How smartwatches measure VO2 max — and why your mechanical watch can stay on
Founder & CEO, Smartlet - CentraleSupelec engineer - Concours Lepine 2025, Awarded - CES 2026
Table of contents
Key takeaways
| Topic | What you need to know |
|---|---|
| VO2 max definition | Your maximum oxygen consumption — the highest amount of oxygen your body uses per unit of weight during exercise — the gold standard of aerobic capacity. |
| Smartwatch method | Heart rate versus pace or power. This is not a direct measurement from a gas exchange system as seen in lab conditions — it is an optical PPG sensor combined with GPS. |
| Garmin accuracy | Several studies have found that Garmin accurately reports running VO2 max with an error of only 5–10% when compared with laboratory-based measurement. Consistent enough to track trends over time. |
| Training utility | It is the trend in your VO2 max over weeks or months that is useful for training, not your absolute value. Use it to confirm adaptation, not to benchmark against others. |
| Dual wear | Smartlet lets you keep a mechanical watch on the same wrist during training sessions that don't require high-impact movement. |
But what does my calculated VO2 max mean and how did my Forerunner 965 arrive at this figure? Should I train according to it? First we cover how all this works from a physiological and algorithmic perspective. Then we dive into the peer-reviewed literature validating wearable-based activity classification. But at the end of the article we address what really matters to all endurance enthusiasts and gadget lovers out there: do you really have to remove your mechanical timepiece to use these devices?
"VO2 max is not your fitness score. VO2 max is how efficiently you convert oxygen into movement. Everything else is irrelevant after that."
What VO2 max actually measures
Maximal oxygen uptake (VO2 max) is sometimes referred to as the aerobic ceiling that each individual has when they train at their hardest. It is measured in millilitres of oxygen per kilogram of body weight per minute (mL/kg/min). To measure your individual ceiling, you train at your hardest level and determine how much your body can use to fuel itself using the oxygen you breathe while your heart, lungs, and muscles are working at their limit.
VO2 max can be measured by taking laboratory readings. Most people perform running or cycling on a treadmill or stationary bicycle at increasing intensity while wearing a breathing mask connected to a metabolic cart. The cart analyses the oxygen and carbon dioxide concentration of your exhaled breath. When oxygen consumption stops rising despite increasing effort, you have reached VO2 max. The test typically lasts eight to twelve minutes from warm-up to exhaustion.
Lab-measured VO2 max values for sedentary adults typically land between 30 and 40 mL/kg/min. Recreational endurance athletes cluster between 45 and 60. Elite marathon runners and cyclists regularly exceed 70. Tour de France contenders have measured above 85. Oskar Svensson, a professional cyclist, published a value of 96.7 mL/kg/min in 2020 — the highest recorded in a peer-reviewed study.
The number correlates strongly with endurance performance, cardiovascular health, and all-cause mortality. A 2018 study in JAMA Network Open that followed over 122,000 patients found that low cardiorespiratory fitness was a stronger predictor of mortality than smoking, hypertension, or diabetes. VO2 max is the single best objective indicator of cardiorespiratory fitness.
How smartwatches estimate VO2 max without a lab
The Forerunner 965 does not come with a built-in metabolic cart to measure oxygen consumption directly. What it can measure is heart rate — via the optical photoplethysmography (PPG) sensor on its underside — and pace, via GPS and accelerometer. From those two data streams, it calculates an estimate.
The underlying model was first developed by Andrew Rowe and his team at Finland-based sports science company Firstbeat Technologies, which Garmin acquired in 2020. The core idea is that the relationship between your running speed and heart rate at a given effort level predicts aerobic capacity. A runner with a high VO2 max of 65 might be able to run faster at the same heart rate than a runner with a lower VO2 max of 45 — that body is more efficient, able to do more aerobic work per heartbeat.
