[2024] Fourth Frontier X2 Review ❌6 Cons❌, ✔️8 Pros✔️ ECG heart rate monitor

fourth Frontier X2 heart rate monitor Strap

Fourth Frontier X2 Review

This is a detailed review of the second-generation Fourth Frontier X2 ECG sports strap launched in 2023. It’s a novel ECG product geared towards those who want to be extra vigilant with their heart health and activities, plus those wanting evidence to support medical dialogues with their doctor. Some unusual and interesting sports physiology insights are also provided.

Last Updated: 17 May 2024

I’ll start with a summary review of the Fourth Frontier X2 ECG strap and then give some deep dives into key features, namely ECG during sport, real-time health alerts, its ECG charts & how to share them. If you’ve come here for the discount, use the code THE5KRUNNER at this link to fourthfrontier.com for 25% off

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A complete solutoin for heart safety in your activities and beyond
  • Price - 75%
  • Apparent Accuracy - 95%
  • Build Quality & Design - 80%
  • Features, Including App - 95%
  • Openness & Compatability - 90%


This review looks at the Fourth Frontier X2, a heart-monitoring chest strap that can record electrocardiograms (ECGs). It’s aimed at people who want to closely monitor their heart health during exercise, or sleep or who have heart conditions.

I tested many features of the X2, including using its ECG readings during exercise, real-time alerts, and sharing ECG reports. The X2 provides accurate heart rate data for sports but I did not have the scientific means to assess its ECG accuracy. It can record full days of continuous ECG data, which other devices like watches can’t do.

fourth frontier x2 review hero


The X2 has some unique aspects. As well as conventional heart rate tracking, it measures and alerts you to “heart strain” during activity.

Fourth Frontier X2 records and saves workouts with ECG data within its pod. You can share ECGs in real-time with others via a link, or generate a PDF report to send to a doctor. Long battery life and integrations with other devices like Apple Watch, Peloton and Garmin are highly useful features.

In summary, Frontier X2 allows close monitoring of heart health during exercise with ECG tracking. While pricey, it provides medical-type data that other devices lack. For those interested in their heart or with conditions, it could offer reassurance during activities.

Using code THE5KRUNNER gets 25% off the Fourth Frontier website price.

Buy: fourthfrontier.com £321.75 / $428


  • Haptic alerts to warn you of imminent heart ‘events’
  • Record or share full ECGs during sport or overnight during sleep
  • Accurate for sports
  • Novel heart insights for sports people based on ECG
  • Links to almost all other devices including Apple, Garmin and Peloton
  • More than adequate battery life and quick recharge time
  • Information-rich app and website for your data and insights
  • near Medical-grade data that others lack


  • Price
  • Sometimes I forgot to start it before an activity
  • The app is nice enough but could be more cleanly structured and presented
  • The chest strap is retro-looking
  • Some people will struggle to get an HR/ECG reading because of their body shape/composition
  • Dubious waterproof rating for serious swimmers
fourth Frontier X2 ECG Strap

Fourth Frontier Design

Fourth Frontier X2 includes two standard heart rate chest straps in a beautifully packaged box. The clip-on pod is highly unusual for this type of product as it has a button, a display and an unusual charging port to the side. Perhaps the aesthetics are somewhat retro and not to my liking but you will rarely see it so it doesn’t matter! You will feel it though, it has rudimentary haptics that usefully act as alerts for various kinds of heart-related events.

The obligatory app is feature-packed with several unique twists that I’ll cover later, and the online platform is also good.

The biggest PRO for Frontier X2 is it’s a complete consumer-grade ECG solution from one company. Better still it appears to be an accurate product that is easy to use and whose battery record over a day of continuous ECG recording. Onboarding is great and there are also free one-to-one video sessions to discuss your data with ECG-product experts. Its PDF reports are perfect for sharing with doctors.

The biggest CON with Frontier X2 is its price. Perhaps the colour display is unnecessary. The app’s design could also be tweaked and improved.

