JustZone2 is an iOS app for indoor cyclists that holds your heart rate in Zone 2 automatically. Connect a smart trainer such as a Wahoo KICKR, set a power target, and a PID controller continuously adjusts resistance to keep you in the zone, then syncs the workout to Strava.

iOS · One-time purchase · No subscription

Zone 2. Locked in.

Connect your Wahoo KICKR. Set a power target. A PID controller keeps your heart rate in Zone 2 for the entire session. Sync to Strava. Done.

See how it works

On the App Store · iPhone · iPad · Apple Watch

Justzone2 active workout screen showing heart rate, power and live chart

Wahoo KICKR ERG Mode via FTMS PID Zone Targeting Strava Auto-Upload Apple Health Apple Watch HR Live Activity Dynamic Island TCX + Power Extension One-Time Purchase Wahoo KICKR ERG Mode via FTMS PID Zone Targeting Strava Auto-Upload Apple Health Apple Watch HR Live Activity Dynamic Island TCX + Power Extension One-Time Purchase

Features

Everything you need.
Nothing you don't.

Zone 2 tracking & ERG control

Bluetooth FTMS

Real-time heart rate monitoring with visual zone indicators. Your Wahoo KICKR resistance adjusts via the FTMS protocol — set your target power and ride.

PID Zone Targeting

±30W auto-adjust

A PID controller samples HR every second using a 45-second rolling average. It nudges resistance to keep you locked in zone — gradually up, faster down. Always comfortable, never jarring.

Strava with full data

TCX + Power extension

Workouts upload automatically with HR, power, cadence, virtual speed, distance, and accurate calories. Strava shows the full picture — not just elapsed time.

Apple Watch + Live Activity

HR via Watch HealthKit mirroring or pair a BLE chest strap. Stats persist on your lock screen and Dynamic Island for the whole session. A Watch complication shows current data at a glance.

Apple Health + workout recovery

Workouts save to Apple Health with HR and power samples. If the app interrupts mid-session, recovery mode offers to resume exactly where you left off — elapsed time and all.

App

Every screen. All you need.

Setup

Workout

HR Zones

Activity Detail

Activity History

Launch

Heart rate, your way

Bring your own sensor.

Chest strap

Polar H10, Wahoo TICKR & any Bluetooth strap

Arm band

Polar Verity Sense & optical arm sensors

Apple Watch

Wrist HR streamed straight to the app

AirPods Pro

In-ear heart rate on AirPods Pro 3

How It Works

Adaptive Zone Targeting

A PID controller monitors your heart rate every second and adjusts your KICKR's resistance — keeping you locked in Zone 2 so you can focus on anything else.

Typical 60-minute session with Zone Targeting enabled

Heart Rate Power Zone 2 Band

The feedback loop

Measure

HR sampled every second from your Apple Watch or BLE chest strap.

Calculate

PID controller measures how far you are from your Zone 2 target — the upper part of the zone by default — using a 45-second rolling average.

Adjust

Resistance nudged gradually — ramps up slowly, backs off faster. Always comfortable.

Repeat

Every second, for the entire session. You keep pedalling. The app keeps you optimal.

Track your progress

Measure your fitness
without FTP tests

Zone Targeting holds your heart rate steady, so the power you produce at that heart rate becomes your fitness gauge. As your aerobic fitness improves, the same effort gives you more watts, and every ordinary Zone 2 ride measures it. No all-out tests.

The app's History trend chart over two years: average heart rate stays level around 134 bpm while average power climbs from about 145 to 180 watts. — **Same heart rate, more power.** Two years of Zone 2 rides: heart rate (red) holds level while power (blue) climbs.

The app's History bubble chart over two years: one bubble per ride in the same heart-rate band, shading from green to red as power rises. — **Every ride, one bubble.** The heart-rate band stays level; the colour shows power, shifting from green to red as the same heart rate produces more watts.

These are the charts from the app's History tab, shown with example data: two years of Zone 2 rides — twice a week in winter, once a week in summer — at 120–140 bpm. Your numbers will differ. Heat, sleep and fatigue move them a little from day to day; the trend over weeks and months is what counts.

Science

Why Zone 2?

It's how the pros build their aerobic engine.

80/20 Polarized Training
Elite cyclists spend 80% of training at low intensity. Zone 2 is where base fitness is built.
Fat Adaptation
Train your body to burn fat as fuel. Improve metabolic flexibility and endurance capacity.
Mitochondrial Growth
Zone 2 specifically targets mitochondrial biogenesis. More mitochondria = more power at threshold.
Sustainable & Enjoyable
Easy effort means you can train more frequently without burnout. Build volume, not fatigue.

Resources

Learn from the experts

Zone 2 Training Explained

The science behind low-intensity endurance training

Benefits of Zone 2

Why Zone 2 is the foundation of endurance performance

Complete Zone 2 Guide

How to find your Zone 2 and structure your training

The Science of Zone 2

Mitochondrial health and fat oxidation explained

Zone 2 Training Tips

Practical advice for getting the most from sessions

Zone 2 for Cyclists

Incorporating Zone 2 into your cycling training plan

From the blog

Zone 2, in depth.

Guide

Getting Started: Your First Ride

Pair your trainer, set your zone, and let the app hold your heart rate in Zone 2.

Zone 2 Life

The Easy Hour: How Zone 2 Fits a Real Life

Why riding slower around a busy schedule left me fitter, calmer and sleeping better.

Under the Hood

The Espresso in the Algorithm

How the PID controller that holds your heart rate works — explained through coffee.

Read the blog Explore the research

FAQ

Common questions.

What is Zone 2 training?

Zone 2 is a low-intensity heart-rate zone — roughly 60–80% of your maximum heart rate — where you can still hold a conversation. Training here builds aerobic base, grows mitochondria, and improves fat metabolism. It's the foundation of endurance fitness, which is why elite athletes spend most of their training time in it.

What is JustZone2?

JustZone2 is an iOS app for indoor cyclists that holds your heart rate in Zone 2 automatically. Connect a smart trainer (like a Wahoo KICKR), set a power target, and a PID controller continuously adjusts resistance to keep you in the zone for the whole session, then syncs the workout to Strava.

Which smart trainers and sensors are supported?

Any trainer that supports the FTMS Bluetooth standard, including the Wahoo KICKR range. For heart rate you can use a Bluetooth chest strap, an optical arm band, an Apple Watch, or AirPods Pro 3.

How do I calculate my Zone 2 heart rate?

A common starting estimate is 60–70% of your maximum heart rate (roughly 220 minus your age). JustZone2 lets you set all five zones directly on a visual editor, and matches Strava's zone percentages by default. See the getting started guide for details.

Does JustZone2 work with Strava?

Yes. Completed workouts upload to Strava automatically with power, heart rate, cadence, distance and calories, and the activity is titled by the zone you trained in.

Is there a subscription?

No. JustZone2 is a one-time purchase with no subscription. It also supports Apple Family Sharing, so a single purchase covers up to six family members — handy if more than one person in the house uses the trainer.

Can I use my smart trainer without a Zwift subscription?

Yes — a smart trainer is a standard Bluetooth FTMS device, so any FTMS app can control it. This comes up most when two riders share one trainer and only one of them pays for Zwift. Here's how that works, and what it costs.