HRV, TRIMP & Resting Heart Rate Guides for iPhone

If you only ever track two numbers, make them your morning HRV and your resting heart rate. Both come from the same sixty-second reading, both are meaningful only as a trend against your own baseline, and between them they explain most of what a wearable's recovery score is telling you. Everything else in this section is a layer on top: TRIMP puts a number on what a session cost, Zone 2 defines the intensity ceiling for your easy days, and VO2 max is a slow-moving fitness marker that Pulse Rate reads from Apple Health rather than computing from the camera. Start with the two foundations, add the rest when the trend line is stable.

The short version

Four ideas that make every guide here easier to read.

These recur across all seven pages. Getting them straight once saves re-reading the same caveat seven times.

Idea one · Baselines beat norms

Your number only means something against your own history.

An HRV of 40 ms can be excellent for one person and poor for another; the same is true of resting heart rate. Published ranges are context, not a verdict. Every guide here asks you to build two to three weeks of readings first, because the comparison that matters is today against your own recent normal.

Idea two · Protocol is the measurement

Same time, same posture, same conditions — or the trend is noise.

HRV moves with posture, caffeine, food, temperature and stress within minutes. A reading taken sitting up after coffee is not comparable with one taken lying down on waking. The protocol sections in these guides exist because an inconsistent measurement is worse than no measurement: it produces a trend that looks real and is not.

Idea three · Load and recovery are one picture

TRIMP without HRV tells you half the story.

Training load describes what you did to yourself; HRV and resting heart rate describe how well you absorbed it. The pattern worth acting on is a load that keeps climbing while morning HRV drifts down — which is only visible if you are tracking both. That is why the TRIMP and HRV guides cross-reference each other.

Idea four · Know what the camera can and cannot do

Optical sensing is a stillness measurement.

A camera reading detects light absorption changes at your fingertip, which works well when you are still and degrades quickly when you are not. That makes it strong for resting and recovery readings and unsuitable for live in-workout heart rate. The accuracy guide covers what affects reading quality and how to spot a bad one.

Which page do I need

Pick the guide that matches your question.

Each row states the metric, what it is actually measuring and where the number comes from in Pulse Rate, so you can jump straight to the page you need.

Metric	What it measures	Where the number comes from	Read
HRV (RMSSD)	Millisecond variation between consecutive beats — a proxy for nervous-system recovery	Measured directly from a camera reading, in milliseconds	HRV basics →
Resting heart rate	Beats per minute at true rest, the slowest-moving and most honest daily signal	Measured from a 60-second camera reading before you get up	Resting HR →
Daily HRV tracking	The habit itself — building a baseline you can actually read	A repeated morning camera reading, charted as a rolling trend	HRV tracker →
Camera accuracy	How far you can trust an optical reading, and what corrupts it	Photoplethysmography at the fingertip, lit by the flash	Accuracy →
TRIMP training load	One number per session combining duration and intensity	Calculated from sessions you log, summed across the week	TRIMP →
Zone 2	The easy aerobic intensity band most endurance training is built on	Estimated from your maximum heart rate, verified by the talk test	Zone 2 →
VO2 max	Aerobic capacity — the slowest-moving fitness marker of the set	Read from Apple Health, not computed from the camera	VO2 max →

All 7 pages

Everything in this section.

Start here — the two foundations

Everything else in this section assumes you have a few weeks of these two numbers. Read them in this order.

Taking a reading you can trust

How to build the daily habit, and what determines whether an optical reading is worth logging at all.

Training metrics

Once the baseline is stable, these three turn your readings into training decisions.

Before you read any of it

What these guides will not tell you.

Health content is where overstatement does the most damage, so the boundaries are stated up front rather than in a footnote.

Nothing here is a diagnosis. Pulse Rate is a wellness app, not a medical device, and no reading it takes should be read as a clinical result.
The app does not take an ECG, measure blood oxygen, or measure blood pressure. Blood pressure is manual logging with trend history only.
VO2 max is read from Apple Health, not calculated from a camera reading. A phone measurement is not a laboratory VO2 max test.
Camera measurement needs you to be still. These guides deliberately place every reading at rest, because an optical reading during movement is unreliable.
Heart-rate measurement is on demand. There is no continuous background monitoring, so a missed morning is a genuine gap in the trend.
Published ranges quoted in these guides are context from cardiology guidance, not thresholds that apply to you personally.

Other sections

Comparisons: Whoop, Oura, Garmin, Polar and more

For athletes: runners and cyclists

Blood pressure logging and trends

FAQ

Your questions, answered.

I am starting from zero. Which guide comes first?

Read the HRV beginners guide, then the resting heart rate guide. Those two metrics are the foundation — almost every recovery score sold by every wearable is built from them. Training load, Zone 2 and VO2 max only start to mean something once you have two or three weeks of those two numbers behind you.

What is the difference between RMSSD and SDNN?

Both describe variation between heartbeats, measured in milliseconds. RMSSD emphasises short-term, beat-to-beat variation associated with the parasympathetic "rest and recover" branch, and it is the steadier morning-to-morning number, which is why it is the usual readiness metric. SDNN describes overall variability across the whole recording window and is better for spotting slow multi-week shifts. Pulse Rate reports RMSSD from a single camera reading, and uses it as the recovery input to your wellness score.

How long before any of these numbers are useful?

A single reading tells you almost nothing, because HRV swings substantially day to day in healthy people. Two weeks of consistent morning readings gives you a rough personal baseline; three to four weeks makes the trend line trustworthy. Every guide in this section is written around trends rather than single values for that reason.

Which of these can an iPhone camera actually measure?

Heart rate and HRV are measured directly from a camera reading — the app detects pulses optically at your fingertip using the flash. Training load is calculated from sessions you log. VO2 max and heart-rate recovery insights are derived from data already in Apple Health rather than computed from the camera, and Pulse Rate does not claim to produce a lab-grade VO2 max figure.

Do these guides only apply to Pulse Rate?

No. RMSSD, SDNN, TRIMP, Zone 2 and resting heart rate are standard exercise-physiology metrics that predate every app on the market. The definitions and protocols in these guides hold whatever you measure with — the guides simply show where each number appears in Pulse Rate and how to take a clean reading with a phone camera.

Is any of this medical advice?

No. Pulse Rate is a wellness app rather than a medical device, and these guides are educational. The ranges quoted come from published cardiology guidance and are there to give context, not a diagnosis. If a reading worries you or a change persists, that is a conversation for a clinician, not an app.

Every number your heart
gives you, explained.