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What the numbers mean, and how they're built.
Strain
Strain measures how much cardiovascular load you built up today, on a 0–10 scale. It is built entirely from your heart rate — how far above your resting rate it sat, and for how long.
Strain is cardiovascular load only.
It reflects what your heart did, not how hard your muscles worked. A heavy lifting session at a low heart rate scores low — that's by design, not a bug.
Heart rate simply cannot see mechanical load. A set of heavy deadlifts is enormously taxing on your muscles and nervous system while barely moving your heart rate, so no heart-rate-based score can capture it. Every app in this category has the same blind spot, including WHOOP and Athlytic.
Use Strain to judge your cardio, and your own read for strength work.
The scale
- Rest — minimal activity, focus on recovery 0.0–1.7
- Light — active recovery day 1.7–3.5
- Moderate — typical workout day 3.5–5.5
- High — hard effort or long duration 5.5–7.5
- Very High — race effort or multiple workouts 7.5–10
How it's built
Every heart rate reading through the day is scored on two things, and nothing else:
- How far above resting you were. Measured as a share of your heart rate reserve — the gap between your resting and maximum heart rate — rather than raw beats per minute, so it means the same thing for you as for anyone else. Harder minutes count disproportionately more than easy ones: a minute at a genuinely high effort is worth far more than three easy minutes, which is what your body actually experiences.
- Whether it was part of sustained activity. A stretch of at least ten minutes with your heart rate meaningfully above resting counts as a bout, and minutes inside a bout count five times as much as scattered elevation. This is what keeps a real session dominating your day and stops a stressful meeting, a hot afternoon, or a flight of stairs from quietly inflating it.
On a typical day with one workout, the bout carries roughly 90% of your Strain. That's deliberate — what you actually did should be the score, and the rest of the day should barely move it.
You don't have to record a workout.
Rheostat never reads your workout list. There's no "recorded" and "unrecorded" — an hour of yard work, a long dog walk, or a hike you forgot to hit start on produces the same sustained heart rate pattern as a logged run, so it scores the same. Most apps in this category credit the log; we credit your heart rate.
Two numbers it depends on
- Your resting heart rate baseline, not today's resting figure. Using today's would quietly cancel itself out — a stressful day raises your resting rate and would subtract away the very elevation that represents the load. Rheostat anchors to your settled 60-day normal instead.
- Your maximum heart rate — the highest Rheostat has actually observed from your own hardest efforts, or
220 − age, whichever is higher. The age formula is off by ±10–12 bpm for most people, and someone who never approaches their true maximum would otherwise shrink their own scale and see every score inflated.
Why it climbs through the day
Strain accumulates. It starts near zero each morning and rises as you generate heart rate elevation, so it's only meaningful at the end of the day. Comparing a midday Strain against another app is comparing nothing — some apps front-load credit the moment a workout ends, while Rheostat integrates as the day happens. It also means the order doesn't matter: a morning run then an evening walk gives the same total as the walk then the run.
Days you didn't wear your watch
With no heart rate readings, there's nothing to score, so the day reads at or near zero. That's honest rather than flattering — Rheostat doesn't invent a floor to make an empty day look plausible. A day with a few scattered readings will read low for the same reason.
Readiness
Readiness is a 0–100 estimate of how recovered you are this morning, built from three signals compared against your own history:
- Heart rate variability (HRV) — the primary signal. Measured during sleep.
- Sleep — duration, quality, and how broken up it was.
- Resting heart rate — lower than your baseline is generally good; elevated often means incomplete recovery, illness, alcohol, or heat.
Every one of those is scored against your personal baseline, never against population averages. A 45 ms HRV might be excellent for one person and a warning sign for another; only your own trend tells you which.
Rheostat won't hand out fake perfect scores.
Some apps in this category cluster everyone in the 90s because high numbers feel good. If your recovery is genuinely mediocre, Rheostat says so — a score that's always green tells you nothing, and the whole point is to catch the days you should back off.
