Running Index Demystified: How Polar Turns Every Run Into Insight
Darius Silaghi
When it comes to understanding your running performance, a single number can sometimes tell a story that pace charts and heart rate graphs only hint at. Polar’s Running Index does exactly that: it condenses speed and heart rate data from nearly every run into a single, validated score that reflects your endurance fitness. But it’s more than just a snapshot—it’s a long-term trend, giving runners a way to see genuine progress over weeks and months, even when their pace or heart rate seems stubbornly steady.
What's all this running index?
Unlike VO₂max estimates from other platforms, which often feel abstract or require specific tests, Running Index updates automatically and frequently, translating real-world runs into actionable insight. It accounts for the inevitable variations in terrain, conditions, and day-to-day energy levels, so a slightly lower score on a trail run doesn’t mean you’re weaker—it simply reflects the extra effort required. Over time, these scores reveal patterns and improvements that can be surprisingly motivating: you may notice your Running Index climbing steadily, even if your stopwatch doesn’t show dramatic changes.
In this article, we’ll dig into the science and technology behind Running Index, explore how it compares to other fitness metrics, and examine what it really tells us about our potential as runners. I’ll also share my own data and experiences, showing how this metric has tracked my progress in ways that pace and heart rate alone never could.
This article was created with the support of Danubius Track Club and Polar Hungary. Danubius Track Club provides professional training conditions for countless athletes, while Polar’s cutting-edge technology enables modern, precise, and conscious training and recovery.
The Physiological Background of Running Index
Running performance essentially measures how fast you can cover a distance, and it’s shaped by three main physiological factors: aerobic capacity, body weight, and running economy.
Aerobic capacity (VO₂max)
Aerobic capacity reflects how efficiently your body can take in, transport, and use oxygen during sustained exercise. In endurance running, this is critical: the lungs bring in oxygen, the heart pumps it through the bloodstream, and the muscles use it to produce energy. The higher your aerobic capacity, the more oxygen your muscles can utilize, and the better your endurance performance. In practical terms, this means a runner with higher VO₂max can maintain faster paces for longer distances.
Body weight
Performance isn’t just about the engine; it’s about the weight it’s moving. Two athletes with the same VO₂max will perform differently if their body weights differ. To account for this, aerobic capacity is normalized relative to body weight—measured as milliliters of oxygen per kilogram per minute. This helps explain why lighter runners can often sustain faster paces with the same oxygen-carrying capacity.
Running economy
Running economy measures how efficiently your body converts oxygen into forward motion. Simply put, it’s the oxygen cost of running a given distance. The lower your oxygen cost, the more efficiently you run, and the faster you can go at a given effort. Factors influencing running economy include biomechanics, muscle fiber composition, and training history. Experienced runners often have better running economy, which means they can run faster with less energy expenditure.
How Running Index Uses These Factors:
Polar’s Running Index integrates these physiological principles into a single, real-time score. Rather than requiring lab tests, it estimates your aerobic efficiency during actual runs by analyzing your speed and heart rate. If you run faster at the same submaximal heart rate—or if your heart rate decreases at a given speed—your Running Index improves. Conversely, slower speed or higher heart rate at the same pace will lower the index.
This approach allows Running Index to capture not just raw VO₂max potential, but also improvements in running economy and cardiovascular efficiency. It reflects how well your body performs in real-world conditions, rather than theoretical maximums.
Why it Matters:
Understanding this physiological basis is key because it shows that Running Index isn’t just a number—it’s a reflection of your aerobic fitness, efficiency, and endurance adaptation over time. It helps runners see progress beyond pace charts or heart rate alone and gives coaches and athletes a meaningful tool to track performance trends.
The Technological Background of Running Index
While the physiological background explains why Running Index reflects performance, the technology shows how it does so in practice. Polar designed Running Index to work seamlessly during real-world runs, translating your speed and heart rate into a meaningful measure of aerobic efficiency.
Sport-specific requirement:
Running Index is only calculated when you are in a running sport profile on your Polar device. That means if you start an outdoor walk or cross-train in another mode, Running Index won’t be recorded. This ensures that the calculation is tailored specifically to running mechanics and effort.
