Indie Spotlight: SYMPATHETIC turns a Polar H10 into a breathing coach

Most runners use a Polar H10 only while they run. Sympathetic, a free iPhone app, uses it once the session is over, to show how a few minutes of slow breathing slow the heart.

The app combines guided breathing with live feedback from the strap. A runner follows the pacer and watches heart rate rise and fall with each breath, then tries a different rhythm to see how the response changes. It brings the curiosity runners already have about training data into the recovery routine.

Lucas Camillo is a longevity researcher and medical student at the University of Cambridge. He built Sympathetic and uses resonance breathing in his own recovery routine.

How Sympathetic works

TL;DR: Sympathetic turns an iPhone and a Polar H10 into a guided breathing and biofeedback setup. It is free, needs no account or subscription, and works either as a quiet breathing guide or as a closer look at the heart.

A visual pacer with sound and vibration cues makes the rhythm easy to follow. Breathing speed, the balance between breathing in and breathing out, and session length can all be adjusted. With an H10 connected, the same session also shows heart rate and heart rate variability, or HRV: the small changes in time between one heartbeat and the next. Seeing those changes unfold gives the breathing exercise an immediate, visible response.

Why breathing belongs in a recovery routine

During exercise, the nervous system raises heart rate to meet the effort. Afterwards, parasympathetic activity slows the heart again. This is the branch often called “rest and digest”, and the vagus nerve is its main connection to the heart.

Slow, controlled breathing can increase the parasympathetic influence on the heart. A runner who still feels wired after training gets a practical way to practise settling down, and a session fits easily after a cool-down, on a rest day or in a quiet few minutes in the evening.

Finding a rhythm that suits the heart

Heart rate normally speeds up a little when breathing in and slows when breathing out. This pattern is called respiratory sinus arrhythmia, or RSA. Despite the clinical name, it is a normal part of how breathing and the heart interact.

Resonance breathing uses a slow pace, often around six breaths a minute, to make that pattern larger. It works with the baroreflex, the body’s feedback system for regulating blood pressure. At the right pace, breathing and the baroreflex reinforce one another and produce larger, more regular swings in heart rate. The best pace varies between people.

Sympathetic puts this on screen. Its guided assessment compares breathing settings, and the heart-and-breath view shows heart rate rising and falling across each breath.

Worked example: 5.5 or 5.3 breaths a minute

In one recorded session, Lucas spent two minutes at 5.5 breaths a minute and two minutes at 5.3. Both settings gave similar readings: a median heart rate of about 71 to 72 beats per minute and almost identical HRV. Breathing slightly slower brought no clear improvement, so the assessment kept the starting pace.

A useful pace is not always a slower one. Here the feedback gave Lucas a reason to stay with the setting he already used. Breathing itself changes HRV, so readings from these sessions should be kept apart from the morning or overnight readings a runner uses to follow recovery over time.

Seeing the ECG behind the numbers

Polar‘s H10 does more than send a heart rate number to a running watch. It can also record a single-lead ECG, the heart’s electrical activity beat by beat, which few chest straps offer; Fourth Frontier is the other main option. Sympathetic records that trace and lets the runner scroll through it and zoom in.

Lucas used this to look through an overnight recording of more than eight hours and noticed what looked like skipped beats. The pattern was Mobitz type I, also called Wenckebach, which can be a normal finding during sleep in healthy athletes.

A long recording can be explored a short section at a time, and the original trace can be exported. Anything that looks unusual on a trace is a question for a doctor.

Trying it after a run

Once breathing has settled after the cool-down, a short session is a simple place to start. Open the Breathe screen, choose a comfortable pace and duration, and follow the guide without taking unusually deep breaths. A slower pace should never be forced if it feels uncomfortable.

For a runner who already owns an H10, a first session is just connecting the strap and following the pacer for a few minutes. The breathing guide also works without a sensor, so there is no need to buy a strap to try the practice.

Compatibility and price

• Sympathetic is free on the App Store, with no account and no subscription.
• It needs an iPhone running iOS 18 or later.
• Heart rate, HRV, ECG and the heart-and-breath view need a Polar H10. It is the only sensor supported at present.
• Breathing time can be saved to Apple Health as Mindful Minutes.
• Recordings stay on the phone unless the user exports them.

Quick answers

Does Sympathetic need a chest strap?
Only for measurements. The breathing guide works on its own, while heart rate, HRV, the ECG and the heart-and-breath view need a Polar H10.

Does Sympathetic work with Apple Watch or other heart rate straps?
Not at present. The app takes its sensor measurements from the Polar H10 only.

Can Sympathetic save sessions to Apple Health?
Breathing time can be saved as Mindful Minutes. The ECG and heart rate measurements are not exported to Apple Health, but the original ECG trace can be exported from the app.

What breathing pace should a runner start with?
Resonance breathing is often around six breaths a minute, but the best pace varies between people. Sympathetic’s guided assessment compares settings, and staying at a comfortable pace is better than forcing a slower one.

How much does Sympathetic cost?
Sympathetic is free, with no account to create and no subscription. It needs an iPhone running iOS 18 or later.

Related reading

• Heart Analyzer: what Apple Watch actually records in Apple Health
• SportVitals: transparent training load and HRV for Apple Watch
• Cinder: recovery for Apple Watch

Author: Lucas Camillo, developer of Sympathetic, edited by the5krunner.

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Last Updated on 13 October 2026 by tfk the5krunner

tfk the5krunner

tfk is the founder and author of the5krunner, an independent endurance sports technology publication. With 20 years of hands-on testing of GPS watches and wearables, and competing in triathlons at an international age-group level, tfk provides in-depth expert analysis of fitness technology for serious athletes and endurance sport competitors. ID