The Basis Peak was one of the first consumer wearables to make continuous wrist heart rate a normal feature. Twenty-four hours a day, every minute, the optical sensor on its back ran a measurement and pushed it to the watch and the cloud. In late 2014 that was new. The Fitbit Charge HR launched a few weeks after it. The original Apple Watch did not have continuous HR at all when it shipped six months later.
That made the Peak an early reference point for how good, or how bad, wrist photoplethysmography (PPG) actually was. Eight contemporary reviewers ran it against chest straps. One of them, DC Rainmaker, did the work properly. The picture that came out of those tests still describes most of the failure modes that consumer wrist-HR devices have today, which is part of why we keep coming back to the Peak as a case study.
What the Peak’s sensor actually was
The optical heart-rate module on the back of the Peak was a single-channel green-LED photoplethysmograph paired with a photodiode. Green light around 525 nm penetrates a few millimeters into the skin, scatters off red blood cells, and returns to the photodiode with a small periodic modulation that tracks the pulse wave. This is the same physical principle every wrist tracker uses. The differences across devices are in the LED count, the wavelengths, the sampling rate, the photodiode sensitivity, and the algorithms that turn a noisy optical trace into a heart-rate number.
The Peak’s implementation was modest by current standards. One LED, one wavelength, a sampling rate Intel never published in detail but which appears from the firmware behavior to have been in the low tens of hertz when the device was in continuous-monitoring mode, and bursts at higher rates during active workouts. Modern sensors (the Apple Watch’s optical stack, Garmin’s Elevate v4 and v5, Fitbit’s PurePulse generations, the Polar OH1) run multiple LEDs across green, red, and infrared, sample at 100 Hz or more, and combine the channels with motion data from the accelerometer to reject artifact.
The Peak also included two sensors most of its contemporaries did not have: a galvanic skin response (GSR) electrode pair and a skin-temperature sensor. Those did not directly improve HR accuracy, but they were part of why we think of the Peak as the ancestor of modern multi-signal sleep and recovery devices. The richer physiological context is the half of the Peak story that the contemporary HR-focused reviews mostly skipped, and we cover it in the Basis Peak review.
What it measured well
For resting heart rate and steady-state cardio the Peak was, by 2014 standards, good.
Lauren Goode, then at Re/code, ran the Peak against a chest strap on steady-state runs and reported readings within a few BPM. Ars Technica’s Casey Johnston had a similar experience for daytime wear and seated readings. The single most rigorous comparison work came from Ray Maker, who writes as DC Rainmaker, and who in 2014 and 2015 ran the Peak head-to-head against a Mio Alpha (then the gold standard for wrist optical), a Polar H7 chest strap, and a Garmin HRM-Run, both on the bike and on the run. His finding for steady-state cycling and running was clear: the Peak tracked the chest strap within about 2-3 BPM at constant effort, sometimes lagging the strap by a few seconds at transition points but recovering quickly.
This is the regime where wrist PPG genuinely works. Blood flow through the wrist is stable. The arm is not swinging hard. Skin contact is consistent. The optical signal is clean and the algorithm has time to lock on. For people whose use of a Peak was “wear it all day, glance at resting HR, see what it does overnight,” the accuracy story was fine. That use case maps onto what we would now call resting heart rate tracking, and the Peak did that part of the job well enough that the readings are still defensible against a 2026 device for the same use case.
Where it broke
Two regimes broke the Peak’s optical sensor reliably: high-intensity interval work and any sustained activity in cold weather.
On the HIIT side, DC Rainmaker’s reports were specific. During cycling intervals with rapid sprints, the Peak frequently dropped out (meaning the watch displayed a dash where the BPM number normally was) for stretches of fifteen to forty seconds. When it did report a number under those conditions, it lagged the chest strap by ten to fifteen seconds on rapid HR changes. During a sprint that took heart rate from 130 to 175 in ten seconds, the Peak would still be reading 140 when the strap had caught up. On the recovery side it was worse: the strap would show heart rate falling, the Peak would stay locked on a stale value for fifteen to twenty seconds before catching down.
This is not a Peak-specific failure. It is the canonical wrist-PPG failure mode. The optical signal is small (a percent or two of total reflected light) and depends on a clean, motion-free path from LED to photodiode. Three things break that path. Motion artifact is the first: arm swing, wrist flexion, and grip changes (think kettlebell handles, dumbbells, or a bike’s hood position) all introduce signal at frequencies near the heart-rate band, and the simpler the rejection algorithm, the more likely it is to lock onto motion as if it were a pulse. Blood-flow change is the second: during high-intensity exercise, peripheral vasoconstriction and vasodilation are not steady, and the pulse waveform itself changes shape. The third is contact: the watch sliding fractions of a millimeter under sweat changes the LED-to-skin geometry.