Why is Fourth Frontier X2 special or different?

Fourth Frontier X2 starts as a regular, well-featured sports HR chest strap from Garmin or Polar, except it’s on steroids. Lots of them.

Fourth Frontier X2 adds new sports features and medical-like features around a more capable sensing unit/display.  This is what sets it apart

  • Sports Features – colour display, haptics, heart strain metrics and alerts, proprietary workout caching and retrieval, breath rate, training zones, training load
  • ECG Features – liver monitoring online or long-duration ECG traces in PDF format for sharing the cardiac events that are automatically identified
  • ECG Alerts – haptic alerts (vibrations) as you cross a performance threshold or when you experience an unusual ECG event


Deep Dive: The ECG Chart & Workout Metrics

For almost all of my early tests, I wore two chest straps – an X2 and a Polar H10 / Garmin HRM PRO PLUS. That probably affects the ECG quality, so what follows is an example of a more recent test, wearing just the Fourth Frontier X2. The main summary shows I had normal rhythms 80.7% of the time during this easy run. 7.3% of the remainder is ‘noise’ and we can probably ignore that. Which leaves 11.9% of rhythms that my doctor might be more interested in.

Looking at the images below, tap the red View ECG button for a full view of the ECG, you’ll notice the ‘O’ markers on the heart rate chart on the second image. These are automatically detected events that might warrant further investigation.


The ECG trace shows 20-second chunks of the activity. If we zoom in at around 11 am we can see on the third image that there are 4 ‘O’ markers and I’ve circled on the ECG the events that these might refer to. If you feel something strange while wearing Fourth Frontier, you can double press its button and a manual event is also visually logged. You can see that on the fourth image from a different activity as a vertical black line.

More: How to read an ECG (GeekyMedics.com)

The workout/activity metrics do offer additional heart-related insights that the main competitors do not provide (Garmin, Polar, Suunto, Apple).

The Workout Summary starts by showing the Training Load. Training Load scores the difficulty of your workout by giving more weight to the time spent at higher intensity levels. Usually, this metric is calculated from heart rate or power zones. However Fourth Frontier cited to me some research that showed that training load derived from breath rate zones is more accurate than HR and that is the method they use. I’d never come across that before but I guess it superficially makes sense as the breathing rate can be determined from signatures in the HRV/ECG track and breath rate will further not be impacted by, say, caffeine or fatigue in the ways that HR is. Interesting.


Next up is the company’s Body Shock score. This is similar to some power-related metrics that you might get with STRYD and comes from inferences of mechanical load from the X2’s internal accelerometers. Persistent levels of high shock are deemed to indicate your susceptibility to injury. Fair enough, we’ve seen similar measures before from other companies but it’s nice to get this all in one package.

Finally, I’ll highlight the Heart Strain assessment, which I’d not previously been aware of. I believe I’m right in saying that when the amplitude of the ECG’s ST segment is compared to the amplitude of the PR segment (image below) and a difference of 0.2 millivolts or more is found, then we can infer electrolyte imbalances, conduction defects or oxygen deprivation. Those all sound potentially serious so it must be good to be warned when this happens

ST Segment on ECG Trace track Image|Wikipedia


From one of my workouts, you can see my heart strain viewed over time in ‘zones’ in the following chart. Strain remains in the blue region and that’s good. Handily you can customise the Frontier X2’s alerts before you start a workout to instantly notify you if the strain exceeds a threshold,  I have mine set to double buzz at 0.3mv of strain. If I was buzzed, I’d definitely ease off!


Deep Dive: Creating A Richer, Safer Activity & Recording Experience

One of the great things about Fourth Frontier X2 is that you can simply press its button twice and go off and do your workout. You don’t need a watch or an app or anything else. All your activities are saved on the strap and you get feedback, alerts and the ability to log events via the button, haptics and display. Optionally, you can take the Fourth Frontier app if you like to get a GPS track or you can pair the Fourth Frontier X2 to any other sports device or app without losing your ability to cache an ECG of the activity back to the Fourth Frontier app.

