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How Zolt estimates VO₂ max from a bike ride

Apple Watch skips rides for cardio fitness. Here is how Zolt turns power, heart rate and your weight trend into a VO₂ max estimate, and where it can be wrong.

Zolt can estimate VO₂ max from a bike ride, as long as the ride has watts. A power meter, smart trainer or smart bike tells us how much work you did, the number a run gets for free from pace. Add your heart rate and your body weight, and two published methods turn that into VO₂ max. This post covers both, the evidence behind them, and where they fall short.

Apple Watch does not estimate VO₂ max from rides. Apple estimates cardio fitness during outdoor walks, runs and hikes, and says indoor workouts and gym equipment do not count (Apple Support). If most of your cardio happens on a bike, your VO₂ max history can go months without a new reading.

What a ride needs

DataWhere it comes fromWhy it matters
Power, in wattsPower meter, smart trainer or smart bike, saved to Apple Health or Health ConnectThe work you did. Without it we skip the ride.
Heart rateWatch or chest strapShows whether an effort was near your max, and drives the steady-ride method
Body weightYour Zolt weight trend on the day of the rideVO₂ max is per kilogram, so weight is in every result
Max heart rateYour hardest recent workouts, or 208 minus 0.7 times your age when those look too lowOnly used by the steady-ride method

We use your smoothed weight trend instead of a single weigh-in. On a 75 kg rider, a 1.5 kg water swing moves a VO₂ max estimate by about 2 percent, and the trend removes most of that noise. If you have not weighed in during the week before a ride, that ride gets no estimate.

Method 1: your best 5 minutes

If a ride has a hard 5-minute effort, we take the highest 5-minute average power, divide it by your weight, and use the equation from Sitko and colleagues:

VO₂ max = 16.6 + 8.87 × (5-minute watts per kilogram)

They built it from lab tests of 46 road cyclists who also rode a maximal 5-minute effort. The 5-minute power explained roughly 60 to 77 percent of the variation in measured VO₂ max (Sitko et al., 2022).

Take a 74.6 kg rider who holds 288 W for 5 minutes. That is 3.86 W/kg, and the equation gives 50.8 ml/kg/min.

Two things limit this number.

The effort has to be close to all-out. Five easy minutes predict a low VO₂ max, because the equation assumes you were at your limit. We only use a 5-minute block when your heart rate in its last minute reached at least 90 percent of your max. Otherwise the ride falls back to Method 2. Field data can stand in for a lab test here: in professional riders, the best efforts of 5 minutes or longer from training and racing came within 5 percent of formal time trials (Pallarés et al., 2022).

The equation also reads low. An independent group tested it on 49 male cyclists and found it underestimated measured VO₂ max by 6.6 ml/kg/min on average. For 95 percent of riders, the real value was between 3.8 below and 17.2 above the prediction (Borszcz et al., 2024). So Zolt shows this estimate with a floor instead of a range. The rider above sees "Likely 47 or higher."

Best 5 minutes: a prediction and a floor

VO2 max predicted from 5-minute power, with the floor Zolt showsPredicted VO2 max rises from about 39 at 2.5 watts per kilogram to about 61 at 5. Measured values in an independent study fell between 3.8 below and 17.2 above the prediction, so Zolt reports the prediction minus 3.8 as a floor. A rider at 3.86 watts per kilogram has a prediction of 50.8 and a floor of 47.3040506070802.533.544.555-minute power, W/kgVO₂ max, ml/kg/min50.8 predicted47 floor
Sitko predictionWhere measured values fell (Borszcz)Floor Zolt shows
The solid line is the Sitko equation, 16.6 + 8.87 × watts per kilogram. The shaded band is where Borszcz and colleagues found 95 percent of measured values, from 3.8 below the prediction to 17.2 above it. Zolt shows the dashed floor. The marked rider held 3.86 W/kg for 5 minutes: predicted 50.8, floor 47.

Method 2: heart rate against power

Most rides are steady, not all-out. For those we use the idea behind the Åstrand bike test and Firstbeat's cycling estimate, which Garmin uses: heart rate climbs in a near-straight line with power, so we can follow that line up to your max heart rate.

