How Long Does It Actually Take to Improve Your VO₂ Max?

I trained consistently for six weeks. The runs felt easier. My recovery improved. Hills that used to crush me started feeling manageable.

Then I checked my Apple Watch. The VO₂ max number hadn't moved.

This is probably the most common frustration in fitness tracking. You feel the changes happening, but the metric that's supposed to capture your progress just sits there unchanged. It's disorienting. Are you actually getting fitter? Is something wrong with the watch? Should you train differently?

After digging into the research, I realized the answer is simpler and more humbling than I expected: cardiovascular fitness just takes a long time to change. Not because something is wrong, but because of what improvement actually requires at the biological level.

Why cardiovascular fitness changes slowly

When your VO₂ max improves, your body is doing construction work. Your heart's left ventricle grows larger and stronger, pumping more blood per beat. New capillaries form throughout your muscles. Mitochondria multiply inside your cells. Blood plasma volume increases.

These aren't quick adjustments. They're tissue remodeling projects that unfold over weeks and months.

This is different from strength training, where your nervous system can rapidly learn to recruit more muscle fibers—making you stronger before your muscles have actually grown. Cardiovascular fitness has no such shortcut. Every improvement requires physical construction at the cellular level.

Once I understood this, I stopped treating each watch reading as a verdict on my training.

What the research says about timelines

The most comprehensive study on this is the HERITAGE Family Study, which put 481 sedentary adults through an identical 20-week training program.

The average improvement was about 400 mL/min. But here's what struck me: the range was enormous. Some people gained over 1,000 mL/min. Others gained almost nothing. Same program, same duration, wildly different results.

The researchers found that genetics explained much of this variation—there was 2.5 times more variance between families than within families. Twin studies estimate that 50-72% of VO₂ max variation comes down to your DNA.

This doesn't mean training is pointless. It means comparing your timeline to someone else's isn't useful. Your body will respond on its own schedule.

So what can you actually expect?

If you're starting from sedentary, you'll likely see the fastest gains. Research shows significant improvements in as little as 8 weeks. Over several months of consistent training, 10-20% improvement is reasonable.

If you're already moderately fit, expect a longer wait. Exercise physiologists typically don't retest clients sooner than 3 months because meaningful changes take that long. You might wait 4-8 weeks before seeing even 1-2 mL/kg/min of movement.

If you're already highly trained, honest truth: you could train perfectly for a year and see only 1% improvement, or nothing at all. This isn't failure—it's proximity to your genetic ceiling.

The encouraging news is that small gains matter more than they seem. A single MET increase (~3.5 mL/kg/min) is associated with 10-25% better survival odds. You don't need dramatic improvements for meaningful health benefits.

Why your watch readings bounce around

Part of the frustration comes from how Apple Watch measures VO₂ max. It doesn't measure directly—that would require a lab. Instead, it estimates cardio fitness by analyzing your heart rate relative to your pace during outdoor walks, runs, and hikes.

This means lots of factors create noise in your readings:

Heat and humidity raise your heart rate independent of fitness, so a summer run might show a lower VO₂ max than a cool spring day—even if you're fitter. Poor sleep, alcohol, or a mild cold will inflate your heart rate and drag down your estimate. And the watch itself has limits: a 2025 validation study found Apple Watch underestimates VO₂ max by about 6 mL/kg/min on average.

Individual readings will bounce around. The trend over months is what matters.

How to know you're improving before the number moves

While waiting for your VO₂ max to budge, look for these signs that adaptation is happening:

Lower heart rate at the same pace. If your easy runs used to average 145 bpm and now average 138 bpm at the same speed, your heart is pumping more blood per beat. This is the foundation of VO₂ max improvement.

Faster recovery during intervals. Notice how quickly your heart rate drops during rest periods. Faster recovery means better cardiovascular efficiency.

Same effort, faster pace. If your comfortable conversational pace crept from 10:30/mile to 10:00/mile without feeling harder, that's real progress.

These changes often appear before your VO₂ max reading updates, because they're less affected by day-to-day noise.

One thing that will definitely slow you down

VO₂ max drops faster than it rises. Two to four weeks without training can cause a 4-14% decline. Building that back takes 8-12 weeks.

This asymmetry has a biological logic—your body sheds capacities it isn't using to conserve resources. But the practical implication is clear: consistency matters more than intensity. Missing a few days won't hurt. Missing a few weeks will set you back significantly.

The training that actually works

If you want to maximize your rate of improvement, the research points to two things.

First, high-intensity intervals at 85-95% of max heart rate provide the strongest stimulus. The 4×4 protocol—four 4-minute intervals at 90% max HR with 3-minute recoveries—is well-validated.

Second, that hard work only helps if it's built on a base of easy volume. The 80/20 polarized approach—80% of training easy, 20% hard—works better than grinding in the middle. Most people spend too much time in Zone 3, which feels productive but isn't optimal for building fitness or recovering properly.

A realistic month-by-month picture

Weeks 1-4: Focus on showing up. Adaptations are starting but won't appear in metrics. This is where consistency matters most.

Weeks 4-8: You might see early movement in readings, or you might not. Either way, runs should feel easier and recovery should improve.

Months 2-3: Measurable changes typically emerge for moderately fit people. A 35-year-old starting at 38 mL/kg/min might see early movement toward 40-42.

Months 3-6: This is where meaningful improvement happens. With dedicated training, reaching 42-45 mL/kg/min from a 38 baseline is realistic—a significant shift in cardiovascular age.

Beyond 6 months: Returns diminish as you approach your ceiling, but small gains compound. Consistent training over years produces results that wouldn't seem possible at the start.

The bottom line

VO₂ max moves slowly because cardiovascular adaptation is a construction project. Your genetics influence how much you can improve and how fast. Your watch readings will bounce around in the short term, but the trend over months tells the real story.

Check your current benchmark to see where you stand. Track your trend over months, not days. And pay attention to how your runs feel—that's data too.

The timeline for real change is measured in months. Once I accepted that, tracking my VO₂ max became a lot less stressful and a lot more useful.