That said, if you want to get richness from its small OLED display, you’ll be left wanting. Sure it’s a better display than on any other chest strap (because there aren’t any others!) but it merely shows time, distance, shock score and battery…all upside down from the perspective of looking down when wearing it correctly with the button downwards! But it gives you freedom from wearing other gadgets even during team sports, so that’s a good thing.


 Fourth Frontier X2 Discount Featured Polar H10 Garmin HRM PRO PLUS

Fourth Frontier X2 mimics the same basic features as other heart rate monitors except you have to turn it on by pressing the button and whilst it will cache your workout, it follows a proprietary mechanism that only retrieves back to its app ie it doesn’t follow the Garmin ANT+ standard.

Turning to the more advanced features in the images below. We can see that upper and lower alert limits can be set for heart strain, breathing rate and heart rate. When those limits are crossed the double- or single-buzz haptic occurs. Simple but effective…and safe!


In the second of those images, if you look closely, the SHARE LINK option gives you a link (URL) that you can share with anyone who can then see your ECG in real time. This has obvious benefits for sports coaches and medical professionals alike. Again, a cool feature.

Alternatively, you can go to the online dashboard, select a portion of your activity up to 3 hours long and generate a PDF file to email.

Example: My ECG in PDF from a recent activity

Finally, you can keep track of key heart health markers over time in the Health Trends section of the app. This is a month of data from me and mostly includes activity rather than sleep or 24×7 wearing. The data is probably affected by wearing alongside another chest strap so don’t read too much into it. I’ve not looked into these trends too much as a result but the presentation looks nice. As an example, on the Average Heart rate chart, you can see the upper and lower limits in each exercise. Similarly, the OTHER RHYTHM gives insight into whether your events are becoming larger and more frequent. Simple but nice.




Who Is Fourth Frontier X2 designed for?

Its primary purpose could be your main sports strap but there are cheaper, basic straps for that job. If any of the following describe you then why not pay a bit extra for peace of mind and a strap that addresses your less common needs?

  • SAFE EXERCISER: You are an older athlete (over 40) and want the security of monitoring for serious health issues as you exercise and at other times
  • BIOHACKER: You are a biohacker or sports data geek. You can’t get ECG tracks from workouts or sleep from your current tech
  • CARER: You want to monitor the heart health of other people –  for example, you might be interested in using one product to monitor your parents periodically. [Paula Radcliffe is an ambassador for the company and has used the product for her mother]
  • HEART CONDITION: You have a heart condition for which you receive medical advice. You want an extra layer of monitoring that you control or perhaps you think you have a heart condition and want to gather some evidence as preparation for a more meaningful consultation with your doctor.

Fourth Frontier X2 Review Discount Featured Polar H10 Garmin HRM PRO PLUS

Other Uses of an ECG Heart Rate Monitor

Fourth Frontier can do some of these already and for those it can’t, it can still provide the ECG data for use by other tools:

  • Differentiate noise from cardiac arrhythmia
  • Determine ST Segment characteristics of each beat
  • Heart rate variability measurements
  • DFA a1 measurement
  • Early atrial fibrillation detection
  • Distinguish atrial fibrillation from ventricular arrhythmia
  • Derive respiratory rates and thresholds

Fourth Frontier recently added new premium features at $14.99/mo

  • Metabolic milestones – adds ventilatory thresholds (VT1 and VT2). These are similar to anaerobic and aerobic thresholds for athletes (AeT, AnT, LT1, LT2, MLSS, LTHR).
  • VO2max test
  • Heart Rate Recovery – as a measure of cardiovascular fitness which shows fitness improvements when trended
  • These Autonomic Health features are being considered for addition: Parasympathetic nervous system (PNS) and sympathetic nervous system (SNS) indexes to assess changes in recovery and stress; Time domain HRV parameters – Mean RR, SDNN, RMSSD, etc: Frequency-domain HRV parameters including LF and HF powers in absolute and normalized units and LF/HF ratio; Nonlinear HRV parameters including Poincaré plot SD1 and SD2 values.

