Frontier Zone Review: ECG Chest Strap for Athletes

Let’s start this review with a short, true story.

Walking with my partner in over 30-degree heat last weekend (July 2026), we watched a man in his fifties collapse during the Richmond Riverside Half Marathon. He looked fit. We helped get the medic from 2Km away to the man, who eventually recovered in the hospital.

Why might this have happened? One plausible answer is that sweating causes dehydration and thicker blood, requiring the heart to work harder. Any lingering coronary problem might be pushed to the forefront. In exercise-induced collapses (ischaemia), ST-segment changes can be seen in an ECG before symptoms become severe. That is a specific event the Fourth Frontier Zone is designed to monitor.

For exercise-induced cardiac strain, it may provide an early warning to act on. And as we get older, all our heart risks increase regardless of how fit we used to be.

The ECG trace from regular 24×7 use can also speed up doctor consultations, and its VO2max and ventilatory training zones provide useful insights to whatever app or sports device you pair the Zone heart rate monitor with.

Use code 5KRUNNER for 10% off at fourthfrontier.com (£179 with discount, RRP £199).

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What is the Fourth Frontier Zone?

The Fourth Frontier Zone is a chest-worn ECG strap that continuously records your heart’s electrical activity during exercise and daily life. It connects to the Fourth Frontier app or to sports devices via Bluetooth, so you can view standard HR training metrics.

An ECG strap differs from most sports straps in that it records at tens of times per second to create the ECG trace you might sometimes see on hospital TV series. No regular sports watches or apps see that level of detail and typically only have up to 5 data points per second – never enough to recreate the ECG trace but enough to give you HRV insights.

Frontier Zone During Sport

The chest strap works like any other sports heart rate monitor and adds a configurable buzz or double-buzz, which occurs when you exceed any of these ranges for 20 seconds

• Heart rate: upper/lower zone threshold
• Breathing rate: upper/lower threshold
• Strain (ST segment): Above 0.3 mV (recommended), providing a margin above the 0.2 mV clinical threshold.

There are no alerts for irregular beats (arrhythmia), as there can be extended periods of noise that might also trigger false alarms. These periods are shown in real time as ‘other’ in the app and don’t necessarily indicate a problem.

Frontier Zone During The Day

Several smartwatches and some Garmin watches do have a manual ECG feature. Their usefulness is limited, however, as they require you to consciously initiate a 30-second recording while the event is happening.

Frontier Zone works differently by creating a complete 24-hour ECG trace from a full charge. You start that with either a double-press of the button or via the smartphone app.

The entire day is continuously recorded, producing a long ECG trace. If there is ever a time of day when you are concerned about how your body feels, you press the button, which flags a 20-second section on the full ECG trace. This is simply a log of something happening to help you or your doctor find the event later.

 

Zone vs older Frontier X2: why is it now cheaper

The Frontier Zone is the cheaper, more modern replacement for the older X2 model. Most people will buy the Zone model, which contains the same ECG features.

Frontier Zone gains the following compared to X2

• Proper USB-C charging
• Battery life boost from 14 days to 30 days in HRM mode.

What you lose

• The small screen on the pod is gone, which removes the ability to glance at time, distance, or shock score without a phone. I was always unsure how you could see the screen during sport when wearing a t-shirt over it! This is no big loss.

What’s unchanged

• continuous recording,
• live streaming,
• ST segment strain monitoring,
• breathing rate,
• HRV,
• body shock, and
• Optional premium subscription tier.

At half the price, the Zone matches X2 on everything that matters for sport and heart safety.

 

Frontier Zone vs X-Plus

The X-Plus model is a prescription-grade product. You’ll likely already have a diagnosed condition and won’t plan to use this in sport.

X-Plus adds the following

• The X Plus holds FDA 510(k) clearance.
• The X Plus detects and quantifies arrhythmia burden, including PVC burden and AFib burden to clinical standards. The Zone flags irregular beats as “Other” without classifying or quantifying them.
• The X Plus generates structured weekly clinical reports for a cardiologist’s workflow. The Zone exports a PDF trace for discussion.

X-Plus lacks the following

• ST-segment strain monitoring
• Athlete-facing training metrics
• A meaningful water resistance rating: the X Plus is rated IP22 (splash-proof only)
• Multi-device Bluetooth: the X Plus connects to one device; the Zone connects to three simultaneously

X Plus costs $649 in the US and requires clinical involvement.

Design and fit

The pod clips onto a standard chest strap, and a longer one is also included. The pod itself is slightly larger than many sports HRMs but easily small enough to sit unnoticed under a jersey or base layer. Wearing and removing are simple, but orientation matters: the pod must be worn with the button facing down, or the ECG signal inverts and doesn’t work correctly.