Core data inputs
The algorithm relies primarily on speed and heart rate data. If your watch has an altitude sensor, it also factors in slope to compensate for uphill and downhill running. Without altitude data, the index can still be calculated, but it’s worth noting that hilly terrain may slightly lower your score because uphill running increases heart rate at any given pace.
In addition, Running Index uses your maximal and resting heart rate to model aerobic efficiency:
- Maximal heart rate (HRmax): the highest heart rate you can achieve during all-out effort.
- Resting heart rate (HRrest): the lowest heart rate measured at complete rest (not during sleep).
These values are stored in your Polar Flow profile and help the algorithm understand your personal cardiovascular range, allowing it to estimate aerobic performance more accurately.
How Running Index is calculated
Running Index is updated automatically while you run, using a robust algorithm that filters out small interruptions. Whether you pause to tie a shoelace or enter a tunnel that temporarily disturbs GPS signals, your Running Index remains accurate.
Technically, Running Index is an estimation of VO₂max expressed in ml·kg⁻¹·min⁻¹. Importantly, you don’t need to push to your maximum effort for the calculation to be valid. As long as your run exceeds 12 minutes and your pace is above 6 km/h, the algorithm can generate a reliable score.
Feedback and context
After your run, Polar provides your Running Index along with personalized feedback. This feedback places your performance in context, comparing it to a reference group of runners of the same age and gender. Over time, these scores track your endurance fitness trends and can even help predict race times based on how efficiently your body uses oxygen at submaximal efforts.
Why it Matters technologically
By combining physiological principles with smart, real-time analysis of heart rate, speed, and terrain, Running Index gives runners a practical, actionable number that reflects true performance potential. Unlike raw pace or heart rate alone, it accounts for efficiency, cardiovascular capacity, and running conditions—making it a valuable tool for monitoring long-term progress and planning training.
My Experience with Running Index
I’ve been using Polar devices for almost 10 years now, and one of the best things about Running Index is how it lets me look back at that decade of data. Seeing trends over time—both the peaks of my athletic performance and the less-than-great periods—makes the numbers feel much more meaningful. It’s one thing to notice a faster pace today; it’s another to see how my endurance and efficiency have evolved over years.
2021, getting COVID after peak shape (luckily JUST after indoor nationals)
For me, Running Index has been an interesting complement to my current VO₂max estimates, which generally hover around 80–85 ml·kg⁻¹·min⁻¹, though they naturally fluctuate day-to-day. Of course, a true VO₂max measurement requires a lab test with specialized equipment—but even Polar offers a more controlled alternative in its Fitness Test (which I’ll cover in another article). Running Index isn’t a replacement for that, but it gives a practical, on-the-road view of how my body is performing in real-world conditions.
I noticed that the numbers fluctuated early on, especially as the device “learned” my heart rate patterns and running habits. Different types of runs also produced slightly different values: an easy, happy run could yield a surprisingly high Running Index, just as a tough, exhausting but successful workout could. The common thread seems to be runs where I kept my heart rate relatively steady for a sustained period—the more consistent and prolonged the effort, the higher the Running Index.
I’ve found that using a Polar Verity Sense or H10 chest strap produces the most reliable data, which makes sense given their precision in capturing heart rate. While Running Index isn’t a “go-to” number for every run, it has become a powerful tool for comparing myself to my past self. It’s especially rewarding to see long-term trends, to notice when I was at my strongest, and to identify periods when my performance dipped—turning historical data into actionable insight for training and motivation.
How to Use Running Index to Guide Your Training
Polar’s Running Index (RI) is a simple yet powerful tool because it’s automatically generated on nearly every run. That simplicity makes it easy to accumulate a rich dataset over time, and it’s in that long-term perspective where RI truly shines. Here’s how runners can use it effectively:
Track long-term trends, not daily fluctuations
Your Running Index will naturally vary from run to run due to fatigue, terrain, weather, or even how you feel that day. Rather than stressing over small fluctuations, focus on the broader trends over weeks and months. A steady upward trajectory indicates real improvements in endurance and efficiency, even if your pace charts seem static.
