Recovery and strain,
measured against you.
One number each morning: how recovered you are, scored against your own baseline — never against population averages. Built on the Apple Watch data you already have.
Readiness · 0–100
Your overnight heart rate variability, sleep, and resting heart rate, compared to your personal normal. High means you can push. Low means ease off.
Strain · 0–10
How much cardiovascular load you actually accumulated, read straight from your heart rate through the day. You don't have to record a workout for it to count.
Honest by design
Plenty of recovery apps cluster everyone in the 90s, because a high number feels good. That makes the score useless — if it's always green, it's telling you nothing.
Rheostat is built to be right rather than flattering. If your recovery is mediocre, it says so. The entire value is catching the days you should back off, and you only get that from a score willing to disappoint you.
Your data never leaves your iPhone.
No accounts. No servers. No analytics, no tracking, no advertising SDKs. Rheostat makes no network requests at all — you can run it in Airplane Mode and every feature works.
This isn't a policy promise we're asking you to trust. There's simply nowhere for your data to go. Read the privacy policy →
Measured against you, not a formula
- Your baseline, from your own sleep. HRV is taken only from readings while you were genuinely asleep — daytime readings are inflated by posture, caffeine, and stress.
- Your real maximum heart rate, observed from your own hardest efforts, rather than
220 − age— a formula that's off by ±10–12 bpm for most people. - Your history, from day one. Rheostat builds your baseline from the Apple Health data already on your phone, so if you've worn a watch for a while you get real scores immediately.
It counts the work you forgot to log
Strain reads your heart rate, not your workout list. A long walk, an afternoon of yard work, a hike you never hit "start" on — if it kept your heart rate up, it scores exactly the same as a workout you recorded. Most apps in this category credit the log; Rheostat credits the heart rate.
What it needs
An iPhone running iOS 16 or later, and an Apple Watch worn overnight. Sleep and overnight HRV are what make recovery scoring possible — without them there's nothing meaningful to measure.
Straight about what it can't do
Strain measures cardiovascular load. A heavy lifting session at a low heart rate scores low, and that's correct — heart rate can't see mechanical load. No wrist-based score can. We'd rather tell you that up front than have you discover it and assume the app is broken. How Strain works →
Pricing
$3.99 per month, or $29.99 per year — and the annual plan starts with a one-month free trial. One subscription, every feature; nothing is held back for a higher tier. Full subscription terms →
Not a medical device. Rheostat is for general wellness and fitness only. It cannot diagnose, treat, or prevent any condition, and should never be used to make a medical decision or to delay seeking care.
Questions?
Start with the help pages, or email support@lacaroventures.com.