Accuracy

Limitations: I have used Frontier Zones extensively, through many tens of hours of sports-grade tests, which regular readers here will have already seen in published test results comparing other products. However, I do not have a heart condition, which limited what I could test.

Test Protocols: Frontier Zone was used as a reference device alongside a highly accurate biceps PPG device, Polar SENSE, as the core of most tests. These two devices were then compared to one or more sports watches and sometimes also to a Whoop MG strap on the other biceps. I didn’t try to wear two chest straps at the same time. On previous occasions, I never managed to get that to work as one or both devices tend to be worn incorrectly when alongside another.

Test Results

The Zone has been used as the HR reference device across multiple published sessions. The table below summarises results from five tests; full data and charts are linked from each article.

Session	Conditions	Devices compared	Bias	LoA	Verdict
150-min Z2 endurance ride	Hills, steady effort	Whoop MG, Fitbit Air, Amazfit Helio Strap	+1.4 bpm vs Helio Strap (pre-alignment); +0.9 bpm vs Whoop MG (post-alignment)	±4.5 bpm (post timestamp correction)	Excellent
Dartmoor Classic (108 km, 1,800 m, 26°C)	Outdoor cycling, heat	Amazfit Balance 3, Whoop MG, Apple Watch Ultra 3	Balance 3: -0.1 bpm; Whoop MG: -0.2 bpm; AWU3: -0.8 bpm	Balance 3: -3.6 to +3.4 bpm	Excellent
Stockholm trail run (11 miles, urban)	Trail running, city environment	Amazfit Balance 3, Garmin FR970, Apple Watch Ultra 3, Polar Verity SENSE	Over-read at start; Polar SENSE used as reference instead	n/a	Inconclusive
Surrey Hills Z2-3 outdoor ride	Hilly cycling	Fitbit Air	-0.9 bpm	-4.8 to +3.0 bpm	Good
watchOS 27 multi-device session	Mixed sport	Whoop MG, Fitbit Air, Apple Watch Ultra 3, Amazfit Helio Strap, Polar Verity SENSE	Co-reference alongside Polar SENSE	See full article	Good

The Stockholm session is the only outlier. The Zone over-read at the start of the run, which ruled it out as a reference for that session. The cause is consistent with a contact or warm-up issue rather than a structural accuracy problem; every other session returned Good or Excellent agreement.

The 150-minute ride session also produced a finding worth noting for anyone comparing devices. The Zone’s data arrived with a 15-second timestamp offset relative to the other three devices. Before correcting for it, the limits of agreement reached ±10 bpm. After alignment, they narrowed to ±4.5 bpm on the same data. The bias barely moved. It is a data alignment issue, not a measurement error, but it illustrates how published accuracy comparisons can misrepresent a device if timestamp offsets go unexamined.

Full accuracy data: 150-minute ride test.

The Frontier X Web App

I want to take you through a few of the views of the data from some of my workouts. I have a nice combination here of swim, bike, run and yoga.

The yellow section from the blue pie chart on the right indicates how much of the rhythm is classified as ‘other’. In this context, it could be a cause for concern, but in my case, it is almost certainly just noisy data.

You can also see the higher training load figures for a 2-hour run and a 3-hour bike ride, as you would expect. However, you wouldn’t expect high heart strain from yoga, yet this is the highest-strain event of the four.

 

Digging into a specific session reveals considerably more detailed information. This is a 22-second section recorded at maximum exertion towards the end of a 2x 20-minute threshold bike ride. You can see low HRV, which is consistent with the difficulty of the effort, as well as heart strain, load, calories, and more.

 

VO2max: This is also shown as incorrect (my VO2max is twice the stated level). A correct VO2max estimate requires either power when cycling or GPS when running, and the latter requires me to run with the app, which I haven’t done.

I know some cardiologists read this site, so maybe you might comment on this take by an AI chatbot analysing the 20 seconds of the ECG shown above.

• The rhythm runs at approximately 120-140 bpm throughout, broadly regular but with visible beat-to-beat variation consistent with the respiratory and mechanical demands of hard cycling.
• The QRS complexes are narrow, and the dominant morphology repeats consistently, with a sharp positive R wave peaking around 2 mV in the first strip and slightly lower in the second. The baseline between beats is relatively clean with no obvious artefact contamination.
• There are occasional beats in which the morphology deviates slightly from the dominant pattern, most visibly in the second strip, around 13-14 seconds and again around 17-18 seconds. These could represent isolated ectopics, but the deviation is subtle and transient contact variation during cycling cannot be excluded.
• The T waves are visible following most QRS complexes, are positive and of moderate amplitude, and show no obvious inversion or gross ST deviation on this single lead. ST assessment on a single-lead wearable trace during exercise is unreliable, so that observation carries limited weight.
• There is no evidence of heart block, no prolonged pauses, and no runs of consecutive abnormal beats. The overall picture is organised sinus tachycardia at a rate appropriate for a hard threshold cycling effort, with a small number of morphologically variant beats that do not constitute a pattern.

