What’s the Most Useful Method for Estimating Your Cardiorespiratory Fitness: Apple Watch versus Stopwatch

The skeptical cardiologist is a strong advocate of regular aerobic exercise to forestall the inevitable decline in cardiovascular fitness with aging. There is compelling evidence that aerobic exercise can reverse the age-related impaired relaxation of the left ventricle. In addition, any amount of running is associated with a much lower risk of cardiovascular death.

Even small amounts of aerobic exercise contribute to maintaining overall cardiorespiratory fitness and thereby sustaining healthy cardiovascular performance into our later years.

Given the importance of regular aerobic exercise for healthy aging, I’ve incorporated an “exercise vital sign” into the EMR I utilize and every patient gets quizzed on how active they have been along with plenty of encouragement to meet my recommended goals.

Although simple and useful for monitoring patients over time, I find the exercise vital sign lacks objectivity. For example, a patient recently told me that he was exercising for 1 hour on his treadmill every day. The easy thing for me to do would be to click 7 days at 60 minutes per day. The patient would have 420 minutes of MVPA per week. Case closed.

However, when I asked him the speed at which he walked he didn’t know but he knew that the distance was 2 miles. For me, a 2 MPH walk is incredibly slow and it is hard to imagine that this qualifies as even moderate exercise. I asked him to boost his speed gradually up to 2.5 MPH and to make sure that he was at least a little breathless during the exertion and that his heart rate was reaching at least 120 beats per minute.

Given the limitations of subjective patient assessments of their exercise patterns, is there a reasonably simple and objective way that patients and physicians can easily assess the impact of their exercise on their cardiorespiratory fitness status?

With this information, an individual and their cardiologist could assess where they stand cardio fitness-wise in comparison to age and gender peers. If below average, this would motivate them to work to improve their fitness and thereby improve their longevity and healthspan. This parameter of cardiorespiratory fitness would ideally be much more objective than the patient’s self-reported exercise.

Maximal Oxygen Uptake

Maximal oxygen uptake (MVO2) is considered the gold standard for measuring cardiorespiratory fitness. To accurately measure MVO2 individuals must perform a special treadmill or bicycle test during which oxygen consumption and carbon dioxide exhalation are measured. Such testing is used in heart failure patients and elite athletes but is not feasible for cost and logical reasons in the general population.

As a consequence, over the last 60 years, dozens of methods for estimating MVO2 have been developed which utilize submaximal exercise and which don’t require the measurement of metabolic gas exchange.

I’ve scanned the literature on the best methods for estimating MVO2 and have found one method that is reasonably accurate and that most people can do with either a wearable activity monitor (WAM) or a stopwatch and a measured distance.

Submaximal Exercise for Estimating MVO2-One Mile Walk Fitness Test (aka Rockport)

The first description of what is now called the Rockport walking test was in 1987 by Kline, et al. They noted the limitations of previous testing protocols, including limited accuracy, difficulty in performing, and lack of data in a broad population. They had 343 “healthy volunteers” aged 30 to 69 years undergo determination of VO2 max by standard cardiopulmonary exercise testing and compared those values to parameters obtained from a one-mile walk.

Subjects were asked to perform two one-mile walks on a measured track as fast as possible. The mean of the HR during the last quarter of the mile and the walk duration were entered into a regression equation and equations were developed to predict MVO2

Note that this came from a “xeroxed” hardcopy of the original paper journal article!

For some reason in subsequent papers this has been called the Rockport one-mile overground walk test (if anyone knows why please let me know.)

The Rockport formula (which I’m inclined to call the Kline formula or the Amherst formula) is as follows:

Note the excessive number of significant digits, implying more accuracy than reality permits. Also note that walk heart rate refers to the average heart rate during the last quarter of your walk which is higher than the total average. If you keep your pace constant the end heart rate will be very close to the final quarter average.

Performing a Rockport Walking Fitness Test

I performed the Rockport test on myself this morning. I prepared by putting on good walking shoes, changing into running shorts, selecting a good podcast (TWIC) and sticking AirPods into my ears. I then walked over to a track that goes around the baseball field next to my condo and after warming up for a few minutes began the test by activating the outdoor walk workout on my Apple watch.

