The Skeptical Cardiologist

Physical Activity/Fitness Level: Can Your Wearable Monitor Track and Improve This Vital Sign?

This is an update of a post first written in 2016. As we approach New Years Day 2021 undoubtedly many of you will be resolving to begin or expand your exercise program. Herein I touch on methods that physicians and patients can use to assess current fitness level, begin a fitness program, and gauge performance using wearable activity monitors relative to activity guidelines

The skeptical cardiologist routinely probes his patients’ activity and exercise levels and encourages them to engage in 150 minutes of moderate exercise weekly.

Initially, I was skeptical of the benefit of treating such assessments as a vital sign (like blood pressure or heart rate)  as a 2016 AHA scientific statement suggested but I now think it makes sense. I was concerned about seeing still another item on a chart checklist that would have to be recorded in the EHR but fitness is such an important parameter it truly is worth putting in with heart rate, blood pressure, and weight.

The 2016 AHA statement suggested that ideally, physicians should be measuring their patients’ fitness by obtaining maximal oxygen consumption (VO2 max) by utilizing an expensive and rarely utilized cardiopulmonary exercise test. Failing that, the AHA statement recommended doing a treadmill stress test. Despite this recommendation, made in 2016, I have seen little evidence that clinicians are routinely performing treadmill exercise tests on their patients to estimate their cardiorespiratory fitness (CRF.)

In the absence of stress test information, rather than utilizing my simple question to patients: “How active have you been?”,  the 2016 AHA statement recommended doctors utilize a formal questionnaire to estimate their patients’ CRF such as the one provided at World Fitness Level.

Estimating Your CardioRespiratory Fitness Without a Stress Test

World Fitness Level was developed by Norwegian exercise physiologist Ulrik Wisloff the leader of the Cardiac Exercise Research Group (CERG) whose laudable goal is “to uncover how to treat and prevent major societal health problems, such as obesity and cardiovascular disease.”
You can read more about their methods on CERG’s excellently referenced NTNU website The Fitness Calculator estimates your fitness age measured as maximum oxygen uptake (VO2max). On the NTNU website you will learn that  VO2max is the most precise measure of overall cardiovascular fitness and can find your own risk of dying early from cardiovascular disease along with ideas on how you can reduce the risk.

In 2016 I went online to take this CRF estimator (which is based on this paper)
I was asked for the following information:

The info on location, level of education and ethnicity is only utilized for CERG research whereas gender/age/heigh/weight are relevant factors for the CRF estimator.


When you have finished answering the questions you are given an estimate of your fitness age. When I did this online in 2016 and answered truthfully I got the result to the right: I had the fitness of a 41 year old with an estimated VO2 max of 49 !

(interestingly this estimate corresponds exactly with VO2 max derived from a recent stress test I had completed.)
I used the app (which appears no longer to be available) and changed a few parameters:

The change was dramatic and depressing: I went from 39 years old to 67 years old in the bat of an eyelid! Thus, the key to youth is obviously exercising at intense levels a lot which among many other things results in a low resting HR.

A Simplified Assessment of Physical Activity

Interestingly, a 2018 AHA statement on “Routine Assessment and Promotion of Physical Activity in Healthcare Settings” came up with a totally different approach to assessing CRF, one quite similar to mine.

Make PA assessment a priority in all visits, in particular when the focus is CVD (cardiovascular disease) or preventive care, using one of two simple, standardized tools:

Physical Activity Vital Sign (PAVS):

– “On average, how many days per week do you engage in moderate or greater intensity physical activity (like a brisk walk)?”

– “On average, how many minutes do you engage in this physical activity on those days?”

●If the product of those responses (MVPA in minutes per week) indicates a lack of compliance with the aerobic component of the US PA guideline recommendation of 150 min/wk, individuals should be advised of the health benefits of regular PA and encouraged to gradually increase either their frequency or duration of activity.

An equally effective approach is contained in a single sentence question:

In the past week/past month, on how many days have you done a total of 30 minutes or more of physical activity, which was enough to raise your breathing rate? This may include sport, exercise, and brisk walking or cycling for recreation or to get to and from places, but should not include housework or physical activity that may be part of your job.”

Starting Exercise To Increase Your Fitness Level and Lower Your Fitness Age

Whatever approach we use to assess our patient’s fitness level/physical activity whether it is a stress test, the world fitness level calculator, or the single question method it is important to have an equally valid method for boosting the patient’s fitness level/physical activity level.

The 2016 AHA statement includes this complicated table with PA dosage recommendations which comes from the American College of Sports Medicine:

The types of exercise which improve CRF are ones which “involve major muscle groups” and are continuous and rhythmic such as brisk walking, jogging, and dancing. I have typically called this aerobic exercise.

Strengthening exercises, although important for many reasons, do not improve CRF.

