The Forerunner 265 Gets HRV Wrong and Heart Rate Right

A new study has tested Garmin's two-minute HRV Health Snapshot on the Forerunner 265 and concluded that its heart rate variability data is "unsuitable for research purposes and patient or athlete monitoring." Resting heart rate, by contrast, was accurate.

The work was conducted by Kayla Porter under the supervision of Andrew Flatt, Associate Professor at Georgia Southern University. It was published as an Honors College thesis in April 2026. Marco Altini of HRV4Training also referred to the result in his weekly newsletter as a recurrence of a familiar pattern of findings.

Resting Heart Rate Is Fine. HRV Is Not.

Thirty participants completed simultaneous two-minute recordings using a Forerunner 265 on the wrist and a gold standard Polar H10 connected to an ECG. The protocol covered three postures: supine, seated, and standing. Resting heart rate showed excellent agreement across all three. RMSSD and SDNN, the two HRV metrics, didn't.

Flatt's own framing on LinkedIn captures the shape of the error: "Garmin overestimates lower HRV & increasingly underestimates higher HRV." The bias varies with the value being measured. It just pulls the numbers toward the middle, compressing the signal HRV is supposed to reveal. Disagreement was greatest in the standing position.

The study is described by Flatt as an early look at ongoing work. It has not yet been peer-reviewed.

Why HRV Is the Harder Number

Heart rate is straightforward to extract from an optical wrist signal. HRV depends on the precise time interval between successive beats (RR value). Errors of a few milliseconds in beat timing produce visible errors in the HRV calculation, usually RMSSD. Wrist motion, sensor pressure, blood flow (perfusion), and skin tone all distort the optical signal in ways that affect timing more than overall rate.

A chest strap measures the heart's electrical signal directly. The two technologies sit in different accuracy classes for HRV, and software cannot fully bridge the gap. At least not yet.

A Familiar Pattern

This is the latest in a sequence of independent results pointing in the same direction. A clinical ECG comparison earlier this year, conducted by Amin Sinichi, ran 62 subjects through a full day of Garmin recording against a hospital-grade reference and reported errors of more than 100 milliseconds in some participants. A peer-reviewed comparison published in The Physiological Society's journal placed the Garmin Fenix 6 behind the Oura Ring and Whoop on overnight HRV. Independent testing on this site of Garmin's flagship chest strap, the HRM 600, produced its own RMSSD underestimation against the Polar H10 of around 6.6 ms despite using electrical rather than optical measurement.

What This Means for Past Research

The Forerunner 265's Elevate Gen 4 sensor is shared with the FR255, FR955, FR965, Fenix 7, Epix Gen 2, and Venu 2. A substantial body of research over recent years has used Garmin HRV from these devices as either a primary or secondary outcome. Porter and Flatt's findings, taken alongside the prior literature, raise legitimate questions about how much of that work can be relied upon.

The deeper point is the sensor type. Optical PPG at the wrist is an obvious constraint, and that constraint applies to current and future Garmin watches as well as older ones.

Practical Takeaway

For owners who track resting heart rate, the Forerunner 265 is fine. The data is solid.

For HRV-based training decisions, the recommendation on this site remains unchanged. Take the measurement at rest, ideally on waking, with a chest strap. The Polar H10 remains the standard against which other devices are compared.

Test Your Own Watch Against a Chest Strap

The Porter and Flatt protocol uses an academic ECG and a controlled posture sequence. Most watch owners will not want to do that. The useful question for a reader to answer is simply: does my watch, on my wrist, in my conditions, produce HRV numbers I can act on?

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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