The Skeptical Cardiologist

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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