How Wearables Measure Stress and Strain — and What the Numbers Mean
Garmin Body Battery, WHOOP Strain, Oura Stress, WHOOP Recovery — these scores sound like they measure the same thing. They don't. Here's what each one actually calculates, why they contradict each other, and how to read them together.
Your Garmin says you’re at 85% Body Battery. Your WHOOP says 14 Strain. Your Oura says you’re stressed. They’re all looking at the same body. They’re all “right.” And they’re all measuring completely different things.
The Problem: Every Wearable Defines “Stress” Differently
The fitness wearable industry has a terminology problem. Garmin calls it “Body Battery” and “Stress Level.” WHOOP calls it “Strain” and “Recovery.” Oura calls it “Stress” and “Resilience.” These words sound interchangeable. They are not. They measure different physiological phenomena, on different timescales, using different sensor combinations, processed through different proprietary algorithms. Comparing your Garmin Stress Level to your WHOOP Strain score is like comparing your blood pressure to your cholesterol — both are health metrics, both involve your cardiovascular system, and putting them on the same axis tells you nothing useful. Yet every wearable comparison article does exactly that, and every user who wears two devices wonders why they disagree.
They disagree because they’re answering different questions.
The Physiological Basis
All of these scores trace back to the autonomic nervous system (ANS). Your ANS has two branches: the sympathetic nervous system (SNS), which mobilizes energy for action — elevated heart rate, faster breathing, increased alertness — and the parasympathetic nervous system (PNS), which conserves energy and promotes recovery — slower heart rate, deeper breathing, digestion, repair.
At rest, a healthy body shows strong parasympathetic tone. Heart rate variability (HRV) is the primary proxy: high HRV means parasympathetic dominance, low HRV means sympathetic dominance. But HRV alone can’t distinguish between sources of stress. A hard interval session, a bad night’s sleep, a looming work deadline, and a brewing infection all suppress HRV. The body’s stress response is nonspecific.
Some devices add additional signals. Oura’s Gen 3 ring includes an electrodermal activity (EDA) sensor that measures skin conductance — tiny sweat gland activations driven by the sympathetic nervous system. Skin temperature deviations track circadian rhythm disruptions and early illness. WHOOP relies heavily on heart rate duration curves during exercise. Garmin uses a licensed algorithm from Firstbeat Analytics (now Garmin-owned) that models energy expenditure and recovery as a battery metaphor.
Same body. Different sensors. Different math. Different answers.
How Each Platform Calculates Its Score
Garmin Body Battery (0-100)
Body Battery is an energy reservoir model, not a stress metric. It’s based on Firstbeat’s physiological analytics engine, which estimates your body’s energy state by tracking four inputs: heart rate variability (for recovery/charging), stress level (for draining), physical activity (for draining), and sleep quality (for charging). The metaphor is literal — your body is a battery that charges when you rest and drains when you’re active or stressed.
A number near 100 means you’re fully charged. A number near 5 means you’re running on fumes. It declines through the day as you accumulate stress and physical activity, and recharges overnight. The rate of decline tells you about your day’s demands; the rate of recharge tells you about your sleep quality.
What it’s good for: answering “how much do I have left in the tank right now?” What it can’t tell you: how hard your last workout was, whether you should train tomorrow, or whether you’re overreaching across weeks.
Garmin Stress Level (0-100)
This is a real-time, momentary reading derived from HRV analysis. Garmin samples your heart rate variability continuously throughout the day and classifies each 3-minute window on a scale from 0 (rest) to 100 (high stress). Low values mean high HRV (parasympathetic dominance). High values mean low HRV (sympathetic dominance).
The critical limitation: it cannot distinguish the source of stress. Caffeine, a flight of stairs, an argument with your partner, and a fever all look identical — they all suppress HRV. A reading of 75 after your morning espresso does not mean the same thing as a reading of 75 during a meeting that’s going sideways, even though the device reports both identically. Context is everything, and the device has no context.
WHOOP Strain (0-21)
WHOOP Strain is a cardiovascular load metric. It measures how hard your cardiovascular system worked during a session or across the day, based on the duration you spend at different percentages of your maximum heart rate. The scale is logarithmic — going from 0 to 10 is easy, going from 18 to 21 requires sustained near-maximal effort.
Crucially, Strain does not incorporate HRV, sleep, or recovery data. It’s a pure workload measure. You can have terrible recovery and still register high Strain — because you trained hard regardless of whether you should have. It answers “how much cardiovascular work did I do?” and nothing else. Thinking of it as a stress metric is a category error that WHOOP’s own marketing doesn’t always help clarify.