Cold weather adds a fourth. Vasoconstriction in the extremities reduces the volume of blood the LED can see. The Peak’s single green-LED stack, brighter than nothing but dimmer than current multi-LED arrays, did not have enough signal margin to keep working when the wrist was cold. Wearing one in Boston in January 2015, the watch would routinely fail to find a pulse for the first ten minutes of an outdoor run. The HR display sat empty until the wrist warmed up.
A fifth confounder is worth mentioning because it shows up in every modern wrist sensor’s documentation: tattoos. Green light is absorbed strongly by dark inks. A wrist tattoo under the sensor will either depress the signal to noise levels or produce confidently wrong readings. The Peak was not labeled for this, and almost no 2014-era device was, but the physics applied to it too.
What the November 2015 firmware update changed
The November 2015 over-the-air firmware update was Intel’s attempt to fix two of the Peak’s most-complained-about issues: cold-weather touchscreen lag, and exactly the HR dropouts described above. Intel did not publish the algorithmic changes in any detail. What can be inferred from the device’s behavior after the update, and from owner reports on the Basis support forum at the time, is consistent across sources.
The update appears to have done three things to the optical sensor. It raised the duty cycle of the LED, so the sensor was on for longer windows during active workouts. It increased the LED drive current, making the green light brighter and pushing more photons into tissue. And it pushed the sampling rate up, sometimes substantially, in the modes triggered when the accelerometer reported motion. The goal was straightforward: more signal, more samples, better tracking of rapid HR changes. Contemporary owners and a handful of reviewers reported modest improvement during running, with the dropouts becoming shorter though not disappearing.
The same set of changes is the most plausible technical explanation for what came next. A brighter LED running at higher duty cycle dissipates more power on the back of the watch. A higher sampling rate keeps the analog front end and the microcontroller working harder. In a sealed stainless-steel case with a fixed lithium-polymer cell and a thermal-management envelope designed around the original sensor duty cycle, the new firmware almost certainly pushed the device closer to its thermal limits. The March 2016 firmware update tried to claw some of that back. It did not work, and in June 2016 Intel started the refund program that became the formal recall.
We are careful with the phrasing here. Intel never publicly identified the November 2015 firmware update as the proximate cause of the battery-overheating failures. CPSC notice 16-235 names the hazard (“the battery in the watch can overheat, posing burn and blister hazards”) but not the firmware mechanism. Owner accounts and the temporal correlation are strong enough to assert that the update changed thermal behavior; they are not strong enough to assert it caused the burns, and we are not going to claim that.
What this tells us about wrist optical HR in 2026
The Peak is interesting now because the failure modes it exhibited are the same ones that still show up in current wearable accuracy studies. The sensors are better. The algorithms are dramatically better. The physics has not changed.
The most useful piece of peer-reviewed work on this is Bent et al., “Investigating sources of inaccuracy in wearable optical heart rate sensors,” published in npj Digital Medicine in 2020. The Duke group tested six consumer devices, including Apple, Fitbit, Garmin, and Xiaomi models, against an ECG reference, across a range of activities and across participants with a spread of skin tones. The headline finding was that mean absolute percentage error (MAPE) exceeded 5% during exercise for every device tested, with errors substantially worse at higher intensities. Skin tone was associated with accuracy in some device-and-condition combinations, though the effect was not uniform across devices and was often smaller than the effects of motion type and exercise intensity. Activity type, particularly arm-loaded movements like cycling on the hoods, resistance training, and rowing, produced the largest errors across the board.
That paper is the canonical citation we use when somebody asks whether a wrist tracker can replace a chest strap for training. The short answer remains no. The longer treatment is in our broader wearable sensor accuracy piece, and for the consumer-facing version of the same question, fitness tracker accuracy compiles the test data we trust.
There is a separate question, distinct from raw beat-to-beat accuracy, of whether wrist optical sensors can capture useful derived metrics. Heart-rate variability is the obvious one. The Peak did not expose HRV at all; the field had not yet converged on consumer HRV as a feature. Modern devices that report HRV (Whoop, Oura, Garmin, Apple Watch) generally compute it overnight, when the wrist is still and the signal is clean. That works around exactly the failure modes the Peak made obvious. The framework for what HRV actually measures and what it does not is in what is HRV.
What we would say if a 2026 reader asked us about the Peak’s HR data
Owners who pulled CSV exports from the Basis cloud before the December 31, 2016 shutdown sometimes ask us how to think about the data. Resting heart rate trends are usable. Five-minute and daily averages during normal wear are usable. Workout sessions tagged as anything more strenuous than steady-state cardio should be treated with skepticism, particularly any session before the November 2015 firmware update. Cold-weather sessions of any kind are unreliable. We have looked at a handful of those exports and the pattern is consistent: smooth, defensible trends at rest, ragged and obviously gappy traces during interval workouts.
The Peak’s contribution to the wrist-HR story was not that it solved any of these problems. It did not. The contribution was that it forced a generation of reviewers and engineers to confront how hard the problem actually is. Every measurable improvement in wrist optical HR between 2014 and 2026 has been a response to the failure modes the Peak made visible. The watch is unusable now, but the lessons it taught the category are still load-bearing.