Fourth Frontier History

Fourth Frontier is a company based in the UK, USA and India with its products made in Taiwan and sold globally. Its founders are ex-Qualcomm and ex-Ashoka University and they initially raised £10.5m to expand the product offering in July 2021.

The original X model was launched in 2019 and the X2 model followed in Q4.2022.

fourth frontier x2 does it work with apple garmin peloton
Will pair to just about any smartphone app that is compatible with external HR straps

Alternatives to Fourth Frontier X2

The single lead ECG of Fourth Frontier can generate a 24-hour ECG track, that one feature sets it apart from the competition. Devices like Garmin or Apple need manual readings or automatic short-duration readings. I’m sceptical of wrist-based wearables accurately recording heart rate tracks, even for workouts, let alone a medical-grade ECG.

Wellue has a consumer-grade ECG that requires taping to the skin and seems impractical for anything other than occasional use.

Garmin HRM-TRI, HRM-PRO and HRM-PRO PLUS will cache sports data when swimming and send it back to your Garmin watch when the workout is finished. Cache retrieval requires a high-end Garmin sports watch as well as a strap.

The Polar H10 chest strap is rated as an ECG source, however, you will need 3rd party software to view and manage the ECG data as the Polar Flow ecosystem doesn’t handle it. Polar produces higher resolution data but that doesn’t seem to me to add the ability to see or detect additional ECG events.

Many non-watch wristbands take your heart rate and have a rudimentary display. Perhaps most of these would have a better display than FourthFrontier X2 but none would have ECG-grade heart rate tracking.

You can get many of these individual features in various paid-for apps like Kubios and 3rd party products like the Polar H10. Frontier X2 packages everything you need in one place, guaranteed to work together.


fourth frontier x2 review
Will pair to all modern Garmin watches and bike computers with Bluetooth support


Fourth Frontier X2 Accuracy Comparisons

Most of my reviews cover a deep dive into accuracy and here are results and thoughts on the Fourth Frontier x2.

Sports Accuracy

These are some examples of Fourth Frontier X2’s heart rate during exercises against Apple Watch, Garmin HRM-PRO Plus and Polar Sense. Spot on! Sometimes the Garmin HRM-PRO Plus seems to be slightly different to the other two comparators.

From the perspective of a sports-grade HR monitor, Frontier X2 does the job. You will always get the occasional errant HR track with Garmin, Polar and Fourth Frontier, and those charts show minor examples of that. However, I caution the reader that I wore two chest straps simultaneously to get this data and there is a very good chance that one strap caused the movement of the other and hence could be the source of the minor errors.

Edit: On some workouts, I needed a 10-second adjustment to the X2’s time stamp. I’m unsure if this was latency or because of the other devices in use at the same time.

Bottom line: I’m fine to use any of the chest straps or even the Polar OH1 armband as my main source of HR data for sports.


HRV Accuracy

Here I’m looking at how 3 pieces of tech correlate to a 2-minute waking HRV reading made by a Polar H10 on the HRV4Training software. the correlations were based on readings taken over 2 months.


fourth frontier hrv accuracy from HRV4Training

The results show a low correlation for all 3 devices (Eight Sleep, Garmin 965) both in terms of the absolute values (day-to-day) and the more important baseline (trend).

Rationale. Waking HRV and overnight average HRV measurements will give different values but they are trying to measure the same physiological thing ie your ability to handle your ‘stress’ levels. Both use scientifically valid sampling methods and I would expect correlations. Garmin and 8 Sleep use the overnight average method with the former using Photoplethysmography technology and the latter using ballistocardiography. Eight Sleep and Frontier X2 were used simultaneously to take a 2-minute reading before getting out of bed.

