VO2 Max

What VO2 Max Measures

VO2 max reflects how much oxygen the body can move from the lungs into the blood and then deliver to working muscles at maximum effort. It's driven by cardiac output (how much blood the heart pumps per minute) and by how efficiently the muscles extract and use the oxygen that arrives. Because both the cardiovascular system and the muscles themselves set the ceiling, VO2 max is treated as a whole-body measure of cardiorespiratory capacity rather than a single organ's output.^[2]

How It's Measured

The gold-standard method is a cardiopulmonary exercise test (CPET), a graded exercise test done on a treadmill or stationary bike in a lab or clinic. The workload increases in stages until exhaustion while a mask captures the oxygen and carbon dioxide in every breath, so VO2 max is calculated directly from measured gas exchange.^[1] A watch or fitness tracker works differently: it doesn't measure gas exchange at all. Instead, it estimates VO2 max from heart rate and pace or speed data collected during outdoor walks or runs, run through a prediction algorithm. A 2025 validation study compared Apple Watch estimates against lab-measured VO2 max in the same 30 people and found the watch systematically underestimated it, off by an average of 6.07 mL/kg/min.^[3] Lab testing and wearable estimation are related but distinct measurement methods, not interchangeable numbers.

Why People Track VO2 Max

VO2 max is widely used as a marker of cardiorespiratory fitness, and exercise capacity more broadly is an independent predictor of health outcomes at a population level, which is part of why it gets tracked as a trend over time rather than a one-off number.^[2] Some people also watch it alongside cycle-phase data out of curiosity about whether it shifts across the month, though as the research below shows, that pattern isn't consistent.

How Go Go Gaia Tracks VO2 Max

Go Go Gaia doesn't calculate VO2 max itself. It syncs the estimate your Apple Watch already generates through Apple HealthKit, then charts it as a trend over time alongside your cycle phase and other biometrics like resting heart rate and HRV, so you can see how it moves in context rather than as an isolated number.

What the Research Shows

The evidence on whether VO2 max shifts across the menstrual cycle is genuinely mixed, and a 2025 narrative review in Frontiers in Endocrinology lays out both sides rather than settling on one. One study it cites found VO2 max about 2% lower in the mid-luteal phase compared to the late follicular phase, but the review also points to multiple other studies, in both untrained women and athletes, that found hormone fluctuations across the cycle didn't affect VO2 max at all.^[4]

A 2025 study in Reproduction & Fertility adds a possible reason for the disagreement: it found VO2 max varied significantly across cycle phases in people with ovulatory cycles, but stayed essentially flat across the cycle in people with anovulatory cycles.^[5] Whether a given study finds a cycle-phase effect on VO2 max may depend on who's in the sample and how ovulation is confirmed, not just on measurement precision. No consistent, reproducible pattern has emerged yet.

Related Terms

Frequently Asked Questions

Educational information based on published sources. Not medical advice. For personal concerns, please consult your doctor.

Does VO2 max change across the menstrual cycle?

The research is genuinely mixed. One frequently cited study found VO2 max about 2% lower in the mid-luteal phase compared to the late follicular phase, while several other studies found no significant change across the cycle at all. A 2025 study found the pattern only in people with ovulatory cycles, not in those with anovulatory cycles. There's no single, consistent finding in the literature.

How accurate is a smartwatch's VO2 max estimate compared to a lab test?

They're different measurement methods. A lab test (cardiopulmonary exercise test) directly measures the oxygen and carbon dioxide in your breath while you exercise to failure. A smartwatch estimates VO2 max from heart rate and pace or speed data during outdoor walks or runs, without directly measuring gas exchange. A 2025 validation study found Apple Watch underestimated lab-measured VO2 max by an average of about 6 mL/kg/min.

How does Go Go Gaia get my VO2 max number?

Go Go Gaia doesn't calculate VO2 max itself. It syncs the estimate your Apple Watch already generates through Apple HealthKit and charts it as a trend over time, alongside your cycle phase and other biometrics.