  1. Find the steady minutes. We need at least 20 minutes where power holds steady and heart rate is above 70 percent of your max. Garmin asks for a heart rate strap, a power meter and several minutes at a moderate intensity before it updates a cycling VO₂ max (Garmin). We ask for more, because a longer stretch is less sensitive to one noisy minute.
  2. Convert each steady minute's power to oxygen cost with the ACSM leg-cycling equation: VO₂ = 10.8 × watts per kilogram + 7 ml/kg/min (Swain, 2000).
  3. Draw a line from rest (your resting heart rate, at a resting oxygen use of 3.5 ml/kg/min) through each steady minute and extend it to your max heart rate. This uses the finding that the share of your heart-rate range you are using matches the share of your oxygen range (Swain and Leutholtz, 1997). The estimate is the middle value across your steady minutes.

In the example ride, 24 steady minutes put the line at 290 W when heart rate reaches 188. That works out to about 49 ml/kg/min.

Heart rate vs power: following the line to max

Steady minutes extended to max heart rateTwenty-four steady minutes sit between about 138 and 157 beats per minute, at oxygen costs of about 32 to 38 ml/kg/min. A line from resting heart rate through those minutes reaches about 49 ml/kg/min at a max heart rate of 188.010203040506080100120140160180Heart rate, bpmOxygen cost, ml/kg/minRestAbout 49 at 188 bpm (290 W)
Steady minuteLine extended to max heart rate
Each dot is one steady minute: its power converted to oxygen cost with the ACSM equation, plotted against heart rate. The line starts at rest (here 55 bpm and 3.5 ml/kg/min) and runs through the steady minutes. Where it meets max heart rate, 188, the power is about 290 W, which works out to about 49 ml/kg/min for a 74.6 kg rider. The resting heart rate and the spread of the dots are for illustration.

The weak link is max heart rate. Firstbeat's own cycling validation, with 29 riders on power meters, reports about a 5 percent average error when max heart rate is known. Setting max heart rate 15 beats too low raised the error to about 9 percent (Firstbeat's VO₂ max white paper, listed on its fitness level page). A Swedish study of the classic Åstrand test found a coefficient of variation of 18 percent, and 9 percent for an improved model (Ekblom-Bak et al., 2014).

So Zolt starts from heart rates you have actually reached. We look at the peak of every workout in the last 6 months and take the second highest, so one glitchy reading cannot set it. If that number is below 95 percent of what your age predicts, your recent workouts probably were not hard enough to show your max, and we use 208 minus 0.7 times your age instead (Tanaka et al., 2001). If a ride takes you higher than either, we use the ride's peak.

The estimate shows as a range of 10 percent either side. When max heart rate comes from your age, the range widens to 15 percent, because an age formula can miss for any one person. The example rider has no hard workouts on record, so their ride reads "Likely between 42 and 56."

What each method gives you

A floor from the best 5 minutes, and a range from steady ridingThe best 5 minutes method estimates 50.8 and reports likely 47 or higher. The heart rate and power method estimates 49 and reports likely between 42 and 56.35404550556065Best 5 minutesLikely 47 or higherHeart rate vs powerLikely between 42 and 56
A hard 5 minutes gives a floor, because the equation tends to read low. Steady riding gives a range of 15 percent either side here, because this rider's max heart rate comes from their age. Both rows are the same example rider.

Runs and walks

Apple Watch already estimates VO₂ max on outdoor runs and walks. Zolt adds its own estimate for each run and walk you open, built the same way as Method 2 with pace in place of power, so you can see where a number came from and how sure it is.

  1. Find the steady minutes. A minute counts when your pace has held within 10 percent for at least 2 minutes, so your heart rate has caught up with it, and your heart rate sits between 70 and 95 percent of your max. We need at least 10 of them.
  2. Convert pace to oxygen cost with the ACSM equations: 0.2 × metres per minute + 3.5 for running, and 0.1 × metres per minute + 3.5 for walking (Swain, 2000). When the route has elevation, running pace is adjusted for hills first.
  3. Extend the heart rate line to your max heart rate, as for rides, and take the middle value.

The range starts at 10 percent either side and widens when an input is weaker: 5 more points when max heart rate comes from your age, 5 for a treadmill, where the watch measures pace from your wrist instead of GPS, and 3 when we have no resting heart rate for you and assume 60.

Intervals, hill repeats and stop-start city runs often have too few steady minutes. Those runs get no estimate, and the sheet says why.

