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 about their heart health and activities and those who wish to have evidence to support medical dialogues with their doctor. Some unusual and exciting sports physiology insights are also provided.

Last Updated: 6 November 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.

Buy: fourthfrontier.com £321.75 / $428

A complete solutoin for heart safety in your activities and beyond

• Price - 75%
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• Apparent Accuracy - 95%
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• Build Quality & Design - 80%
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• Features, Including App - 95%
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• Openness & Compatability - 90%
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87%

Summary

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 monitor their heart health closely 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 accuracy in ECG. It can record full days of continuous ECG data, which other devices like watches can’t do.

 

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 devices like Apple Watch, Peloton and Garmin are handy features.

In summary, Frontier X2 allows for the 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, get 25% off the Fourth Frontier website price.

Buy: fourthfrontier.com £321.75 / $428

Pros

• 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

Cons

• 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

• Buy direct - 25% discount auto applied

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 them, so it doesn’t matter! You will feel it, though. It has rudimentary haptics that usefully act as alerts for 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 its complete consumer-grade ECG solution from one company. Better still, it appears to be an accurate, easy-to-use product whose battery records over a day of continuous ECG recording. Onboarding is excellent, 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 most significant 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 unique 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—and medical-like features around a more capable sensing unit/display, which 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

I wore two chest straps for my early tests – 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 in 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 a.m., 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 a vertical black line on the fourth image from a different activity.

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

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

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 some research showing that training load derived from breath rate zones is more accurate than HR, which 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 was unaware of. I believe I’m right in saying that when the amplitude of the ECG’s ST segment is compared to that of the PR segment (image below), a difference of 0.2 millivolts or more is found. 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.

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. You can easily customise the Frontier X2’s alerts before starting a workout to notify you instantly if the strain exceeds a threshold. I have mine set to double buzz at 0.3mv of strain. If I were buzzed, I’d ease off!

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

One of the great things about Fourth Frontier X2 is that you can 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 to get a GPS track or 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 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 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 good.

Fourth Frontier X2 mimics the same basic features as other heart rate monitors, except you have to turn it on by pressing the button. While it will cache your workout, it follows a proprietary mechanism that only retrieves it from its app; i.e., 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, it’s 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 crucial heart health markers over time in the Health Trends section of the app. This is a month of my data, mainly including activity rather than sleep or 24×7 wearing. The data is probably affected by wearing it alongside another chest strap, so don’t read too much into it. I’ve not looked into these trends too much, but the presentation looks nice. For example, on the Average Heart rate chart, you can see each exercise’s upper and lower limits. 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, why not pay 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 your current tech’s ECG tracks from workouts or sleep.
• 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 evidence in preparation for a more meaningful consultation with your doctor.

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: This adds ventilatory thresholds (VT1 and VT2), 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 Autonomous 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 normalised units and LF/HF ratio; and nonlinear HRV parameters including Poincaré plot SD1 and SD2 values.

Fourth Frontier History

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

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

Alternatives to Fourth Frontier X2

Fourth Frontier’s single-lead ECG 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 and a strap.

The Polar H10 chest strap is rated as an ECG source. However, you will need third-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 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.

These features are available in various paid-for apps like Kubios and third-party products like the Polar H10. Frontier X2 packages everything you need in one place and is guaranteed to work together.

Fourth Frontier X2 Accuracy Comparisons and Detailed Accuracy Test Results

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

Sports Accuracy

These are examples of Fourth Frontier X2’s heart rate during exercises against Apple Watch, Garmin HRM-PRO Plus, and Polar Sense. Accuracy is spot on! The Garmin HRM-PRO Plus sometimes seems slightly different from 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; those charts show minor examples. However, I caution the reader that I wore two chest straps simultaneously to get this data, and there is a perfect chance that one strap caused the movement of the other and, hence, could be the source of the minor errors.

Edit: I needed to adjust the X2’s timestamp by 10 seconds on some workouts. I’m unsure if this was due to latency or if the other devices were used simultaneously.

The bottom line is that I’m okay with using any chest straps or even the Polar OH1 armband as my primary source of HR data for sports.

HRV Accuracy

Here, I’m looking at how three 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 two months.

from HRV4Training

The results show a low correlation for all three 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, i.e. your ability to handle your ‘stress’ levels. Both use scientifically valid sampling methods, and I 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 I used a 2-minute sampling period – even if the value comes from 100 seconds (excluding the 20-second warm-up), 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 detect (or not) the arrhythmia simultaneously

Testing Notes

Fourth Frontier X2 produces multiple readings every second to create an ECG. However, if you export HR data from the app, e.g., to Apple Fitness, 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. Several firmware updates have since been released, 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 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 eight 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 two weeks of normal usage.
• Charge time: 45 minutes (2 hours) via proprietary micro-USB cable. It 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 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 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, 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, the ECG will be synchronised 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, 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, HR from the wrist can be incorrect in sports with significant arm movements like tennis. 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: I can’t comment further because it has not gone through the official certification process. 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 has 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 friction pads to reduce movement and improve accuracy.

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

A: It’s 125Hz or every eight 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 sports blogger reviews of the Fourth Frontier X2. For example, neither Rob from the Quantified Scientist nor Ray Maker from DC Rainmaker have covered the product. Maybe they will.

Fourth Frontier Subscription

A premium subscription is not required for the features in this review.

The subscription is quite good, though; it gives additional insights into VO2max calculation from VT, sleep stage analysis, and a readiness assessment. Here are all the details.

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 that don’t have obvious symptoms. Prevention is better than cure; 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 twenty years, I’ve been interested in heart issues related to sports performance. As far as I know, the Fourth Frontier X2 is the only product suitable and valuable for athletes and people with known or suspected heart problems. It allows them to continue their usual active lifestyle while producing medical-grade data to share with their cardiologist or use themselves.

The Frontier X2 seems well thought out and has good features. As of today, it does what it’s meant to. 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 accuracy. It stores a workout without needing other tech during the activity. It can also be used advanced, like live-streaming an ECG to anyone worldwide. But you might prefer sending results to your doctor as a PDF file instead!

The product has new features, such as vibrating 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 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 more demanding weekly sessions, as a check-up on myself.

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

Buy Fourth Frontier X2 Discount

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

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