The first time I did the Rockport I walked as fast as I possibly could and ended up with a time of 15:40 with HR averaging 94 beats per minute. During the last quarter the HR averaged 95 beats per minute

Any fitness wearable will (hopefully accurately) give you the information needed: average heart rate during the last quarter of the walk, distance walked, and duration of the one-mile walk. But if you have a track of a known distance and a stopwatch plus the ability to count your pulse you don’t need any digital devices.

When I plugged my numbers into the above equation I calculated my MVO2 at 36.45 ml/kg/min.

Oddly enough, this was very similar to the MVO2 simultaneously measured by my Apple Watch.

Wearable Activity Monitors for Estimating Cardio Fitness: Apple Watch’s Approach

Apple Watch (AW) and other wearables (most notably Garmin) are now reporting out something they call MVO2. But it is clearly only an estimate and relies on the accuracy of the heart rate and distance measured by the device plus the MVO2 algorithm selected for its determinations.

Apple has a very detailed white paper (not published in the peer review literature to my knowledge) on their website that describes the study they did to validate their MVO2 estimates. They concluded:

VO2 max estimation by Apple Watch is accurate and reliable relative to commonly used methods of measuring VO2 max, with an average error of less than 1 MET and an ICC of more than 0.85.

Apple says:

Apple Watch users can view their cardio fitness level in the Health app on iPhone, and receive a notification on Apple Watch if it falls within the low range. Breakthrough technology released in watchOS 7 allows Apple Watch to easily measure low cardio fitness, and today cardio fitness notifications empower users to be more active for dramatic long-term health benefits.

To activate it follow these steps:

After doing all this, if you take and record with AW an outside walk, run or hike longer than 5.75 minute you’ll see VO2 max/Cardio Fitness show up under the Heart section in the Health app. This will take 24 hours after the initial walk.

I’ll talk in detail about the AW MVO2 methods and limitations in a subsequent post. Appropriately, it will not generate an MVO2 evaluation if the walk or run includes an incline or decline of more than 5%. In addition, there are certain conditions under which it is almost certain to be inaccurate, one of which has influenced my personal AW MVO2 numbers.

Here are some of my AW MVO2 readings:

These are my AWMVO2 numbers for the last week. In general, AW provides remarkably consistent numbers whether evaluating a run or a walk. The first two low values are due to a change in the information I gave the device which I will explain in a later post.

During the time in late April, early May when I had COVID-19 and couldn’t exercise my MVO2 by AW dropped significantly.

Personally, I’m insulted by the low MVO2 numbers estimated by AW and the Rockport. However, given that I have a cardiomyopathy and asthma, despite my fairly rigorous and now very regular workouts these may accurately reflect my real cardiorespiratory fitness. I’ll be doing my best to get them higher or at least stop going down.

Making Sense of Your MVO2

Whether you derive your MVO2 from the Rockport or a WAM you want to see how your number compares to average for your age and gender. Apple obviously has made its own determination on what constitutes average, above or below average, and uncomfortably low values. I’m still pouring over the literature in this area and will give my best estimate of normal values by age and gender in a subsequent post.

If you have an above-average number, great! Keep up what you are doing. If it is significantly below average then this should serve as a wake-up call to advance or begin your exercise program. Your goal should be to get up to at least average.

Please leave your personal experience with either the Rockport or a WAM-derived MVO2 in the comments section.

Rockportingly Yours,

-ACP

N.B.If you are unable to walk over a measured distance of level ground due to weather or logistics you may consider utilizing a treadmill for the Rockport. Most treadmills will give you distance and measure your heart rate. Unfortunately, a study suggests that the Rockport equation is not very accurate for treadmill walking. The estimation of VO2 max from treadmill walking requires a different equation. If you are interested in that formula email me.

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26 thoughts on “What’s the Most Useful Method for Estimating Your Cardiorespiratory Fitness: Apple Watch versus Stopwatch”

  1. The ranges that the Apple Watch gives are gendered, and like so many gendered things in health, it doesn’t say what causes the differences. Not much more than an omitted intellectual footnote if you’re cis, but frustrating for those of us who are trans.

    My question is: is the gendered difference in this metric due to pubertal body development, hormones, or a combo of the two? (This doesn’t seem like a case where it would be reproductive tissue/genital based, genetic, or behavioral/environmental, the other common reasons for gender differences in health issues.) Until I know which, I don’t know what the ‘fair’ metric is to judge myself by in this case.

