Fourth Frontier X2 Review: Your Heart’s New Best Friend?

This in-depth Fourth Frontier X2 review dives into the second-generation ECG sports strap (launched in 2023), featuring accuracy tests and real-world insights. Use code THE5KRUNNER for 30% off—grab it now at fourthfrontier.com (£321.75 / $428).

It’s not just a heart rate monitor—it’s a game-changer for anyone serious about heart health, from athletes to those with medical concerns. Curious? I was. I wore the Frontier X2 almost continuously for over a week; this is my review of it, warts and all!

Last Updated: 29 August 2025

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Fourth Frontier X2 Design: Built for Purpose

Unbox the X2, and you’ll find two chest straps, a clip-on pod with a button, a colour display, and a side charging port—all in sleek packaging. The retro look might not win beauty contests, but it’s rarely visible. You notice smart haptics that buzz for heart alerts and a battery that lasts over 24 hours of continuous ECG recording.

The app and online dashboard are packed with unique features—think detailed ECGs and shareable PDFs for your doctor’s review. It’s a comprehensive, consumer-grade ECG solution that’s accurate and easy to use, featuring free expert video sessions to help decode your data.

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Listen to the Podcast Discussion

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Why Choose Fourth Frontier X2?

Think of it as a souped-up sports strap—like a Garmin or Polar, but with ECG superpowers. It’s unique because it blends athletic tracking with medical-grade insights:

• Sports Edge: Colour display, haptics, heart strain alerts, breath rate alerts, breath-based training load, and workout caching, meaning you don’t have to carry the phone with you.
• ECG Power: Live monitoring, long-duration ECG PDFs, and auto-detected cardiac events. The ECG is interactive, meaning you can tap and record an event as a mark in time to review later. Thus, as it’s an ECG, you have frequent readings taken per second – ECG granularity!
• Safety First: Haptic buzzes warn you of thresholds or unusual heart activity.

It’s perfect for biohackers, cautious exercisers, or anyone needing doctor-ready data.

Deep Dive: ECG Charts & Workout Insights

On an easy run, I tested the X2 with no comparison device. It logged normal rhythms 80.7% of the time, with 11.9% flagged as notable, potentially useful for a doctor’s review. Tap “View ECG” in the app, and you’ll see 20-second traces with ‘O’ markers for events worth checking.

Double-press the button during a workout to log a manual event, such as the odd flutter you felt. The ECG zooms in on key moments, like these four circled anomalies around 11 a.m. (learn more).

Workout Gems:

• Training Load: Uses breath rate (not just HR) for a smarter intensity score—novel and research-backed.
• Body Shock: Tracks mechanical stress to flag injury risk, akin to Stryd’s metrics.
• Heart Strain: A unique measurement that highlights when you dangerously over-exert – more specifically, it measures ST segment shifts (0.2mv+ signals issues like oxygen deprivation)—set it to buzz at a slightly higher level of 0.3mv and ease off on the training if it buzzes.

What’s great about the app?

When looking at your live ECG trace, the app presents an intuitive view, albeit with a delay afterwards as it processes the workout.

Rich & Safe Workouts

Double-press, workout, done—no phone needed. The X2 caches everything, buzzing for alerts and showing time, distance, and shock score on its OLED (upside-down, but who cares?). Pair it with your app or Garmin for GPS, and the ECG will still sync back.

Set custom alerts for heart strain, breathing issues, or heart rate—simple, effective, and safe. Share live ECG links with coaches or export 3-hour PDFs from the dashboard (see mine).

Frontier Premium: Optional Subscription

The optional Premium subscription ($14.00/mo, $119/pa) has a 7-day trial period and adds several useful but non-essential advanced features.

• • Improved VO2max estimation
  • Ventilatory Threshold VT1/VT2 zones for Zone 2 training
  • Readiness Score – Measures nightly and waking HRV deviations from baseline to determine readiness to train.
  • Oxygen Uptake – gives aerobic performance insights during exercise
  • Sleep Stage Analysis – Consider 4-stage sleep stage tracking. Deep sleep is important for physical recovery, and REM for cognitive recovery.
  • Metabolic Profile: Analyses energy sources (fat vs. carbs) during exercise.
  • Beat Reliability: Measures ECG signal quality for accurate heart data.

Feature Deep Dive: VO2max, Readiness and Sleep Stages

Since this review was initially published, additional features have been added for premium subscribers, covering the key metric that defines your healthspan: VO2max.

Also added is a detailed look at nightly sleep stages, emphasising optimising DEEP SLEEP for muscular recovery and REM sleep for cognitive recovery.

Readiness is a third new metric that assesses your daily HRV deviation from your baseline; a too large negative deviation usually indicates that you are not coping with life’s stresses and encourages you to slow down to adapt and recover.

Have a deep dive read at the following link.

Fourth Frontier X2 – Leading ECG Strap gets VO2max Feature Boost – Deep Dive

Body Shock

Fourth Frontier X2 beat Garmin to add this new metric to their chest strap. While Garmin examines the impact forces, Fourth Frontier considers how your body responds to the forces, including changes to your ECG.

Have a deep dive read at the following link.

BODY SHOCK – an innovative running metric from fourth frontier x2

 

Who’s It For?

Not just a sports strap—it’s a health guardian. Get it if you’re:

• Over 40 & Active: Extra Monitoring for Peace of Mind.
• Biohacker: Craving ECG-level workout data.
• Carer: Watching a loved one’s heart (Paula Radcliffe uses it for her mom!).
• Heart-Conscious: Managing a condition or gathering evidence for your doc.

Accuracy: Does It Deliver?

Here are my Sports HR test results for a variety of sports, directly compared to other good, known sources of HR. The result? Fourth Frontier X2 matches Polar H10 and Garmin HRM-PRO Plus, with rare glitches (likely from physical movements due to wearing two straps). It’s as reliable as any top chest strap.

HRV: Mixed results vs. Polar H10 on HRV4Training—low correlation, possibly due to timing or setup quirks. Still, it’s functional for trends.

ECG: I can’t verify arrhythmia detection without having an arrhythmia to test! However, the data appears to be legit and doctor-friendly.

April 2025: The FDA approves Frontier X Plus as a prescription device, allowing medical professionals to prescribe it to their clients. I.e. it’s accurate.

Why Buy Fourth Frontier X2?

Heart disease hits 48% of us—silent issues can strike anyone, even athletes. The X2 catches 85% of hidden arrhythmias, offering prevention over regret. It’s the only strap blending sports precision with continuous ECG—Garmin and Apple can’t touch that. At £321.75 / $428 (30% off with THE5KRUNNER), it’s an investment in safety and performance.

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Discount & Final Thoughts

The X2’s retro vibe hides a powerhouse: accurate heart rate (HR), unique ECG insights, and life-saving alerts. It’s not cheap but is unmatched for heart health and sports data. I’ll keep using it for tougher sessions—why risk it?

Sources and Resources

• Fourth Frontier Manual: here
• Fourth Frontier X2 Product Page
• buy Fourth Frontier X2