While you should definitely not use Frontier Zone for self-diagnosis, you can analyse your own ECG trace and then arrive armed with information and a background understanding before talking to a medical professional. A route likely to get you in front of the right experts more quickly.

ECG and heart strain

The app classifies my heartbeat as either normal (sinus rhythm) or one of several other categories. The small proportion flagged as “Other” is common and usually benign, covering ectopic beats, minor arrhythmias, and movement artefacts. If a proportion clusters at a specific point in a workout, or rises persistently over time, it is worth discussing with a doctor.

The strain metric is an ECG-only feature that sets Fourth Frontier apart from every other sports chest strap on the market. It measures ST-segment deviation in real time. The ST segment is the brief pause on the ECG trace between the heart contracting and beginning to recover. A deviation of 0.2 millivolts above baseline is the threshold used to interpret stress (myocardial oxygen deprivation). The Frontier Zone alerts the wearer via vibration at a configurable threshold, typically set at 0.3 millivolts to allow a margin above background noise.

As of July 2026, no other consumer sports chest strap offers real-time ST segment monitoring with vibration alerts during exercise. Garmin HRM 600, Coros HRM, and Polar H10 do not.

More detail on ST segments and what they indicate: ECG monitoring at the London Marathon.

Breathing rate and ventilatory zones

The Frontier Zone derives breathing rate from the ECG signal using two mechanisms: the chest wall moves with each breath, slightly shifting the electrode position and altering the electrical signal; and heart rate varies subtly in sync with respiration, a phenomenon known as respiratory sinus arrhythmia. Fourth Frontier’s premium subscription uses this breathing rate data to estimate VT1 and VT2, the ventilatory thresholds that define the boundaries between aerobic, threshold, and high-intensity effort. These are better intensity markers than heart rate zones based on lactate thresholds; most notably, they are more stable over long sessions, where cardiac drift makes HR-based zones progressively unreliable.

Some other points on breathing rates:

• Resting and overnight breathing rates are indicators for wellness and recovery metrics, e.g. a higher overnight breathing rate might indicate the onset of illness or recovery occurring
• Garmin uses a similar method to assess breathing rate, but has not done the extra work to compare it against VT1/VT2.
• An alternative product for ventilatory zones that adds a strain gauge to the strap is TymeWear VitalPro.

Premium subscription

The base device is fully functional without a subscription. The optional Frontier Premium tier ($14/month, $119/year, with a 7-day trial) adds VO2 max estimation, VT1/VT2 ventilatory zones, a readiness score based on nightly HRV deviation, sleep stage analysis, oxygen uptake zones, and a metabolic profile showing fat versus carbohydrate contributions during exercise. None of these features requires additional hardware. They are algorithmic outputs from the ECG data the device already collects.

This extra set of subscription data is quite interesting.

• People tend to want to know their VO2max, which is a key marker of fitness and longevity (healthspan). It kinda states how metabolically efficient an average kilogram of your body is – so if you get fitter or lose weight, the figure should rise, but interestingly, someone with the same VO2max as you could be a significantly faster runner if their technique is efficient.
• The readiness metric here, based on overnight readings, is comparable to all other brands in the space. It’s not a readiness measure per se but rather a measure of how well you are coping with the general stresses of everyday life. It is linked to sporting readiness, but there are better measures for this, namely a waking, seated  1-minute reading.
• Understanding fat/carb usage during sport is key to losing weight and getting fitter. It’s a complex subject, but it simplifies to understanding that training below VT1 is highly fat-burning and training above VT2 is highly carb-burning. Many people make the mistake of training heavily in the intermediate zone.

Who is it for

The Fourth Frontier Zone makes most sense for active people over 40 who train regularly, for anyone managing a known heart condition who wants continuous monitoring during sport, and for coaches or carers who want shareable ECG data for clinical review. The ability to export up to three hours of ECG trace as a PDF, reviewable by a cardiologist, is a genuine differentiator from anything else in the sports wearable market.

It makes less sense for athletes whose primary driver is a sports-focused ecosystem with advanced training features, GPS and power sensors, and structured workouts.

That said, Fourth Frontier Zone works seamlessly with Garmin, Apple Watch, Polar, Suunto, Amazfit, Coros, and other sports apps. You keep all the benefits of those ecosystems and add the ECG, strain and peace-of-mind metrics via the Zone.

Buyers whose clinical needs require arrhythmia detection validated to medical-grade standards should look at the Frontier X Plus, which carries FDA 510(k) clearance and requires a prescription.

