VO₂ Max
VO₂ max is a measure of your aerobic fitness — the maximum amount of oxygen your body can take in and use during hard exercise. It’s one of the most established indicators of cardiorespiratory fitness and endurance capacity, expressed in milliliters of oxygen per kilogram of body weight per minute (mL/kg/min). A higher VO₂ max generally means your heart, lungs, and muscles are more efficient at delivering and using oxygen, which translates to a stronger aerobic engine and better endurance.
How Reckon estimates it
A true VO₂ max is measured in a lab using a graded exercise test and specialized equipment. Reckon provides an estimate instead, built from data it can capture during your normal activity — primarily the relationship between your pace and your heart rate. The underlying idea is straightforward: the faster you can move at a given heart rate (or the lower your heart rate at a given pace), the higher your estimated aerobic capacity. Reckon analyzes this relationship across your activities and refines the estimate over time.
Because it’s derived from wrist-based sensors rather than lab equipment, your number is an approximation and won’t necessarily match a clinical test exactly.
It’s also worth understanding that this is a submaximal estimate — it’s built from everyday activity rather than an all-out maximal effort, so it relies on an estimate of your maximum heart rate to project what your capacity would be at full intensity. Because of this, the accuracy of your VO₂ max depends on the accuracy of that maximum heart rate: if your max heart rate is estimated too high, your VO₂ max will be artificially inflated, and if it’s estimated too low, your VO₂ max will be artificially deflated. The error carries through in the same direction.
Why the trend matters more than the number
As with most fitness metrics, the direction your VO₂ max is heading matters more than the exact value. An estimate that’s a few points off from a lab measurement still tells you what you need to know as long as it’s tracking consistently. A VO₂ max that climbs over weeks and months means your aerobic fitness is improving; a steady decline can be an early signal of detraining, accumulated fatigue, or illness. Focus on your own trend rather than comparing your number to other people or reading too much into any single value.
Getting a reliable estimate
The estimate improves with use, especially from sustained efforts where both pace and heart rate are meaningful, like runs and rucks. For the most consistent results, wear the watch snugly so heart rate readings stay accurate, and keep in mind that factors like heat, altitude, or significant fatigue can temporarily skew an individual reading. Known Issue We are currently working on a software update that affects how long your estimated VO₂ max remains visible. At the moment, the estimate only appears on the day of the run. Once resolved, your estimated VO₂ max will stay visible for 72 hours after a completed run.