    Reply
  2. Doctor: Thanks for a great article! I am hoping you can use your credentials as a published cardiologist to get Apple to correct their VO2 max targets for Old People like me.

    Here’s the problem: iPhone > Health app > Cardio Fitness > Scroll way down to “What it Means if Your Cardio Fitness is Low” > Scroll down to “VO2 max: What’s your cardio fitness level?” Note all of the graphs for High, Above, Below, Low fitness. On all of them, the range is highest for your 20s and declines steadily thereafter decade by decade, but then it ends at 60+. In other words, they assume that if you’re in your 70s, 80s, 90s or centenarian, your fitness should be the same as when you were 60! That’s just wrong.

    I’m 76 years old, BMI 22.2, blood pressure and cholesterol normal, and those figures have been static for two decades. Nonetheless, I had quadruple bypass surgery 18 months ago. I now regard myself as being in “lifetime cardio rehab” mode, meaning I get ~ 300 minutes of aerobic exercise per week in the 65% – 85% max heart rate range for my age, averaging about 75% for the 5 hours of exercise. Since building my fitness back after the CABG surgery, Apple’s VO2 max for me runs between 27 and 28, and always shows as “Below Average,” since that’s how I’d rate if I were 60 years old. Not fair!

    Some months ago, I tracked down Apple’s paper with their methodology. I’m a layman, but I was impressed, it seemed like sound science to me. Given your credentials, perhaps you could contact the authors of that paper and advocate for them to extend their targets to include senior-senior citizens as well as youngsters and middle-agers? Thanks!

    Reply
  3. I’m taking bisoprolol (5 mg/day) for an irregular heartbeat, and I’m 77 years old. The bisoprolol limits my maximum heartrate.
    How does that factor into the results of this proxy for an MVO2 test ? (Thanks — your blog is excellent!)

    Reply
    • Charles
      Thank you. bisoprolol could definitely influence the results. I am working right now on a post that discusses beta-blockers, heart rate, and MVO2 estimates.
      Apple clearly thinks it is a big factor and incorporates this information, if provided into their Max VO2.
      However, how much a beta-blocker lowers the heart rate with exercise varies depending on the type and the dosage.
      Diltiazem also lowers heart rate response to exercise and could influence MVO2 determinations.
      Dr. P

      Reply
  4. Very interesting. Any suggestions on what questions to ask of a clinic if you want to have a VO2 Max Test by a clinic that measures it and take you through the test?

    Reply
  5. I had an actual, gen-U-ine VO2MAX test done (on a bike) just over 10 years ago……as I was approaching 60. It was a hair over 45 ml/kg/min…..if I remember correctly, it was in the “excellent” category for women a couple of decades younger.

    My Garmin has estimated my current VO2MAX as toggling around 38-40. Still not too shabby…..especially since I was out of training action for a good few months after lapiplasty last October.

    P.S. that’s at altitude (lived at over 6,000ft for almost 6 years) and with Afib and Sotolol

    Vivienne

    Reply
    • vivienne,
      Congrats on the genuine MVO2 score from the past!
      I’m pretty sure the afib would deflate current score but the sotalol would inflate it.
      Does Garmin ask about rate influencing medications?
      Dr. P

      Reply
      • Not that I’ve seen. I doubt the algorithm could handle that complexity, to be honest, but I’ll take another look. By the strangest of coincidences, I’ve been listening to a Peter Attia podcast on just this topic this past week for my Z2/MAF workout sessions on my treadmill …

        https://peterattiamd.com/mikejoyner/

        Reply
  6. My AW says my VO2 has averaged from 48.5 to 49.9 each month for the past year. My age is 59. Dog walk everyday 2.5 miles ~50 minutes. Pickleball 2-2 1/2 hours 3 times per week (AHR from 130 to 140). Strength training twice a week for 1 hour. Am I exercising enough? Am I in shape? I guess my AW thinks so.

