A pace target assumes every day is the same. After a short night, a stressful week or a hard session earlier in the week, the usual easy pace costs far more than an easy effort. Hills and heat do the same. Holding the number turns the easy day into a moderate one, and slowing down feels like failing the session.

Heart rate follows what a run costs, whatever the cause. Kiiro, an iPhone app for Apple Watch runners, uses it on both sides of training: a training plan sets what to run next, and the analysis shows whether it is working.

How Kiiro works

TL;DR: Kiiro syncs Apple Watch runs from Apple Health and builds a training plan from the runner’s own history. Each run has a heart rate target instead of a pace, and each week adjusts to what was run. Kiiro also shows whether the training is working, through training load, Fitness level, Load balance, same-route comparisons and personal bests.

The heart rate plan

The plan starts from what the runner has been doing: the last three weeks and an eight-week baseline of how often, how long and how hard. It also takes the available days, weekly time, long-run day and whether the aim is to build or maintain fitness. It keeps most running easy and adds harder work in small steps.

Each session has a clear job.

Easy Run and Long Run

These runs are the base of the plan. The instruction reads: “Keep this one relaxed and finish with plenty left.”

Endurance Run

An Endurance Run puts a steady block in the middle of an easy run.

Controlled Intervals

Controlled Intervals are repetitions of 3 to 10 minutes, run controlled and not flat out, with standing or walking breaks. The instruction reads: “Let heart rate rise naturally during each rep.”

Every session comes with a heart rate target and no pace target. The pace that comes with it depends on the day.

On the watch

Planned sessions appear in the Workout app on Apple Watch, with a heart rate alert on each step when the runner drifts above or below its range.

Scoring and wrist heart rate

After the run, Kiiro scores duration and time in range. Pace plays no part, so a slow run on a heavy day still scores well. The score allows 3 bpm either side of the range. Below-range heart rate in the first 60 seconds of a repetition does not count, because heart rate lags. No repetition is shorter than three minutes.

Week to week

Each new week, the plan makes at most one main change based on what was run: more weekly time, a longer or extra interval block, or shorter recoveries. Otherwise it holds the week, or cuts back when recent load runs high against Fitness level. A missed or extra run can prompt an offer to rebuild the rest of the week.

Is it working? The analysis

Every run gets a training load from its heart rate, so a long easy run and a short hard session land on the same scale. From that, Kiiro builds three views:

• Fitness level and Load balance are a 42-day and a 7-day average of training load. Load balance shows how far the recent average sits above or below fitness, named from Fresh through Balanced and Building to Fatigued.
• Route progress matches runs on the same route automatically, so the same loop at the same pace with a lower heart rate shows up as progress.
• Personal bests are found inside every run, alongside trends for VO2 max, resting heart rate and heart rate variability from Apple Health.

Worked example: Jaro’s numbers

These numbers are Jaro’s own: a week of a Kiiro plan from late September 2026, and the build to an April 2026 marathon before that.

A week that adapted

The table shows the plan adapting after an unplanned run on a rest day.

Day	Before	After
Mon	Rest	Unplanned run, 48 min
Tue	Intervals 4 × 3 min	Rest
Wed	Easy 45 min	Intervals 4 × 3 min
Thu	Easy 45 min	Easy 45 min
Fri	Endurance 45 min	Easy 45 min
Sat	Rest	Rest
Sun	Long 70 min	Long 65 min
Week	245 min	243 min, with Monday’s run

Monday’s run counted toward the week, so the total stayed about the same. The next week went back to 245 minutes, with one change: five interval repetitions instead of four.

A hilly long run

In November 2025, during the build to the marathon, Jaro ran an 18.3 km long run with 288 m of climbing to an easy range of 123 to 142 bpm. Pace ranged from 5:54 to 8:03 per kilometre, while heart rate stayed between 130 and 141 bpm.

Progress on a familiar loop

Kiiro tracks runs on the same route, so progress shows up on a familiar loop. These are the averages on Jaro’s 8.5 km loop before and after the build.

The 8.5 km loop	Pace	Heart rate
November 2025, 5 runs	6:47 /km	138 bpm
March to April 2026, 5 runs	6:21 /km	136 bpm

By spring, the loop was 26 seconds per kilometre faster at a heart rate 2 bpm lower, and the build ended with a 3:42 marathon in April 2026.

Integrations, compatibility and price

• Kiiro works only with Apple Watch. It needs a Series 4 or later, any SE or Ultra, and an iPhone on iOS 18 or later. The best Apple Watch for sport guide covers the current models.
• Kiiro reads runs from Apple Health with heart rate, route, elevation, power and form metrics, plus VO2 max, resting heart rate, heart rate variability and cardio recovery.
• Runs sync to Kiiro’s servers for the plan and the analysis. Kiiro does not export files or send runs to other services.
• The plan needs Kiiro Pro, which costs $14.99 a month or $99.99 a year in the US. The analysis is free.

Background

Kiiro is built by three runners: Jaro on the backend and plan engine, Ivan on the iOS app and Eduard on design. It grew out of Jaro’s frustration with pace plans, and with jumps in training load that were hard to sustain. The name comes from kiire, Estonian for fast.

Kiiro is on the App Store.

More: kiiro.app

Quick answers

Is heart rate training better than pace training for running?
Heart rate tracks what a run costs on the day, so the target still fits after poor sleep, on hills or in heat. A fixed pace does not adjust for any of those. Kiiro gives every session a heart rate range and scores the run on time in range, with pace playing no part.

Does Kiiro need a chest strap?
No, the watch’s own heart rate sensor is enough. A Bluetooth strap paired with Apple Watch works too, and Kiiro uses its readings for the scoring and the analysis. A strap helps most on interval days.

Which Apple Watch models work with Kiiro?
Kiiro works with Apple Watch Series 4 or later and any SE or Ultra, paired with an iPhone on iOS 18 or later. It does not work with Garmin or other watches, or with Android phones.

How does the Kiiro plan change from week to week?
Each new week the plan makes at most one main change based on what was run. That change is more weekly time, a longer or extra interval block, or shorter recoveries. Otherwise it holds the week, or cuts back when recent load is high against Fitness level.

Can the Kiiro plan fit around a busy week?
Yes, Reschedule marks the days still free this week, and Kiiro rebuilds all the remaining sessions around them. The usual schedule stays as it is.

Can Kiiro be used without the plan?
Yes, the analysis works on its own for every run synced from Apple Health. It covers training load, Fitness level, Load balance, route progress and personal bests, and it is free.

How much does Kiiro cost?
The analysis is free. The heart rate training plan needs Kiiro Pro, which costs $14.99 a month or $99.99 a year in the US.

Related reading

• Somvia: threshold, critical power and race times from Apple Watch data
• SportVitals: transparent training load and HRV for Apple Watch
• Fit File Forge: 120 classic workouts ready for Garmin

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Author: Jaro, Ivan and Eduard, the team behind Kiiro, edited by the5krunner.

Last Updated on 6 October 2026 by tfk the5krunner