Reading the score
- PRIME — well recovered; a good day to take on load 80–100
- READY — solidly recovered; a normal day 60–79
- STEADY — moderately recovered; fine for most things, maybe not a personal best 40–59
- RECOVER — under-recovered; your body is still dealing with something 20–39
- REST — substantially under-recovered; worth asking why before training hard 0–19
A score in the 50s is a normal, unremarkable day, not a bad one. Because everything is scored against your own baseline, roughly half your days sit in the middle by construction — that's the scale working, not you underperforming.
Why it locks for the day
Your score is calculated once in the morning and then held for the rest of the day. Recovery describes the state you woke up in; a number that drifted every time you opened the app would be noise, not signal. Rheostat does allow it to settle for a few hours after you wake, because Apple Watch data syncs gradually — so an early-morning score can still sharpen once the rest of the night arrives.
A note on HRV sampling
Apple Watch takes only a handful of HRV readings per night — often four or so. HRV also climbs through the night, so the last reading before you wake is frequently the highest. That makes the nightly average genuinely sensitive to a single sample, and it's part of why two similar-feeling nights can score a few points apart. It's a limitation of the measurement, not of the comparison, and it's why Rheostat leans on your settled baseline rather than reacting sharply to one night.
Your baseline
A baseline is your personal normal — a rolling average of your own recent nights, recalculated daily. Every score you see is a comparison against it.
- HRV baseline uses only readings taken while you were actually asleep. Daytime HRV is inflated by posture, caffeine, stress, and movement, so including it would quietly distort your normal.
- Resting heart rate baseline uses the median of your daily lows.
- Sleep baseline uses your nightly durations.
The window is 60 days by default, and it rolls forward once a day — so last night enters your normal, and the night from two months ago drops out.
Rheostat can build your baseline immediately from the Apple Health history already on your iPhone. If you've worn an Apple Watch for a while, that means real scores on day one instead of waiting weeks.
Your first days
If you're new to Apple Watch and have little history, the first two weeks of scores are directional rather than precise — there isn't yet enough of your data to know what normal looks like. Wear the watch to sleep as consistently as you can; sleep data is what makes the whole system work.
Why my score changed
The usual causes, roughly in order of how often they're the answer:
- Alcohol. Reliably suppresses HRV and raises resting heart rate, often for two nights.
- Short or broken sleep.
- A hard session the day before, especially a late one.
- Illness coming on. Elevated resting heart rate and suppressed HRV often show up a day before you feel anything.
- Heat, altitude, dehydration, or a late heavy meal.
- Stress and travel — your nervous system doesn't distinguish work stress from training stress.
- Where you are in your menstrual cycle, if that applies to you. HRV typically falls and resting heart rate rises through the luteal phase. It's a normal, expected shift rather than a recovery problem, but it does move the score.
Subscription
Rheostat is $3.99 per month or $29.99 per year, and the annual plan begins with a one-month free trial. There's one tier — every feature is included. Full terms are on the pricing page.
Managing or cancelling
Open Settings → Subscription in Rheostat, which takes you straight to your Apple subscription management. You can also get there from iOS Settings → your name → Subscriptions. Cancelling during the free trial means you're never charged, and you keep access until the trial ends.
Deleting the app does not cancel a subscription. Cancel first, then delete.
Restoring a purchase
New phone, or reinstalled the app? Use Restore Purchases on the subscription screen. Your subscription follows your Apple Account, so make sure you're signed in with the same one you bought it on.
Refunds
Apple handles all billing and refunds, not us — request one at reportaproblem.apple.com. If something in the app went wrong, email us too; we'd rather fix it.
Glossary
ACWR
Acute:Chronic Workload Ratio — your recent training load (about 7 days) divided by what you're used to (about 28 days). Near 1.0 means you're training at a level you've adapted to. The widely quoted 1.5 "danger zone" rests on weaker evidence than it's usually given.
Baseline
Your personal normal for a metric, built from your last 60 days. Every score compares you to you, never to population averages.
Bout
A sustained stretch of elevated heart rate. It's how Rheostat detects activity without needing you to record a workout — a long walk and a logged run that produce the same heart rate pattern count the same.
Confidence
How much data a score is standing on — how complete the night's readings were and how settled your baseline is. Low confidence doesn't mean a score is wrong, only that it rests on less.