    Reply
  7. I am a 78-year-old male with a VO2 of 41 according to my Polar watch. I try to jog at least once 5k per week on a hilly trail near our home in Ohio. I exercise by heart rate and let the pace take care of itself. Lately the pace is 3.5-5.5mph. This 5k averages around 53 minutes.
    I usually skip a day and then fast walk, jog, or hike the alternate days. In the winter, I may switch to treadmill or Concept2 rowing during inclement weather. Following Clevland Clinic guidelines I keep my heart rate at under 65% of my max of 142, using the 220 formulae, meaning under 92 bpm, avg 85bpm. My resting heart rate is 44, partly due to the 2mg clonidine patch, but primarily to my fitness level.
    My first episode of Afib (irregular heart rate, not racing heart rate} was February 2021 after clearing snow. Labetalol was prescribed, but recently switched to Clonidine because I could not tolerate the labetalol brain fog. I have had occasional episodes since, with the last one this past February 2022, again while clearing snow. I have found that if I exercise at a higher heart rate, it takes the rest of the day for the HR to settle down to resting. So, it’s LSD, long, slow, distance for me. What do you think?

    Reply
    • Bill,
      Congratulations on maintaining such an active fitness regiment into your late 70s!
      You mentioned that you follow “Cleveland Clinic guidelines” and keep. your heart rate <92 bpm (65% of predicted maximum). Was this target HR an individual recommendation to you (based on your history of exertional afib) or a general population recommendation.This seems quite a low target for general population.
      I suspect the VO2 max calculation from your Polar watch is influenced by the rate-slowing properties of the clonidine you are on. Does the watch ask for input of any rate-slowing cardiac medications.
      Dr P

      Reply
  8. i dont trust the Rockport test. my guess is that the average 4 minute miler has a pulse rate of 215 at the end of a race. It is reported that Jim Ryan’s maximal heart in a race was 150. Do you think his Rockport measured max o2 was that much greater than the average 4 min miler.?

    Reply
    • Gebhard,
      The Rockport wasn’t calibrated for elite endurance athletes.
      Extremes of low or high MVO2 are going to be difficult to estimate without true cardiopulmonary metabolic stress testing.
      Apple only gives numbers up to 60 and I’ve seen multiple high endurance athletes complain on Reddit about Garmin and AW not going high enough.
      Dr. P

      Reply
  9. Apple Watch readings have VO2 max readings ranges for various age groups but nothing for those of us aged 80 and above. I have a Bowflex exercise bike for workouts but need to do long walks on the flat to get the watch to read out VO2 max. I tend to get readings in the 28.0 range and see up and down variations from that based on how often I do the Bowflex work. Watching the readings over time is a great motivator. Anything we can measure, we can improve.

    Reply
    • Chuck,
      I’ll see if I can track down expected for your age range but my gestalt is that 28 is way above average.
      Most of the age ranges I have seen normals for extend up to 70 or 80.
      apple likely did not do any testing in that range.
      Dr. P

      Reply
  10. As I understand mvo2 supposed to show your level of cardiac fitness, but actually it show your fitness only if you use it during exercise, if not, it will say that your fitness is below average, as it is in my case. I cannot take Apple Watch into gym and pool, I exercise every day for 40 min, pretty intensive with HR -120-140 per min. I use Apple Watch other time of the day, and result showing
    – vo2 max below average- 28. Something wrong here. What do you think, Dr.?

    Reply
    • Yuri,
      Apple Watch does not require your to be actually exercising .
      This morning I took a walk on the beach and it yielded the same MVO2 it gives me when I run.
      How old are you?
      Dr P

      Reply
      • According to Apple it measures MVO2 when exercising. The gentleman gets no reading because he does not wear the Apple Watch during workouts. I always wear mine when exercising in a pool. Wind the stem after the swim to expel any water.

        Reply
  11. Very timely article as I just discovered this feature on my apple watch. Have you ever used the HRM from Fourth Frontier? I would be interested in your thoughts.

    Reply
    • AC
      Never used the Fourth Frontier. Are you using it? Seems like other readers have recommended I take a look at it.
      Dr P

      Reply
      • Yes, I have been using it for about 3 months now. It took some getting used to as it seemed a little “fiddly” at first. But it seems to give more accurate Heart Rate info than my garmin watch, as it should. Hope you ‘ll try and write about it sometime.

        AC

        Reply
    • Ed,
      Thanks. Does Garmin attempt VO2 max reading off Peloton cycling workouts?
      What Garmin device are you utilizing?
      Dr. P

      Reply
      • Sadly Garmin only gives VO2 max from exercises using gps data. I did get out on the outdoor bike regularly during that time so I think it was updating from that data even though the hard work was really on the Peloton. I use the Garmin Forerunner 945.

        Reply

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