Verdict

At £179 (10% discount code 5KRUNNER), the Fourth Frontier Zone is the most capable ECG sports strap available to a consumer without a prescription. It matches the Frontier X2 on every feature that matters for sport, improves on it in hardware, and costs less than half the price. The ST segment strain monitoring alone justifies the purchase for any athlete in a demographic where undiagnosed coronary artery disease is common and exercise-induced cardiac events go undetected until they happen in public, on a hot day, during a race.

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Quick answers

Is the Fourth Frontier Zone a medical device?
No. The Fourth Frontier Zone is classified as a health and fitness product and is not intended for diagnostic or treatment purposes. Fourth Frontier’s medical-grade product is the Frontier X Plus, which holds FDA 510(k) clearance and requires a prescription.

What is the difference between the Frontier Zone and the Frontier X2?
The Zone uses USB-C charging instead of a proprietary port, offers up to 30 days of typical battery life versus 14 days on the X2, has no screen, and discounts to £179, compared with the X2’s £449. The core ECG feature set is identical on both devices.

What does the Frontier Zone's strain metric measure?
Strain on the Fourth Frontier platform refers to ST-segment deviation, not training load. The device measures how far the ST segment shifts from its baseline during exercise. A persistent ST deviation of 0.1 to 0.2 millivolts can be clinically significant, depending on the context and lead placement. Fourth Frontier sets the default vibration alert at 0.3 millivolts to reduce false alarms during exercise.

Does the Fourth Frontier Zone work without a smartphone?
The Zone can record data autonomously without a phone present during a session. Data syncs to the app and web dashboard afterwards. Live ECG streaming and real-time metric display require a connected phone. Vibration alerts function independently of the phone.

Can I share Fourth Frontier Zone ECG data with my cardiologist?
Yes. The web dashboard allows export of up to three hours of continuous ECG trace as a PDF. A cardiologist can review this in the same way they would a single-lead ambulatory ECG recording. The device is not a replacement for clinical investigation but the exported trace is a meaningful supplement to a clinical conversation, particularly if recorded during a hard effort when symptoms were present.

Does the Fourth Frontier Zone support ANT+?
No. The Zone uses Bluetooth Low Energy only and can connect simultaneously to up to three devices: one app and two external heart rate monitors. It does not support ANT+.

How does the Frontier Zone compare to the Polar H10 and Garmin HRM 600?
The Polar H10 and Garmin HRM 600 are accurate sports chest straps that transmit heart rate via ANT+ and Bluetooth. Neither records a continuous ECG trace nor monitors ST segment deviation, and neither provides breathing rate derived from ECG. The Frontier Zone does all three. The trade-off is that the Zone is Bluetooth only with no ANT+ support, costs more than a basic HRM, and requires its own app for ECG features. If you need ANT+ compatibility or the lowest possible cost, the H10 or HRM 600 is the correct choice. If you want ECG data and cardiac strain monitoring during exercise, neither device offers them.

How long does the Fourth Frontier Zone battery last?
Typical battery life in HRM mode is up to 30 days. Maximum continuous ECG recording is 24 hours per charge. A 48-hour ambulatory trace would require a sync and recharge midway, resulting in two separate sessions rather than a single unbroken recording.

What is the correct way to wear the Fourth Frontier Zone?
The pod must be worn with the button facing downward, positioned centrally on the chest just below the breastbone. Wearing it reversed inverts the ECG signal, biasing ST-segment readings in the wrong direction and rendering the strain metric unreliable. The correct orientation is marked on the device.

Does the Frontier Zone work with Garmin, Apple Watch, and other sports watches?
Yes. The Zone connects via Bluetooth Low Energy as a standard heart rate monitor to compatible devices including Garmin watches, Apple Watch, Wahoo bike computers, Peloton, Zwift, and most other BLE-capable fitness devices. ECG features, strain monitoring, and breathing rate are available only in the Fourth Frontier app. The external device receives only heart rate data.

What is the Fourth Frontier Premium subscription and is it worth it?
The optional Frontier Premium tier costs $14 per month or $119 per year, with a 7-day free trial. It adds VO2 max estimation, VT1 and VT2 ventilatory training zones, a readiness score based on nightly HRV deviation, sleep stage analysis, oxygen uptake zones, and a metabolic profile showing the fat-versus-carbohydrate contribution during exercise. None of these requires additional hardware. Whether it is worth it depends on how much you use the training zone and readiness features. The base device is fully functional without the subscription.

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Sources and resources

• Fourth Frontier support and manual
• Fourth Frontier: the science behind continuous ECG
• 150-minute ride accuracy test
• ECG monitoring at the London Marathon
• Fourth Frontier X2 review
• Tymewear VitalPro review
• Garmin HRM 600 review
• Frontier Zone product page

 

Last Updated on 10 July 2026 by the5krunner