While not an exact measurement, the Firstbeat algorithm incorporates additional variables including terrain via GPS elevation data, temperature stress, current level of fatigue, and running economy estimated from cadence and stride length. The result is not a single-session calculation. The watch accumulates data across multiple runs, refining its estimate each time you provide a clean submaximal or maximal effort.
Garmin calls its output "VO2 max estimate" rather than VO2 max, which is honest. The Forerunner 965 also provides a "Fitness Age" metric that contextualises the number against population norms by age and sex. The watch gives higher confidence ratings to estimates generated during outdoor runs with stable GPS signal and consistent effort, and lower confidence to estimates from treadmill runs or sessions with heavy heart rate variability from elevation change.
The Forerunner 965 generates more reliable VO2 max estimates after five or more outdoor runs with stable GPS and consistent pacing. The watch explicitly flags low-confidence readings. If your number shifted dramatically after one single session, wait for the next two or three runs before adjusting your training zones.
Garmin Forerunner 965: how accurate is the number?
The accuracy question has been studied repeatedly. The most relevant research for Garmin's Firstbeat-based algorithm comes from several independent groups of researchers around the world.
A 2021 study in the International Journal of Sports Physiology and Performance tested Garmin devices against laboratory spirometry in 41 recreational runners. Mean absolute percentage error was 4.9% for outdoor running sessions. Based on a mean VO2 max value of 50 mL/kg/min, individual estimates from Garmin would be expected to cluster around the true value with most falling between approximately 47.5 and 52.5 mL/kg/min.
A 2019 paper in PLOS ONE evaluated Firstbeat's algorithm across multiple devices and found that it performed well for tracking within-individual changes over time — the trend was more reliable than the absolute value. This is the finding that most matters for training: if your Garmin VO2 max moves from 51 to 55 over twelve weeks of structured training, that improvement is real and proportional, even if the absolute values don't perfectly match a lab test.
Accuracy degrades in specific conditions. Altitude training introduces error because the algorithm doesn't account for reduced oxygen partial pressure. Significant weight changes alter the mL/kg/min calculation in ways the watch doesn't always track quickly. And optical heart rate measurement during high-intensity intervals with heavy wrist movement introduces noise that propagates into the estimate.
For most endurance athletes — runners, cyclists, triathletes — the practical conclusion is this: the Forerunner 965 gives you a number accurate enough to set training zones, track adaptation, and compare yourself over time. It is not accurate enough to use as a medical diagnostic or to compare yourself to athletes tested in different lab conditions.
"The trend is truth. A single reading of VO2 max is a photograph. Twelve weeks of VO2 max data is a film."
Using VO2 max to set training zones
VO2 max is the ceiling. Training zones are the floors below it. Once the Forerunner 965 has a stable VO2 max estimate, it can calculate lactate threshold pace, aerobic threshold, and the five-zone model that most endurance coaches use.
Zone 1 — recovery — sits below 55% of VO2 max. Zone 2 — aerobic base — runs from roughly 55% to 75%. Zone 3 — tempo — covers 75% to 85%. Zone 4 — threshold — from 85% to 95%. Zone 5 — VO2 max intervals — at or above 95%. The Forerunner 965 translates these percentages into heart rate ranges and, for running, pace ranges personalised to your weight, age, and recent training history.
The practical training implication: the majority of effective endurance training — 80% by volume in the polarised model used by many elite coaches — sits in Zone 1 and Zone 2. These are the intensities where your aerobic system adapts without excessive cortisol or recovery debt. VO2 max improves by making Zone 2 feel easier, and by periodically targeting Zone 4 and Zone 5 to stress the ceiling itself.
