Metrics, in plain words

What Strava's Fitness score actually measures

What is the Fitness score actually measuring?

Your training load, averaged over about six weeks. Nothing else.

Each run gets a load number from your heart rate and how long you ran. Those daily numbers feed a 42-day exponentially weighted average. That average is the line labelled Fitness.

The model is older than the app. Banister and Calvert published it in 1980, and it has been in coaching software ever since under the name chronic training load. One runner on a forum put the correct answer in nine words: it _does not track fitness but chronic training load_.

Why does it fall when I am getting faster?

Because volume and speed are different things, and the chart only knows one of them.

Consider two consecutive months. In month one you run 60 km (37 mi) a week, all easy. In month two you cut to 40 km (25 mi) and add two hard sessions. Your total load falls. Your 5K time improves. The Fitness line goes down.

The last row is the one that catches people out. A good taper always lowers this number. If it did not, the taper would not be working.

Why is it built on heart rate, and why does that matter?

Because heart rate is the only intensity signal most watches have for every run. It is also the weakest link in the chain.

The load calculation uses your heart-rate reserve, which needs your maximum and your resting rate. Most people have never measured either. If your maximum is a formula guess that is 10 bpm too low, every run scores as harder than it was, and your Fitness line is inflated for months.

Three practical consequences:

• A run recorded without a heart-rate strap or watch may score nothing at all.
• Hot weather raises heart rate at the same pace, so summer inflates the line.
• Two runners doing identical training will have different numbers.

If you want to sanity-check your own zones, the heart rate zone calculator shows what five different methods say your maximum is, and how far apart they are.

How do I tell which case I am in?

This is the decision rule. Look at the last four weeks and answer in order.

Only the last row is a training problem. The other four are the metric describing something other than what its name suggests.

What should I look at instead?

Three things, in this order.

Your race and time-trial times. The only direct measure of fitness on this list. A 5K every six weeks tells you more than any chart.

Your pace at a fixed easy heart rate. If you cover more ground at 140 bpm than you did in March, you are fitter. This is the single most useful trend a running app can show you, and it needs no model at all.

Weekly volume and how many weeks you strung together. That is what the Fitness line is really tracking, so you may as well read it directly.

The glossary entry on training load covers what goes into the daily number, and our method page gives the exact time constants we use, which are the same 42 and 7 days.

Would a better name fix this?

Mostly, yes. Almost every complaint about this metric is a complaint about the word Fitness sitting above a chart of training volume.

Call the same line Training load and the confusion goes away. The chart becomes useful immediately: it shows whether you are building, holding or backing off, which is what it was designed to show in 1980.

We name it that way in Inertial for exactly this reason. It is not a better model. It is the same model with an honest label.

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