Running Index vs. VO₂max: friends or rivals?
Ferenc Kovacs Soma – November 18, 2025
If you’ve ever delved into your Polar Flow data, you’ve probably come across two iconic metrics that seemingly measure the same thing: Running Index and VO₂max . Both metrics describe your endurance and performance, but in two different ways. In this article, I’ll explain how these two metrics are related, how they differ, and why you should take both seriously.
VO₂max – or engine size
VO₂max , or maximal oxygen uptake, is the amount of oxygen your body can use per kilogram of body weight in one minute (ml/kg/min). It is one of the most important indicators in sports physiology because it is directly related to aerobic capacity: the higher this value, the more oxygen is delivered to the muscles, and thus the more energy they can produce .
The classic VO₂max measurement is performed in a lab: on a treadmill or ergometer with an oxygen mask, with a gradual increase in load. The test lasts until the athlete is completely exhausted. This allows an estimate of the body's absolute maximum.
Polar's Fitness Test attempts to estimate this from resting heart rate, heart rate variability and demographic data. It doesn't require running - so it's more of a basic fitness indicator than a reflection of daily performance. VO₂max therefore quantifies the maximum potential of an athlete's body .
The Running Index – efficiency measured in reality
The Running Index, on the other hand, is not created in a lab, but on the treadmill. It measures the ratio of speed and heart rate while running, and from this it deduces the body's current aerobic efficiency. In simpler terms: it shows how "economically" the athlete moves at a given intensity.
While VO₂max tells you how big your body's "engine" is, Running Index tells you how you drive the car. A runner with a VO₂max of 80 ml/kg/min is incredibly fit on paper, but if their running economy is poor, they may perform worse on hilly terrain or in hot weather than someone with a lower VO₂max but higher efficiency.
Running Index is calculated at the end of each run and provides immediate feedback on how the runner performed under the given conditions. Running Index is therefore not a theoretical but a practical indicator. This difference is its true power: it can show subtle changes that VO₂max does not always detect.
Two different perspectives on the same system
So VO₂max and Running Index are not rivals, but two different measures of aerobic performance. One measures maximum potential , the other measures actual performance . VO₂max tells you what you can do under optimal conditions, while Running Index tells you how that works in practice – in the heat, in the rain, on hills, with tired legs.
For most runners, the two values go hand in hand: as fitness improves, both VO₂max and Running Index increase. But Running Index is more sensitive to daily fluctuations. A poor night’s sleep, a hot afternoon, or a stressful day can easily pull the value down without VO₂max actually deteriorating.
That's why it's worth monitoring both: if the Running Index fluctuates but the VO₂max is stable, it's probably just a temporary load. However, if both decrease, it could indicate a real deterioration in fitness or overtraining.
How do we read them together?
Let's say your VO₂max value on your Polar Flow graph has been roughly stable for the past three months, say around 60 ml/kg/min, while your Running Index has been gradually increasing on a weekly average. This indicates that your body is using oxygen more efficiently: the engine size hasn't increased, but it's using less fuel to produce the same amount of power. Efficiency is improving.
If your Running Index is stagnant while your VO₂max is increasing, it may be that your lab “motor power” has increased, but your running economy or technique has not yet caught up. In this case, it is worth working on your running technique, frequency, and strength to make the gains you have made on paper show up in real life.
A typical Flow timeline often looks like this: at the beginning of the season, during the conditioning period, VO₂max increases slightly, but the Running Index responds more slowly. During the competition period, when form is at its peak, the Running Index shoots up – while the VO₂max only moves subtly. So the real improvement is not necessarily in the “bigger engine”, but in more efficient operation.
When should we rely on which one?
VO₂max is used to assess long-term trends. It's useful if you want to see if your aerobic capacity has increased during a specific training period (for example, after a winter warm-up).
Running Index, on the other hand, is a tool for everyday feedback . If you run regularly, your RI graph in Flow is a live indicator of how your body is responding to your workouts. Its weekly averages are especially useful: if it's trending down, you're probably more tired than you think; if it's steadily rising, your body is adapting well.
The best approach is to read the two together: VO₂max gives you the horizon, and Running Index gives you the rhythm of your daily steps.