From estimated to measured: Sweat testing in Musette

Two athletes can ride the same route, at the same intensity, in the same weather—and lose very different amounts of fluid. That's a challenge for any personalized fueling plan: before we've measured how much you sweat, we have to start with an estimate.

Musette can now replace that estimate with data collected during your own training.

Until you've run a test, Musette is upfront that your sweat rate is a low-confidence, default estimate.

We've added field sweat testing throughout Musette, with a guided workflow for performing a test, a quick way to enter measurements from a test you've already completed, and a calibration system that uses the results to improve future hydration plans.

Both paths start from the same place.

A sweat test doesn't need to be a laboratory test

The basic idea is straightforward.

Weigh yourself before training. Keep track of how much you drink. Weigh yourself again afterward. From the change in body weight, fluid consumed, and test duration, Musette calculates your measured sweat rate.

You can schedule a dedicated guided sweat test in Musette, or Musette may suggest one when an upcoming workout looks like a good opportunity.

Scheduling a guided sweat test.

The guided workflow walks you through preparation, suggests how much fluid to carry, records the information needed for the test, and helps you complete it afterward.

Before you start, Musette lays out exactly what the test needs from you.

The preparation checklist.

You don't have to run Musette during the workout. There's a live timer if you want it, but you can also prepare your test, go train normally, and enter your finishing measurements when you're done.

The live timer is there if you want it — not required.

Timing a test in progress.

Already performed a test yourself? You can enter those measurements directly instead.

Entering a test you already completed — same underlying calculation, no guided workout required.

Weight before, weight after, fluid consumed — the same three numbers either path collects.

Measured sweat rate isn't quite the whole story

A sweat rate measured during one workout reflects that particular workout. In particular, intensity matters: an easy endurance ride and a harder effort aren't directly comparable, even if everything else is similar.

That's why Musette records two numbers.

Your measured sweat rate is what the measurements from that particular test indicate.

Musette also calculates a reference sweat rate, adjusted to a standard endurance intensity so results from different tests can be compared more consistently.

We also record the temperature during the test as useful context, but don't use it to adjust your reference sweat rate. Instead, Musette keeps your personal sweat-rate calibration separate from the conditions of a particular workout.

Separately, when you flag a segment of a plan with expected conditions—cool, normal, warm, or hot—Musette nudges the fluid target for that segment accordingly. That lets us start with what we've learned about you, then adjust for the conditions you tell us to expect.

Measure the athlete. Adjust for the conditions.

We don't blindly trust one number

This was one of the most important design decisions behind the feature.

A field sweat test contains uncertainty. A scale can be slightly off. Fluid measurements can be imperfect. A workout might not be suitable for calibration. And sometimes a perfectly legitimate result simply doesn't agree with previous measurements.

So completing a test doesn't automatically mean:

"This is your new sweat rate."

Musette first checks whether the measurement is plausible and whether the test is suitable for calibration.

Musette flags whether a result looks physiologically typical as you enter it.

A test can still be worth keeping in your history even when it's too short to calibrate from.

When multiple eligible tests are available, we look for agreement between them. Tests that agree can contribute to your current sweat-rate calibration and increase our confidence in it. A result that differs substantially can still be saved without immediately changing your baseline.

In other words, Musette treats a field test as evidence, not absolute truth.

You can see what Musette is using

We also wanted the calibration process to be understandable rather than something that happens invisibly behind the scenes.

The new Sweat Tests history shows your current sweat rate, our confidence in that calibration, and how many tests currently contribute to it.

Every test you've logged, in one place.

Individual results are identified as Contributing, Superseded, Ineligible, Invalid, or Voided, so you can see which measurements Musette is actually using and which ones it isn't.

Mistakes aren't permanent, either. A result can be corrected without silently rewriting the original observation, or removed from consideration entirely. When that happens, Musette recalculates the calibration from the remaining valid tests.

That matters because these aren't just numbers sitting in a history table. The resulting calibration affects future recommendations.

Better measurements lead to better plans

Once Musette establishes a sweat-rate calibration, that rate becomes an input to future hydration planning.

Instead of continuing to rely on an initial estimate, Musette can base fluid targets on measurements from your own training. Then, for a particular plan, those targets can still be nudged for factors such as the conditions you expect on a given segment.

As additional tests agree with one another, confidence in your underlying calibration can increase.

Musette can also suggest another sweat test when your current sweat rate is still based on a low-confidence estimate, or when it would be useful to confirm an existing calibration.

It's a simple feedback loop:

Train → measure → calibrate → improve future plans.

That's the larger idea behind the feature.

Musette shouldn't just produce a fueling plan and assume its initial assumptions were perfect. Where we can collect useful real-world evidence, the plan should get better as we learn more about the athlete.

What a sweat test doesn't measure

There's one important distinction.

This field test measures sweat rate—how much fluid you're losing—not sweat sodium concentration.

Musette also uses a sweat sodium concentration when calculating sodium targets, but determining that accurately requires a different kind of measurement. We don't infer sodium concentration from changes in body weight or pretend that an at-home sweat-rate test measured something it didn't.

For now, sweat-rate calibration and sweat-sodium estimation remain separate.

Try it on your next suitable workout

If your sweat rate in Musette is still estimated, a field test is one of the most useful pieces of information you can add to your athlete profile.

You can schedule a guided sweat test, perform one during a suitable training session, or enter measurements from a test you've already completed.

And don't worry about getting the "perfect" number from one attempt.

That's why we built calibration around repeated evidence in the first place.