Conclusion: Something might be wrong here. Possibly with my H10, as there have been good correlations in the past, or possibly with the X2 as it takes at least 20 seconds to display an HRV value. That said, that was one reason why I used a 2-minute sampling period – even if the value comes from 100 seconds (excluding the 20-second warm-up) then it is more than sufficient to give a valid reading.

ECG Accuracy

I can’t determine the accuracy of the ECG. This would involve the tester

  1. Having an arrhythmia
  2. Using two tools to simultaneously detect (or not) the arrhythmia


Testing Notes

Fourth Frontier X2 produces multiple readings every second to produce an ECG. However, if you export HR data from the app eg to Apple Fitness then the data resolution is reduced to one reading per two seconds. I believe more frequent and correct HRV data points are stored in Apple Health but I disabled those to ensure my HRV4Training master data remained consistent.

Marco Altini (HRV4Training ) pointed out to Fourth Frontier in 2023 that its rMSSD calculation was incomplete. There have since been several firmware updates but I don’t know if the correction has been made.

My reference device, Polar H10, had a battery change during the test period. The low charge before the battery change may have affected the results. IDK.

Fourth Frontier x2 specifications

Fourth Frontier X2 Detailed Technical Specifications

These are the detailed technical specifications of Fourth Frontier X2 with the capabilities of the previous generation product in brackets

  • ECG Type – Single Lead ECG in the V5 position
  • Sensors: Triaxial accelerometers detecting 0.01-20G forces
  • Sampling rate: 125Hz or every 8 milliseconds
  • Memory: 30 hours of continuous data storage (24 hours)
  • Battery: 150mAh lithium-polymer
  • Battery life: up to 24 hours per charge (10 hours). Typically that’s 2 weeks of normal usage.
  • Charge time: 45 minutes (2 hours) via proprietary micro-USB cable. Can be charged and worn at the same time but awkward
  • Water resistance: IP67 rated to 1m. I would NOT describe this as suitable for swimming but the company does. It will work ‘for a while’
  • Connectivity: Bluetooth Low Energy, Bluetooth 5 with up to 3 simultaneous BLE connections. No ANT+ support
  • Sports Platform Integration: FROM Garmin Connect. TO Apple Health and to Strava. Other integrations are not directly supported by can be achieved using the manual FIT export facility
  • Display: 0.96″ OLED showing time, distance, shock score, battery level (monochrome LCD)
  • Dimensions: 45 x 33 x 13mm, excluding 22mm chest strap (52x38x15mm)
  • Weight: 19g, excluding 30g chest strap
  • Compatibility: Android 6.0+, iOS 11+,
  • Accessories: 2 strap sizes, micro-USB cable, carry case
  • PDF Maximum size – a 3-hour ECG selection

fourth frontier x2 is it any good

Fourth Frontier X2 FAQs

Q: Does Fourth Frontier X2 work with an Apple Watch

A: Yes

Q: Does Fourth Frontier X2 work with a Garmin Watch or bike computer

A: Yes

Q: How do I best use Fourth Frontier X2 with an Apple Watch and my preferred sports app?

A: Pair Fourth Frontier X2 to the Apple Watch as a generic Bluetooth Heart Rate Strap and use your favourite app. Then ALSO double-press the X2’s button so that it also records an ECG track on the X2. At the end of your workout stop the X2 and open the X2 app on your smartphone. After a minute or so the ECG will be synchronised across to the Fourth Frontier app.

Q: Does Fourth Frontier X2 work with Peloton

A: Yes

Q: Does Fourth Frontier X2 work with an iOS and Android app

A: Yes

Q: Is Fourth Frontier X2 worth it?

A: Yes, who can put a price on safety when exercising? Furthermore, all the ECG and medical-related features you’ll need are included. No need for additional apps.

Q: How do you charge the Fourth Frontier X2

A: It can only be charged with the included, proprietary cable that’s compatible with a standard USB port (USB-A). The charging cable has a blue light indicating it is ready to charge your X2.