What we chose not to do

Plugging maximal power into the ACSM equation

The ACSM equation describes steady, submaximal riding. In 580 competitive cyclists, using it at maximal power underestimated VO₂ max by 7.3 ml/kg/min in men and 8.2 in women (Jurov et al., 2023). We use it only for the steady minutes in Method 2.

Using average ride power

Average power tells us the oxygen cost of the ride you did, not the most oxygen you can use. An easy hour and a hard hour would give very different VO₂ max values for the same rider.

Using equations built on other groups

Storer's cycle equations are accurate for sedentary adults on a lab ramp that adds 15 W every minute (Storer et al., 1990). A ride outside is not that protocol.

Where the estimate can be wrong

  • Power accuracy varies. A crank or pedal meter or a direct-drive trainer is usually close. Bikes that calculate power from resistance and cadence can drift, and we found no peer-reviewed validation of them. Read estimates from those bikes with extra caution.
  • Hot rooms, dehydration, caffeine and fatigue push heart rate up at the same power or pace. That makes the heart-rate methods read low.
  • Cycling and running estimates can disagree for the same person. Ride estimates sit in their own list on your VO₂ max history page, apart from your watch's readings and its chart, and we never write them to Apple Health. To hide them, turn off Show ride estimates on that page.

Treat any single ride as a rough reading. The trend across several rides on the same bike tells you more.

Where to find it

Open a ride, run or walk and tap VO₂ max under More detail. The sheet shows which method we used, the 5 minutes or steady minutes it came from, and the weight and max heart rate that went in. If a workout has no estimate, the sheet says what was missing.

Ride estimates also appear in your VO₂ max history. While that list is showing, Zolt works through your recent rides with power a few at a time, so rides you never opened fill in too. Zolt's Coach can read these estimates, and it calls them estimates, with their floor or range.

References

  1. Apple. Track your cardio fitness levels. Apple Support. support.apple.com/en-us/108790
  2. Borszcz FK, Ferreira Tramontin A, de Lucas RD, et al. Is the 5-minute time-trial cycling test a valid predictor of maximal oxygen uptake? An external cross-validation study. International Journal of Sports Physiology and Performance. 2024;19(6):565-575. doi:10.1123/ijspp.2023-0330
  3. Ekblom-Bak E, Björkman F, Hellenius M, et al. A new submaximal cycle ergometer test for prediction of VO₂max. Scandinavian Journal of Medicine & Science in Sports. 2014;24(2):319-326. doi:10.1111/sms.12014
  4. Firstbeat. Fitness level. firstbeat.com
  5. Garmin. About VO₂ max estimates. epix owner's manual. garmin.com
  6. Jurov I, Toplišek J, Cvijić M. Prediction of maximal oxygen consumption in cycle ergometry in competitive cyclists. Life. 2023;13(1):160. doi:10.3390/life13010160
  7. Pallarés JG, Hernández-Belmonte A, Valenzuela PL, et al. Field-derived maximal power output in cycling: an accurate indicator of maximal performance capacity? International Journal of Sports Physiology and Performance. 2022;17(11):1558-1564. doi:10.1123/ijspp.2022-0208
  8. Sitko S, Cirer-Sastre R, Corbi F, et al. Five-minute power-based test to predict maximal oxygen consumption in road cycling. International Journal of Sports Physiology and Performance. 2022;17(1):9-15. doi:10.1123/ijspp.2020-0923
  9. Storer TW, Davis JA, Caiozzo VJ. Accurate prediction of VO₂max in cycle ergometry. Medicine & Science in Sports & Exercise. 1990;22(5):704-712. doi:10.1249/00005768-199010000-00024
  10. Swain DP. Energy cost calculations for exercise prescription. Sports Medicine. 2000;30(1):17-22. doi:10.2165/00007256-200030010-00002
  11. Swain DP, Leutholtz BC. Heart rate reserve is equivalent to %VO₂ reserve, not to %VO₂max. Medicine & Science in Sports & Exercise. 1997;29(3):410-414. doi:10.1097/00005768-199703000-00018
  12. Tanaka H, Monahan KD, Seals DR. Age-predicted maximal heart rate revisited. Journal of the American College of Cardiology. 2001;37(1):153-156. doi:10.1016/s0735-1097(00)01054-8

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