Estimate race readiness
RI can provide insight into your preparedness for events like 5K, 10K, or longer races. By observing how your Running Index changes on runs that mimic race conditions, you can gauge whether your body is ready to perform efficiently at the expected intensity. Use it as one piece of the puzzle alongside your pacing strategy and recent training volume.
Monitor fatigue and recovery
Drops in your Running Index can signal more than a rough day—they may indicate fatigue, inadequate recovery, or stress. Keeping an eye on these dips allows you to adjust training, add rest, or manage intensity before it impacts your overall progress.
Contextualize with environment and conditions
RI is influenced by terrain, heat, altitude, and other external factors. Compare scores against environmental conditions to avoid misinterpreting a lower score as a decline in fitness. For example, a hot, hilly, or trail run may produce a lower RI, but it doesn’t mean your aerobic capacity has dropped.
Set personal max and resting HR accurately
For the most reliable Running Index data, ensure your maximal and resting heart rate are up to date in your Polar Flow profile. Accurate HR values allow the algorithm to estimate aerobic efficiency more precisely, giving you a score that truly reflects your performance potential.
Optimal results require steady, longer runs
The algorithm handles small interruptions and signal disturbances, but the most repeatable results come from longer runs (over 30 minutes) at a steady pace. Short, highly variable runs are less reliable for tracking performance trends.
Embrace the ongoing conversation
Think of Running Index as a dialogue between your body and the data. The goal isn’t to chase a higher number every run, but to track consistent efficiency improvements and learn from your patterns. It encourages you to run smart, notice subtle gains, and make informed decisions about training load, pacing, and recovery.
RI is an estimation, not a lab measurement
RI simplifies running performance into a single number representing VO₂max. It cannot independently measure anaerobic threshold, running economy, or all the physiological nuances of maximal performance. It’s based on submaximal running data supplemented by your HR range to predict maximal performance. Race time estimates, similarly, are guidelines rather than guarantees, influenced by course conditions, fatigue resistance, and proper race preparation.
Why it matters
Using Running Index consistently keeps you accountable to your training, sparks curiosity about your progress, and provides tangible evidence of improvement—even when pace, heart rate, or perceived effort doesn’t tell the whole story. Over time, it transforms everyday runs into a meaningful dataset that helps you train smarter and celebrate progress.
Reflecting on Running Index
Polar’s Running Index distills the complexity of running performance into a single, understandable number, yet behind that simplicity lies a deep connection to your physiology and training. It combines heart rate, speed, and your personal HR profile to estimate aerobic efficiency and provide a practical proxy for VO₂max, all while taking real-world conditions into account. It’s a tool that doesn’t just measure performance—it reveals trends over time, showing when your body is adapting, when recovery is needed, and how consistent effort translates into improvement.
For me, Running Index has been a window into my own running journey. Over nearly a decade of Polar usage, I’ve seen the highs and lows of my fitness, discovered patterns across different run types, and learned to value steady, sustainable progress over chasing a single number. Easy runs and challenging workouts alike have contributed to my understanding of efficiency, and RI has helped me see that improvement is often subtle, cumulative, and most visible in the long term.
My hernia diagnosis from 2022, the stop and the fallback is clearly visible
For anyone looking to deepen their relationship with their running, I encourage exploring your own RI data. Use Polar Flow reports to track trends, compare your past performances, and consider environmental factors. Focus on consistency, not perfection—celebrate incremental gains and learn from fluctuations rather than being discouraged by them. Over time, this approach turns every run into both a workout and a data point, helping you grow smarter, fitter, and more connected to your body.
Looking ahead, the potential for Running Index is exciting. Imagine AI-based recovery modeling that combines RI trends with sleep, training load, and nutrition, offering even more personalized guidance on when to push, when to rest, and how to optimize your performance. The future could make this insight even richer, helping runners not only track performance but truly understand how to maximize it.