Pace lies.
Your heart doesn't.
Pace lies on hills, into wind, in heat, when you're tired, and when you're coming down with something. Heart rate tells you what the effort actually cost. In The Zone shows which of five training zones you're in — live, while you can still do something about it.
iPhone and Apple Watch · $1.99 once · No subscription, ever
What makes it different
Read it by colour
Your whole screen becomes your zone. Green is easy, red is hard. You read your effort from the corner of your eye instead of squinting at a number mid-stride.
Or don't look at all
Set a zone to hold and your watch taps your wrist when you drift — one pattern for too hard, another for easing off. No app does this.
Indoors is not an afterthought
Treadmill, elliptical, bike and rower are first-class here. Every other running app treats an indoor session as a grey rectangle with no analysis.
Routes coloured by effort
Others show terrain. Others show pace. Nobody shows what your heart was doing — the hill where you tipped into threshold, the stretch where you settled.
The treadmill display
Prop your phone on the console and the entire screen turns your zone's colour. It changes as you do. You can tell whether you're working too hard from three feet away, without breaking stride.
Turn it sideways and the number roughly doubles in width — the difference between readable and not, at running distance. The screen stays awake for the whole session.
A watch that tells you, so you don't have to check
Choose a zone to hold. When you drift above it your watch taps one way; when you ease off, another. Two distinct patterns, so you know which way to move without looking at anything.
Most people drift out of Zone 2 without noticing — it's the single most common mistake in endurance training. On a treadmill your phone can call the zones out loud instead, ducking your music rather than stopping it.
Know whether you're doing too much
Training load weighs your time by intensity — twenty minutes at threshold costs five times twenty easy ones — and compares this week against your recent average. It answers the question a total-time figure never can.
It stays quiet until there's enough history for the comparison to mean something. A verdict based on three days would be noise dressed as insight.
Zones that actually fit you
Calculated with the Tanaka formula, from a meta-analysis of 18,712 subjects — meaningfully more accurate than the familiar 220 − age, which increasingly underestimates past 40. Measured your real maximum? Enter it; a real figure always wins.
Measure against percentage of maximum, or against your heart rate reserve with the Karvonen method if you know your resting rate. The two aren't interchangeable at the same percentage, so the app always shows which is in play.
No Apple Watch? No problem
In The Zone works with any Bluetooth chest strap — Polar, Garmin, Wahoo, Coospo — so your iPhone can do the whole job alone. Chest straps measure electrically rather than optically, which is why they hold up during intervals where a wrist sensor lags behind.
Your data never leaves your devices
The App Store lists this app as Data Not Linked to You — a claim most fitness apps can't make. Your heart rate, workouts and routes live on your devices and in Apple Health. The only thing that ever leaves is a single number, your weekly minutes, and only if you choose to use the leaderboard. Read the full policy.
$1.99. Once.
No subscription, no upsell, no premium tier holding your own data hostage. You buy it, you own it.
In The Zone is a training aid, not a medical device. Heart rate zones are estimates. If something feels wrong while exercising, stop and speak to a doctor.