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I am using a Suunto watch to record activities. The data is analyzed in Suunto App which uses training metrics powered by Training Peaks, which is an established leader in this area. The same data is uploaded to Strava that provides its own training metrics. In both cases I can see Fitness / Fatigue / Form metrics that, as I understand, are supposed to be based on the roughy the same formulas. However the metrics look quite a bit different. Let’s do a comparison.

Here is the Training Peaks data from Suunto app for the last 8 weeks. You can see that the Fitness metric has been going mostly up from 43 on 1/6/25 to 56 on 3/2/25 - 30% increase. That seems reasonable to me as I’ve been ramping up the volume. The intensity remained roughly the same because I train for ultra distances. Note how smooth the fitness graph is overall, which is reasonable because the fitness level doesn’t really change that fast. Also note the clean representation of the Form metric, which is essential to targeting the training in the most productive zone from -10 to -30.  Strava also has the Form metric, but only on the web, and it provides zero guidance.


Now let’s look at the Strava Fitness graph for the last 3 months. What we can see in comparison that Fitness graph is a lot more volatile - it goes down significantly more between my long runs where I tend to have the most training load. And overall it remained mostly flat in the same period - it changed from 63 on 1/6/25 to 67 on 3/2/25 - 6% increase. There is no trend that I can see despite the increase in the training volume. Also the Fatigue metric looks a lot more volatile than in the Training Peaks case.
 

So the question is why these two graphs are so different between Training Peaks and Strava? The former makes much more sense to me based on how I trained recently, but the latter makes very little sense. Can anyone from Strava comment on this?
 

Hello ​@Silentvoyager,

 

Both sets of metrics are calculated on the same principle, based on work by Dr. Eric Bannister, but will differ in their implementation. The two main variables which could differ between the two platforms are:

 

  1. Calculation of the training impulse from the power meter or heart rate data. Which data source are you using on each platform? Even if the same data source is used, different formulae exist to convert HR or power over an activity to a training impulse.
  2. Decay timescales for the fitness and freshness (or CTL vs ATL in TrainingPeaks terminology). The shorter the timescale, the spikier the graphs will look. Therefore Strava and TP may use different timescales.

Hope that makes sense but let us know if you have any questions!

​@Lola, thank you for your reply! Most of the activities are runs or trail runs recorded with a modern Suunto watch and external HR sensor. So I'd think Strava should use HR to calculate training impulse, but perhaps instead it uses running power that my watch also produces. Is there a way to know that? 

By the way, is training impulse the same as relative effort, and the same as suffer score?

What I see is that there a major difference in how Strava scores my runs vs. Training Peaks TSS, and that I think is why the progression of the Fitness metric is so different in my case. Let's look at a few concrete examples:

 

1) Short but fairly intense trail run. I raced two segments, including a fairly steep but still runnable uphill. Distance 5 miles, Elevation gain 750 ft: Moving time: 50m, Avg HR 142, Max HR 162.

Strava Relative Effort: 75, Training Peaks TSS: 56

 

2) Z1/Z2 intensity trail run with a lot of hiking parts on steep terrain. Focus on elevation gain, which was rather large for the distance. Distance 13.5 miles, Elevation gain 4750 ft: Moving time: 3h 36m, Avg HR 117, Max HR 139.

Strava Relative Effort: 66!!!, Training Peaks TSS: 178

 

3) Moderate intensity long run with a mix of running and hiking uphills. Focus on high aerobic endurance. Distance 14.8 miles, Elevation gain 2100 ft: Moving time: 2h 37m, Avg HR 138, Max HR 155.

Strava Relative Effort: 180, Training Peaks TSS: 150

 

If we compare these, we can see that the first and the third runs are more or less aligned between Strava and Training Peaks in terms of training effort, although Strava still gives a bit more relative impact to the first one. However the difference is really massive in the case of the second low aerobic long trail run with large elevation gain. Considering how I felt afterwards and how quickly I recovered, it was definitely the hardest. However, on Strava it has the least relative effort of the three examples. It was valued less than a short run with 6x less elevation gain. Training Peaks TSS reflects this effort much more fairly in my opinion.

When people like me train for ultramarathons, there isn't a lot of emphasis on high aerobic endurance, but there is a lot emphasis on muscle endurance, which requires long training days at low intensity with large distances and/or elevation gains. These efforts are still hard and greatly contribute to aerobic fitness. But according to Strava they have the same impact as a 40-50 minute run, which I think is just wrong.

Perhaps Strava's calculation of effort has been optimized for cyclists, but I think it doesn't work well for running and especially ultrarunning.

 

 

 

​@Silentvoyager - thank you for all this info. I reached out to our team internally to get their eyes on this and here is the answer:

 

Regarding the different metrics, bear in mind that the Fitness metric is a fairly crude model of the human body: we are trying to summarise the adaptation of a complex system using a single number. Power-based Fitness scores are currently only available for cycling activities, so your Fitness will be based on Relative Effort, not power.You can find a good explanation of Strava's Relative Effort score and comparison to its ‘Suffer Score’ predecessor here. Two key points to bear in mind are:

  1. It is based purely on heart rate data so it measures the training stress on your cardiovascular system.
  2. It weights intensity more than duration.

You can surmise why this metric will work well for improving your health and training for more traditional road running races, but might not be so well suited to ultramarathon training!

 

Hope this helps!

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