Have you ever wondered how fit you are? How this year compares to last year? What about how 5 days off in a row affects your fitness? These questions can be answered using the Base Fitness metric within the COROS EvoLab software.

Keep reading as we break down this metric ruling over your training data!

What is Base Fitness?

Base Fitness displayed in the Training Status tab of the COROS app.

Base Fitness can be found in the Training Status tab of your COROS app dashboard as a single value as well as a graph over time. Now you may be wondering, what does this value mean? Each activity you complete will have a Training Load score associated with it. The higher this score, the more stressful this activity was for your body. While this number can be insightful for day-to-day monitoring, its importance magnifies when you look at its long-term trend, Base Fitness.

In other words, Base Fitness reflects your long-term stress and your readiness to tackle more challenging sessions.

How is Base Fitness calculated? Base Fitness is a weighted 42-day rolling average of your Training Load scores. Since your body has a poor memory and only remembers your past few weeks of training, this metric accurately reflects your body's current fitness.

How To Monitor Base Fitness?

Emma Bates's Base Fitness graph during her 2023 Boston Marathon training.

Base Fitness is a great metric to track throughout the year to help you understand where you currently stand.


  • During this time, you may reduce volume and intensity, or even focus on cross-training to let your body rest. This ultimately leads to a decrease in Base Fitness.

Build season

  • This time of year when you move from the off-season to training often requires an increase in volume and/or intensity. This often leads to a consistent increase in Base Fitness up to your highest values.

Taper / Race

  • This period is a crucial moment shortly before a race where you reduce volume in order to be fresh come race day. This often leads to slight decrease in Base Fitness.
Can I compare my Base Fitness with a friend? Base Fitness is highly individualized to the sport you practice. Slow, long duration activities like ultras will naturally induce higher Base Fitness scores, while fast-paced activities like Track & Field will naturally maintain a low Base Fitness. We suggest looking at your own Base Fitness patterns to best utilize this metric for your training.

How To Efficiently Use Base Fitness?

Base Fitness and Intensity Trend during a recovery and build season displayed in COROS Training Hub.

Now that you understand how Base Fitness can fluctuate throughout training, the next logical question is "How much can I build?". Although this question is highly individualized, there are some guidelines for you to refer to.

Do not increase too fast.

  • During your build season, we recommend increasing your Base Fitness by 2-6 points/week. Some athletes may be able to build more, but this range allows for a safe and consistent progression.

Stay within the Intensity Trend 'optimized' range.

  • Intensity Trend provides an optimal range where your Base Fitness is progressing at an ideal pace. Keep an eye out for this metric to prevent from jumping over to the 'excessive' range and risk overtraining.

Don't aim for a specific Base Fitness value.

  • That's right. While some athletes may be able to build towards 100, others can build well over 200. This is where the principle of specificity comes in: While having a Base Fitness of 120 before your marathon may be great, it will not be helpful if you ahieved this through 200-meter sprints. Make sure your Base Fitness comes from specific training needed for the event.
When do I know I built enough prior to race day? Base Fitness is an individualized metric with various life stressors beginning to play a major role the higher you build. Take a look at our case-study article on how do pro athletes use the COROS Base Fitness metric to better understand how you can manipulate this metric for your own training.

How Can COROS Coaches Help?

COROS Coaches are a team of dedicated and certified coaches that can guide you in your training and understanding your metrics, for free. Email them at coach@coros.com today and get the discussion going on how to best structure your Base Fitness for your upcoming objective!