If your Garmin VO2 max is rising, it means Zone 2 pace at the same heart rate is increasing — you are running faster for the same oxygen cost. That is the adaptation you are after.
| Zone | % VO2 max | Training purpose | Perceived effort |
|---|---|---|---|
| Zone 1 — Recovery | <55% | Fat oxidation, active recovery | Conversational, cadence may slow |
| Zone 2 — Aerobic base | 55–75% | Mitochondrial density, aerobic efficiency | Comfortable, can hold sentences |
| Zone 3 — Tempo | 75–85% | Lactate clearance, comfortably hard | Two or three words at a time |
| Zone 4 — Threshold | 85–95% | Lactate threshold, race pace | Hard, single words only |
| Zone 5 — VO2 max | >95% | Cardiac output ceiling, top-end power | Maximal, unsustainable beyond 5–8 min |
What distorts the reading
Several factors introduce systematic error into the Garmin VO2 max estimate. Understanding them lets you correct for them rather than chase noise.
Cadence-induced heart rate artifacts. At high cadences — above 170 steps per minute — wrist movement creates optical noise that can spike or suppress heart rate readings. The Forerunner 965 uses a green LED array with an accelerometer for motion compensation, which reduces but does not eliminate this effect. For VO2 max interval sessions at high intensity, a chest strap paired via ANT+ eliminates this variable entirely.
Heat stress. Running in ambient temperatures above 25°C elevates heart rate by five to ten beats per minute independent of oxygen demand, a phenomenon called cardiovascular drift. The Forerunner 965 has limited ability to distinguish thermally elevated heart rate from effort-driven elevation. VO2 max estimates taken in hot conditions tend to read low. Garmin's Body Battery and heat acclimation features partially address this, but the estimate will stabilise once you return to cooler conditions.
Altitude. When training above 1,500 metres, reduced oxygen partial pressure means you work harder for the same pace. The algorithm may interpret this as lower fitness. If you train regularly at altitude, use Garmin's altitude acclimation feature and expect a two-to-three-week recalibration period after returning to sea level.
Watch fit and sensor contact. The optical sensor requires consistent contact with the wrist. A loose band, significant wrist hair, or dark skin tones can all reduce signal quality. Wearing the watch one to two finger-widths above the wrist bone, snug but not constricting, produces the cleanest readings.
Caffeine and stimulants. Caffeine increases resting and submaximal heart rate by about five beats per minute in non-habituated individuals. If you train with a strong pre-workout coffee, heart rate at a given pace will be elevated, and the algorithm may underestimate VO2 max. This normalises with consistent caffeine habits.
Why your mechanical watch can stay on
For those who own both a Garmin and a meaningful mechanical watch, the question becomes: does the smartwatch have to be the only watch on the wrist? The short answer is no, in most training contexts.
The Garmin Forerunner 965 monitors heart rate using an optical sensor positioned on the underside of the watch. It does not require sole occupancy of the wrist. With a Smartlet strap, you place the mechanical watch on top of the wrist and have the Forerunner 965 face the underside of the wrist and skin. The strap is shared — the Smartlet system means you don't need to choose which watch to wear.
There is one context where this changes: Zone 4 and Zone 5 interval sessions involving explosive movements, heavy arm swing, or contact sport risk. In those sessions, the mass of a second watch creates non-trivial torque at the wrist and increases impact energy if the arm strikes a surface. For high-impact activity, keep your Garmin on its standard strap for that session. On your long Zone 2 runs, tempo work, and threshold intervals at controlled intensity, a properly fitted Smartlet setup keeps both watches on wrist without compromise.
The Smartlet Classic adapter — brushed SS316L, compatible with 18–24mm lug widths — threads one strap through both a mechanical watch and a smartwatch. No tools required.
The dual-wear setup for endurance athletes
The practical setup for an endurance athlete who trains with a Garmin Forerunner 965 and wears a mechanical watch:
The Smartlet adapter threads a single strap through the spring bars of your mechanical watch, then the Forerunner 965 mounts to the outer carrier. The mechanical watch sits on top of the wrist — visible, readable, completely independent. The Garmin's optical sensor presses against the skin on the underside, reading heart rate exactly as it would on its own strap.
