instead of training quality. Twenty-five miles of mostly Zone 3 running is worse than fifteen miles that includes two easy days, one long easy run, and one proper interval session.

Not allowing enough recovery. After a hard interval session, your body needs 48 hours or more to adapt. Stacking another moderate run the next day interrupts that process. Easy means easy.

What actually works: the 80/20 approach

Research consistently shows that successful endurance athletes organize their training around a simple distribution: roughly 80% of training time at low intensity (Zone 1 to 2), and 20% at high intensity (Zone 4 to 5). Almost nothing in the middle.

For someone running four times per week, that might look like:

Monday: Easy 40 minutes (Zone 2)
Wednesday: 4x4 intervals (Zone 4 to 5)
Friday: Easy 30 minutes (Zone 2)
Sunday: Long easy run, 60 to 75 minutes (Zone 2)

Total time: about 3 hours. Time in Zone 3: zero.

This polarized approach works because it respects how your body adapts. Easy training builds your aerobic foundation without accumulating stress. Hard training forces your cardiovascular system to grow. The middle zone just makes you tired without improving either.

How to know your zones

The simplest method uses max heart rate. If you don't know yours, the rough formula is 220 minus your age. For a 40-year-old, that's about 180 bpm.

Zone 1 (very easy): 60 to 70% of max (108 to 126 bpm)
Zone 2 (easy): 70 to 80% of max (126 to 144 bpm)
Zone 3 (moderate): 80 to 87% of max (144 to 157 bpm)
Zone 4 (hard): 88 to 92% of max (158 to 166 bpm)
Zone 5 (very hard): 93 to 100% of max (167+ bpm)

For most easy runs, stay in Zone 2. For intervals targeting VO₂ max, you want Zone 4 to 5. You can check what's considered average for your specific age and gender to understand where your current fitness stands.

Better yet, do a field test. Run hard for 20 minutes on flat ground and record your average heart rate for the last 10 minutes. That approximates your lactate threshold, which typically falls around 85 to 90% of max heart rate.

The timeline for seeing results

If you restructure your training today, you should start seeing measurable VO₂ max improvements within four to six weeks, assuming you stick with the polarized approach and actually hit the right intensities during your hard sessions.

The Helgerud study showed significant gains in eight weeks. Other research suggests improvements can continue for 12 weeks or longer before plateauing.

After that, maintaining your VO₂ max requires ongoing high-intensity work. Two interval sessions per week seems to be enough for most people. Skip them for more than two to three weeks and your aerobic ceiling starts to drop.

What I do now

My current routine looks nothing like it did a year ago:

Three easy runs per week where I genuinely keep my heart rate low. I run slower than feels natural, and I've made peace with that.

One interval session, usually the 4x4 protocol on a treadmill set to a slight incline. The incline makes it easier to get my heart rate up without destroying my legs.

One long easy run on weekends when I have time.

My VO₂ max has climbed from 42 to 48 over the past ten months. That's a six-year reduction in biological age according to the HUNT Fitness Study data. For a 36-year-old, going from 42 to 48 moves you from "average" to "above average" territory.

The biggest change wasn't running more. It was running smarter.

Track your VO₂ max progress over time with VO2 Max Pro. Get instant notifications when your Apple Watch records new data, see your biological age, and understand how your training is working.