The intensity should be enough to get heart rate to >50% of the heart rate reserve.

For some patients, just pointing out that they need to increase the duration and intensity of their walks, bike rides or other aerobic activities is effective.

Many patients, however, despite acknowledging that they are out of shape and recognizing the benefit of ramping up their exercise, fail to begin or sustain regular exercise programs.

Others consider their very slow walks with their dogs as satisfying criteria for moderate exertion and thus fail to reach the recommended PA dosage.

On CERG’s website, you can get information on a 7-week program to increase your fitness level. This is a very good start for sedentary individuals looking for a science-based approach to getting more fit. The PDF is available here.

The CERG program emphasizes high-intensity interval training (HIT or HIIT.) The equally acceptable more mainstream approach is exemplified by the CDC guidance on getting started with physical activity (see CDC Move Your Way PDF.)

A key determinant of the dose of PA is intensity: remember that every minute of “vigorous” counts double the minutes of “moderate” exercise toward achieving the desired 150 MVPA minutes.

The CDC offers a simple, low-tech method for gauging whether one is engaged in moderate versus vigorous exercise:

Variations on the “talk test” have actually been validated as measures of exercise intensity, but I would prefer a more objective measure.

Are WAMS the Solution For Measuring and Motivating Physical Activity?

Lately, I’ve become obsessed with the methods by which various wearable monitors might more precisely measure the dose of physical activity.  My recommendations for exercise suggest 150 minutes per week of moderate activity or 75 minutes of vigorous activity and they include the following suggestions on what constitutes moderate or vigorous activity:

Examples of Different Aerobic Physical Activities and Intensities

Moderate Intensity

Vigorous Intensity

But is there a better way of determining whether you are doing moderate or vigorous exercise and reaching PA goals beyond the individual’s recollection of the type and duration of activity?

Can wearable activity monitors (WAMs) like FitBit, Apple Watch and Garmin Vivofit provide a more reliable assessment of PA thereby guiding and motivating patients to achieve the magical level of 150 minutes weekly of MVPA?

The 2018 AHA statement goes into great detail on WAMS and even has a chart rating them according to 5 criteria: validity, how readily the device classifies MVPA minutes versus just counting steps, feasibility (which assesses battery life and cost), health care data integration, and incorporation of elements of behavioral change

They concluded:

Despite the growing use of WAMs, an ongoing challenge is that few of them collect and report PA data in a manner that enables assessment of compliance with the PA guidelines. Most WAMs and mobile apps focus on daily step counting, with the somewhat arbitrary goal of 10 000 steps a day, rather than reporting the World Health Organization, Centers for Disease Control and Prevention, and AHA guideline metric of MVPA minutes per week, largely because step counting is easier technically, with studies showing good accuracy. Estimating PA intensity has been more challenging, with suboptimal results when devices are studied systematically.Many WAMs and smartphone apps report active minutes and calories burned as indicators of PA intensity, but little information has been available on the specific algorithms used and how these measures correspond to the guideline-based MVPA minutes.

You can view a tabular summary of their report by downloading this PDF:

The problem with assessing WAMs is that they are a moving target, continuously upgrading and modifying the algorithms they use. Thus, the ratings in the 2018 AHA document are mostly irrelevant for current devices.

It is my sense, however, that currently available WAMs are trying to incorporate many of the features the 2018 AHA statement mentions.

Specifically, WAMS manufacturers are attempting to provide an assessment of the wearer’s MVPA minutes and then provide feedback that encourages the wearer to achieve the 150 minutes MVPA per week goal.

Personal Activity Intelligence

Professor Wisloff, of CERG fame, has developed a free smartphone app that takes heart rate data from any wearable and generates an assessment of MVPA minutes.

This MVPA assessment method is called Personal Activity Intelligence (PAI) and it has a solid scientific background and a laudable goal:

The PAI algorithm was derived from one of the world’s largest health studies involving 45,000 people over 25 years, and it has been further validated across broader populations of over 730,000 people, with over a million person-years of data. 

The objective was to develop a single, easy-to-understand personalized activity tracking metric that could help everyday people manage their health. The PAI algorithm was the result of this research. 

Published studies have demonstrated evidence associating a PAI Score of 100 and above, with reduced risk of mortality across many cohorts and sets of data

What I particularly like about PAI is the link provided to a complete listing (with the complete papers) of the research supporting the program. For example, this seminal paper:

I’m in the process of evaluating the PAI app and comparing it to WAMs like Apple Watch, Garmin VivoSmart, and the Amazon Halo. I’ll post that analysis along with a discussion on heart rate reserve and methods for calculating MVPA minutes in the near future.

In the meantime, I hope all my non-exercising patients will get off the couch and start accumulating those MVPA minutes and raising their World Fitness Level.

Dynamically Yours,


h/t Mark Goldstein for suggesting I evaluate PAI

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