The system is compatible with any mechanical watch with a lug width between 18mm and 24mm, covering the full range of Seiko, Hamilton, Longines, IWC, and most sport watches. It uses a standard spring bar, so installation takes under two minutes with a standard spring bar tool.
For data quality: position the Garmin sensor snugly against the inner wrist, one to two finger-widths above the wrist bone. The mechanical watch rests on the outer side. The combined wrist mass is slightly higher, so expect minor cadence reads at the very beginning of a session as the Garmin learns your movement signature — this stabilises within the first kilometre.
On your first run with the Smartlet setup, compare heart rate readings from the Garmin optical sensor against a chest strap for the first 15 minutes. If they track within three to four beats per minute, your fit is clean and VO2 max estimates will be unaffected. If there is systematic drift, adjust the strap tension and retry.
Smartlet Titanium — Grade 2 titanium, satin finish — for athletes who train hard and want to carry as little weight as possible on the wrist.
The Smartlet system makes dual wear possible without asking you to leave either watch behind — the Garmin reads your training data, the mechanical watch reads the time.
FAQ
Is the VO2 max number on my Garmin Forerunner 965 accurate enough to trust?
Independent studies place Garmin's Firstbeat-based algorithm within approximately 5% of laboratory spirometry for most recreational runners. That level of accuracy is sufficient for setting training zones and tracking adaptation over time. It is not precise enough to use as a medical metric or to benchmark against athletes tested under different laboratory protocols. The trend over weeks is more reliable than any single session estimate.
Does VO2 max change with training, and how quickly?
Yes. VO2 max is trainable, particularly in the first few years of structured endurance training. Untrained individuals can improve VO2 max by 15–20% in the first twelve weeks of consistent aerobic work. Already-trained athletes improve more slowly — a 3–5% gain over a training block is meaningful. The primary drivers are Zone 2 volume (which builds mitochondrial density) and periodic Zone 4–5 intervals (which stress cardiac output). Your Garmin will reflect genuine improvements within three to four weeks of consistent training, assuming stable heart rate sensor conditions.
What VO2 max do I need to run a sub-3-hour marathon?
A commonly cited threshold is approximately 55–60 mL/kg/min for male runners, slightly lower for female runners due to differences in running economy. However, VO2 max alone does not determine marathon performance. Lactate threshold as a percentage of VO2 max, running economy, and fatigue resistance all contribute significantly. A runner with a VO2 max of 55 who can sustain 90% of that capacity for three hours will outperform a runner with a VO2 max of 65 who falls apart at 75%.
Can I wear a mechanical watch and a Garmin on the same wrist without affecting heart rate accuracy?
Yes, with a properly fitted Smartlet adapter. The Garmin's optical sensor faces inward against the skin. The mechanical watch sits on top of the wrist, separated by the strap. Provided the Garmin is snug and positioned correctly — one to two finger-widths above the wrist bone — heart rate accuracy is unaffected during steady-state and tempo efforts. For VO2 max intervals at maximum intensity with explosive arm movement, use a chest strap paired via ANT+ for the cleanest heart rate data.
What is a good VO2 max for an amateur endurance athlete in their 40s?
Garmin's population data places a 40-year-old male in the "excellent" category at 52+ mL/kg/min and a 40-year-old female at 45+ mL/kg/min. "Superior" ratings begin around 57 for males and 50 for females. These benchmarks come from the American College of Sports Medicine norms. More practically: a VO2 max that is rising year-on-year, at any absolute level, indicates that your training is producing adaptation — which is the outcome that matters most for long-term health and performance.
Does the Garmin Forerunner 965 VO2 max work for cycling as well as running?
Yes, but the estimate is generated separately for cycling and running, and the values will differ. Cycling VO2 max estimates require a power meter for maximum accuracy — without power data, the algorithm uses heart rate and GPS speed, which produces less reliable results on variable terrain. The Forerunner 965 supports power meter pairing via ANT+ and Bluetooth, which substantially improves cycling VO2 max estimate quality.