WHOOP Recovery (0-100%)
Recovery is where WHOOP answers the readiness question. It’s calculated each morning from four inputs: HRV (the dominant factor), resting heart rate, respiratory rate, and sleep performance (duration and quality relative to your need). A Recovery of 90% means your body shows strong parasympathetic recovery signals. A Recovery of 30% means it doesn’t.
This is the closest analog to what Oura and Garmin are trying to capture with their readiness scores. It’s backward-looking — “how recovered are you from yesterday?” — not predictive. WHOOP doesn’t tell you what to do with a low recovery. It just tells you the number.
Oura Stress
Oura’s Stress view classifies your day into “Restorative” and “Stressed” time segments. It uses a combination of HRV patterns, EDA (electrodermal activity) from the Gen 3 sensor, skin temperature, and movement. The EDA sensor adds a dimension that wrist-based devices don’t have — it detects micro-sweat events that correlate with acute sympathetic activation, even at rest.
Oura reports how many minutes of your day were spent in a restorative state versus a stressed state. It doesn’t give you a single composite number — it gives you a time breakdown. This is arguably more honest than a single score, because it acknowledges that stress fluctuates throughout the day rather than summarizing it into one misleading digit.
Oura Resilience
Resilience is Oura’s newest metric and the only one in this comparison that looks at trends rather than snapshots. It tracks how quickly your body returns to baseline after stress — how fast your HRV recovers, how quickly your daytime readings normalize after a poor night. High resilience means your body bounces back fast. Low resilience means stressors have lingering effects.
This is conceptually distinct from anything Garmin or WHOOP offers. It’s not measuring today’s stress or today’s recovery. It’s measuring your body’s adaptive capacity over weeks. It’s the only consumer metric that attempts to capture allostatic load — the cumulative toll of repeated stress cycles.
Head-to-Head: Which Score Answers Which Question
| Question | Best metric | Runner-up |
|---|---|---|
| Should I train hard today? | WHOOP Recovery | Oura Readiness Score |
| How hard was my workout? | WHOOP Strain | Garmin Training Load |
| Am I stressed right now? | Garmin Stress Level | Oura Stress (time segments) |
| How much energy do I have left? | Garmin Body Battery | — |
| Am I trending toward burnout? | Oura Resilience | HRV 30-day trend (any device) |
| Did I sleep well enough to recover? | WHOOP Recovery (sleep component) | Oura Sleep Score |
No single device answers all of these questions. No single score even tries to. And yet users routinely compare Body Battery to Recovery to Strain as though they’re denominated in the same currency.
Why Multiple Devices Give Contradictory Answers
Monday morning. WHOOP Recovery: 82% (green). Garmin Body Battery: 45. Oura: 8 hours of sleep, but “Pay Attention” on the Readiness score.
All three are correct. They’re just looking at different slices of reality.
WHOOP measured your HRV during slow-wave sleep and found strong parasympathetic recovery. Correct — your deep sleep was excellent. Garmin tracked your energy expenditure across a full Sunday that included a long hike and found you haven’t fully recharged yet. Also correct — you burned a lot of energy yesterday. Oura noticed your resting heart rate was 4 bpm above your 2-week average and your HRV trend has been declining for three days. Also correct — there’s an emerging pattern that one good night hasn’t reversed.
WHOOP is answering “did you recover from yesterday?” Garmin is answering “do you have energy right now?” Oura is answering “is there a concerning multi-day trend?” Three different questions. Three different timescales. Three different — and fully compatible — answers.
The contradiction is an illusion created by treating proprietary scores as comparable units. They aren’t. They never were.
How Omnio Resolves the Conflict
We don’t pick a winner. We don’t average the scores. We don’t build another proprietary black box on top of existing proprietary black boxes.
Instead, we pull the raw inputs that all of these scores are built from: HRV, resting heart rate, respiratory rate, sleep duration and staging, skin temperature, training load. We normalize each metric against your personal baseline on each device, so an Oura HRV reading and a WHOOP HRV reading are expressed in the same terms — deviations from your norm.
Then we build composite scores from the best available data. If you wear an Oura Ring and a Garmin watch, we take sleep data primarily from Oura (finger PPG is cleaner for overnight measurement) and training load from Garmin (it has accelerometer + GPS context that Oura lacks). If your Oura HRV and your Garmin HRV diverge on a given night, we weight the source with the higher-confidence measurement window rather than averaging two noisy estimates.
The composite readiness score feeds into adaptive training, which decides what your next workout should look like. Not “you’re 72% recovered, good luck” — but “your HRV trend is suppressed and your acute-to-chronic workload ratio is at 1.2, so today should be moderate intensity with a volume reduction.”
Wearables are good at measuring. They’re bad at agreeing. Omnio doesn’t need them to agree — it needs them to contribute what they’re best at and stay quiet about the rest.
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