Q: Can Fourth Frontier X2 be used in team sports

A; Yes, Frontier X2 can typically be used in team sports. Unlike a sports watch, it does not count as jewellery and is unlikely to hurt others. Furthermore, in sports with significant arm movements like tennis, HR from the wrist can be incorrect. A chest strap almost always gives correct results for most people in a safe way.

Q: Can Fourth Frontier X2 be used with a pacemaker

A: It has not gone through the official certification process for this.  I can’t comment further. Contact the company.

Q: My Apple (or Garmin) has ECG why do I need Fourth Frontier X2

A: Frontier X2 has the benefit of saving a day-long continuous ECG. You can send that to your cardiologist or even live-stream to them. At best, Garmin and Apple produce short readings at rest.

Q: Does the Fourth Frontier X2 use a generic strap to attach the electronic pod to?

A: Yes. X2 comes with a spare strap but you can use most standard chest straps. Indeed some other straps are better – you might try the one that comes with the Polar H10, it has superior sensing pads and superior friction pade to reduce movement and improve accuracy.

Q: What is the sampling frequency of Fourth Frontier X2?

A: It’s 125Hz or every 8 milliseconds.


Fourth Frontier Review – A Review of the Reviews!

Looking at Amazon, Trustpilot and the aggregate reviews on its site, Fourth Frontier X2 scores a creditable 4/5(ish).

There are few good blogger reviews of the Fourth Frontier X2 available, for example, neither Rob from the Quantified Scientist nor Ray Maker from dcrainmaker have covered the product.

Fourth Frontier X2 Review – Thoughts and Summary

Buy: fourthfrontier.com £321.75 / $428 (25% discount included at checkout)

Silent heart attacks (silent myocardial infarctions or SMIs) are thought to account for 45% of all heart attacks. Nearly half of people (48%) will suffer some form of heart disease at some point. The company claims that a product like the Fourth Frontier X2 should catch 85% of asymptomatic arrhythmias ie that don’t have obvious symptoms. Prevention is better than cure and even very active people aren’t immune to heart problems. A FIFA report on sudden deaths (from 2020) estimated that over 617 football players worldwide died from sudden cardiac arrest between 2014 and 2018.

For almost 20 years, I’ve been interested in heart issues related to performance in sports. As far as I know, the Fourth Frontier X2 is the only product suitable and useful for athletes and people with known or suspected heart problems. It allows them to continue their usual active lifestyle while also producing medical-grade data to share with their cardiologist or use themselves.

The Frontier X2 seems to be well thought-out and has good features. It does what it’s meant to as of today. Fourth Frontier plans to add new features in 2024 and beyond, building on its success of selling over 100,000 units.

The Frontier X2 is accurate as a heart rate monitor for sports. Furthermore, I have no reason to doubt the accuracy of the ECG, although I couldn’t verify its ECG accuracy. It works well to simply store a workout without needing other tech during the activity. Or it can be used in advanced ways like live-streaming an ECG to anyone in the world. But you might prefer sending results to your doctor as a PDF file instead!

The product has some novel new features that will vibrate alerts during exercise if it detects potential issues. We both know that without that information, we’d continue to the end of what we started. So whether you’re just curious about your body, want a safety net for yourself or a loved one, or want data for a medical professional, this product can do it all.

I’d say it worked how I expected. So I’m neither impressed nor disappointed. However, I thought Fourth Frontier X2 would just collect dust in a drawer after I’d finished this review. But I’m not getting any younger, and there’s no downside to using Frontier X2 in sports compared to other heart rate monitors. So I’ll probably use it sometimes, maybe just for harder weekly sessions, as a check-up on myself.

It’s a great product once you get past the quirky retro looks!

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You can buy directly from the manufacturer who ships internationally. Use the code THE5KRUNNER for the biggest discount available – a whopping 25%.

Buy: fourthfrontier.com £321.75 / $428 (25% discount included at checkout)


Buy Fourth Frontier X2 USA Discount Coupon




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