Garmin: What the Research Says About Each Metric
Garmin keeps the scientific term for VO2 Max, splits HRV across two differently-scoped features, and has stayed out of the ECG feature race entirely.
Garmin's approach leans toward the athlete, not the general wellness reader. It's the only major brand here that keeps "VO2 Max" as the on-device label rather than renaming it, and it layers training-specific composites like Training Readiness and Training Load on top of the same core metrics every other brand tracks. Below is what independent research and Garmin's own technology documentation say about each metric it tracks, and one feature category it has stayed out of.
How Does Garmin Measure HRV?
Garmin splits HRV across two separate, RMSSD-derived features that measure different timescales: Body Battery reflects same-day energy reserve, while HRV Status tracks a multi-week trend. Both use the same underlying variability data, which creates a real risk of conflating a same-day dip with a longer-term trend shift.
What Garmin calls it
"Body Battery" and "HRV Status," presented as two separate features in Garmin Connect rather than one unified HRV number the way a raw RMSSD reading on Apple Watch would appear.
How Does Garmin Measure Resting Heart Rate?
Garmin reports the lowest 30-minute average heart rate recorded in the last 24 hours, a different window from Apple's daytime-stillness sampling or Oura's overnight-only low.
What Garmin calls it
"Resting Heart Rate," the same label every brand uses, but the 30-minute rolling-window method is specific to Garmin. That measurement-window difference is why the same person can see genuinely different RHR numbers on a Garmin watch versus an Oura ring worn the same night.
How Does Garmin Calculate Its Sleep Score?
Garmin's Sleep Score incorporates a separate HRV-derived stress metric alongside standard sleep-stage data, a real methodological difference from Oura's stage-architecture-heavy formula and Whoop's performance-and-consistency framing.
What Garmin calls it
"Sleep Score." All three major sleep-scoring brands use nearly the same name, but the underlying formulas weigh meaningfully different inputs, so a given Sleep Score value isn't measuring the same combination of things across devices.
How Accurate Is Garmin's VO2 Max Estimate?
Garmin is the only major brand here that keeps the scientific term "VO2 Max" rather than renaming it, and estimates it using the Firstbeat Analytics algorithm, which analyzes the heart-rate-to-pace relationship during outdoor runs and walks.
What Garmin calls it
"VO2 Max," direct and unrenamed. Apple's equivalent estimate is labeled "Cardio Fitness" instead, which means the two brands are estimating the same underlying concept under different names, a real source of confusion for anyone comparing devices side by side.
Worth knowing
Garmin's own technology page documents the Firstbeat methodology directly, a genuine primary source, unlike several other Garmin metrics that rely on secondary comparison sources.
How Accurate Is Garmin's Race Predictor?
Race Predictor projects finish times from 5K to marathon using VO2 Max together with recent training load and performance history, recalculating automatically as those inputs change. Because it's a downstream estimate, its accuracy is bounded by how accurate the underlying VO2 Max reading is: roughly a 5 percent margin of error against lab spirometry under Garmin's own controlled validation, with larger errors reported in independent testing that relies on wrist-based optical heart rate instead of a chest strap.
What Garmin calls it
"Race Predictor." No independent peer-reviewed validation of the race-time prediction itself, separate from the underlying VO2 Max estimate it's built on, was found during research for this page.
How Does Garmin's Pulse Ox Work?
Garmin's Pulse Ox can run either as an on-demand spot check or as continuous all-day and overnight tracking, depending on device settings, a more flexible approach than Apple's on-demand-only model or Oura's overnight-only design.
What Garmin calls it
"Pulse Ox." The ability to toggle between spot-check and continuous modes is specific to Garmin among the brands compared here.
What Does Garmin's Body Battery Actually Measure?
Body Battery combines RMSSD-derived HRV, daytime stress patterns, and sleep quality and duration into a single energy-reserve score. Garmin also layers a separate six-factor Training Readiness score on top, combining sleep score, HRV status, remaining recovery time, acute training load, stress history, and Body Battery itself into one morning number aimed specifically at athletes.
Worth knowing
A real peer-reviewed evaluation found that composite recovery scores like Body Battery, Whoop's Recovery Score, and Oura's Readiness Score are not methodologically equivalent to each other, despite similar-sounding names and overlapping inputs. Comparing Body Battery against a Whoop Recovery Score on the same day isn't scientifically sound, even though both claim to answer a similar question.
How Does Garmin's All-Day Stress Score Work?
Stress Score runs continuously through the day, reading heart rate and HRV through the Firstbeat Analytics algorithm and updating every few minutes on a 0-100 scale: 0-25 rest, 26-50 low, 51-75 medium, 76-100 high. Garmin cross-references motion data to exclude intense activity from the calculation, an attempt to separate physical exertion from psychological stress.
What Garmin calls it
"Stress Score," sometimes labeled "All-Day Stress" in Garmin Connect. It shares the same RMSSD-based HRV foundation as Body Battery and HRV Status but reports on a different scale and a shorter time window than either.
What Garmin Doesn't Do
One real, notable absence.
No widely-offered ECG feature
Unlike Apple's FDA-cleared ECG app or Fitbit's Sense 2 ECG, Garmin has not pursued single-lead ECG certification across its lineup, keeping its heart-rhythm feature set focused on continuous optical monitoring instead.
If you want a device built around training load and endurance performance specifically, with VO2 Max reported directly rather than renamed, Garmin's athlete-first framing is a real fit. If a simpler, single daily wellness score matters more than training-specific detail, Whoop's or Oura's narrower focus may read more clearly.