Deep / REM sleep
Sleep stages associated with physical and mental recovery. Wrist estimates of them are approximate — treat total duration as solid and the stage breakdown as indicative.
Effort
How hard a single workout was, 0–10, from the time you spent in each heart rate zone. It reads intervals that a whole-day average smooths away, so it can deliberately disagree with that day's Strain.
Heart rate zones
Five effort bands set by where your heart rate sits between your resting rate and your maximum — not by raw beats per minute. The same pace can be zone 2 for you and zone 4 for someone else.
HRR (heart rate reserve)
The gap between your resting and maximum heart rate. Used to scale effort to your individual range rather than to absolute numbers. Not the same as heart rate recovery below, despite the shared abbreviation.
Heart rate recovery
How quickly your heart rate falls after exercise — a general fitness indicator, where a bigger drop is better. Confusingly shares the "HRR" abbreviation with heart rate reserve above.
HRV / SDNN
Heart rate variability — the variation in time between heartbeats, in milliseconds. Higher generally means your parasympathetic ("rest and digest") system is in charge. SDNN is the specific measure Apple Watch records; chest straps often report RMSSD instead, so the number isn't comparable across devices or apps — only against your own history.
Readiness
One number, 0–100, for how recovered you are this morning, built from HRV, resting heart rate and sleep measured overnight. It's a measurement, not a target.
RHR (resting heart rate)
Your heart rate at complete rest. Lower is generally better; a persistent rise of several beats above your baseline is worth paying attention to.
Sleep efficiency
The share of time in bed actually spent asleep.
Strain
How much cardiovascular load you took on in a day, 0–10, built entirely from your heart rate. Heavy lifting at a low heart rate scores low — a wrist sensor can't see muscular strain.
VO₂ max
An estimate of how much oxygen your body can use during hard exercise — millilitres per kilogram per minute. A broad fitness measure that moves over months, not days. Your watch estimates it from certain outdoor workouts; it isn't a lab measurement.
Common questions
Why did my heavy lifting session barely register?
Because Strain measures cardiovascular load, and lifting is mechanically brutal but cardiovascularly quiet. See Strain above. It's the most common question we get, and the score is behaving correctly.
Why doesn't my Strain match another app?
Different apps weight heart rate differently and make different assumptions about your maximum. Rheostat uses the highest heart rate it has actually seen from you rather than an age formula, weights sustained activity far above scattered elevation, and accumulates through the day instead of front-loading credit when a workout ends — so a midday comparison is especially meaningless. Use one app's scale consistently; mixing them tells you nothing.
Do I have to record my workouts?
No. Rheostat never looks at your workout list — it reads your heart rate. Activity you didn't record counts exactly the same as activity you did. Recording is still worth doing if you want per-workout Effort scores and a training history, but it doesn't change your Strain.
Does Rheostat need my Apple Watch?
Effectively yes. Readiness depends on overnight HRV and sleep, which means wearing the watch to bed. Without it there's nothing meaningful to score.
Why is today's Strain so low? I worked out this morning.
Strain builds through the day rather than jumping when a workout ends, and Apple Watch data syncs in batches — so it can lag by a while. Judge it in the evening. See Why it climbs through the day.
Does my data leave my phone?
No. Every calculation runs on your iPhone. There are no accounts, no servers, and the app makes no network requests at all. See Privacy.
Can I use it without a subscription?
The annual plan includes a one-month free trial, after which Rheostat requires a subscription. Your Apple Health data always remains yours and stays on your device regardless of subscription status. See Subscription.
Can I export my data?
Your underlying health data lives in Apple Health, which has its own export. Rheostat's calculated scores are stored on your device and are removed when you delete the app.
Still stuck?
Email support@lacaroventures.com. If it's about a score that looks wrong, mention the date and roughly when you opened the app — that's usually enough to work out what happened.
Rheostat is not a medical device. It is intended for general wellness and fitness only. It cannot diagnose, treat, cure, or prevent any disease or condition. Never use it to make a medical decision or to delay seeking care. If something feels wrong, talk to a doctor.