Measuring, Tracking, and Improving Cardiorespiratory Fitness to Maximize Healthy Life Years

I want to make the case today that one’s cardiorespiratory fitness (CRF) should be considered a “vital sign”: akin to those measures of bodily function; temperature, blood pressure, heart rate, and respiratory rate that we routinely measure to assess our patient’s health status.

Previously, on the skeptical cardiologist, I made the case that a patient’s physical activity as measured by the minutes spent in moderate to vigorous physical activity per week (MVPA) should routinely be assessed by PCPs and cardiologists.

That post reviewed methods physicians and patients can use to assess current fitness levels, begin a fitness program, and gauge performance using wearable activity monitors relative to activity guidelines.

Eventually, I was able to get MVPA incorporated into EPIC, but I didn’t feel that it captured my patient’s CRF level accurately.

MVPA is an extremely subjective measure: the patient decides what the intensity of exercise is and tries to accurately recall the time spent on average in each level of exercise.

MVPA minutes are a surrogate for a very important parameter of bodily function: cardiorespiratory fitness, aka cardio or aerobic fitness.

They are tightly linked because regular endurance-type exercise produces a variety of biological adaptations that lead to an increase in CRF, most likely related to an increase in stroke volume and an increase in O2 extraction in the trained muscle.

A 2016 statement from the AHA (Importance of Assessing Cardiorespiratory Fitness in Clinical Practice: A Case for Fitness as a Clinical Vital Sign) reviewed the “large body of epidemiological and clinical evidence demonstrating not only that CRF is a potentially stronger predictor of mortality than established risk factors such as smoking, hypertension, high cholesterol, and type 2 diabetes mellitus (T2DM), but that the addition of CRF to traditional risk factors significantly improves the reclassification of risk for adverse outcomes.”

CRF goes declines with aging in parallel with a marked reduction in diastolic function of the left ventricle (which is the most likely cause of the age-related decrease in stroke volume observed during exercise.)

Most of the “large body of evidence” cited by the AHA statement consisted of observational studies showing associations. It would be nice to have robust randomized trials showing that improved CRF directly results in reduced adverse cardiovascular events and longer health span.

If CRF is a vital sign akin to blood pressure we need 1) an accurate and easily accessible method for estimating it plus 2)  a database showing where we stand in comparison to others our age and gender and 3) Evidence showing that if CRF is improved by exercise then lifespan and health span are improved. 

In 2018 newer studies convinced me that it was possible to forestall the age-related decline in cardiorespiratory fitness and cardiac stiffness by increasing exercise and I began publishing information on methods for estimating it and normal ranges.

The Quest to Find a simple, accessible, universal method of measuring cardio fitness

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 doctor 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 CRF would ideally be much more objective than the patient’s self-reported exercise.

Maximal oxygen uptake (MVO2) (i) is considered the gold standard for measuring CRF. 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 logistical 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 have described in detail how to perform the simplest of these: the Rockport walking test which was developed in 1987 and has been utilized and validated widely.

The Rockport protocol requires you to walk on a level surface outdoors as fast as possible for one mile. You need some measure of the distance walked and some measure of time to do this. Wearable activity monitors will provide both. Record your heart rate during the last quarter of the walk along with the duration of your walk and enter them into the regression equation I describe here.

For those well-heeled enough to afford wearable activity monitors (WAMs), Apple Watch (AW) and other wearables (most notably Garmin but including, Fitbit, Polar, Samsung and Huawei) are now using submaximal heart rate data to provide estimates of CRF or Max VO2.

I have been using an Apple Watch since 2017 and started using it to monitor my MVO2 in 2020. 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.

Early on, the Apple Watch MVO2 estimates I received were erratic and unreliable (I have a draft post from 2020 entitled “Apple Watch’s Max VO2 Is A Useless Measure of Aerobic Fitness”) but beginning in the fall of 2021 they have been quite consistent and reliable.

I describe in detail how to utilize AW to determine your Max VO2 here.

Normal MVO2 Values from Age 20 to Age 90

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.

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.

The best reference database for directly measured CRF determined from cardiopulmonary exercise testing (CPX.)are available in a paper published in Mayo Clinic Proceedings in 2022 entitled “Updated Reference Standards for Cardiorespiratory Fitness Measured with Cardiopulmonary Exercise Testing: Data from the Fitness Registry and the Importance of Exercise National Database (FRIEND).”

This is the database Apple Watch references.

These data are from >22,000 “apparently normal” individuals ranging in age from 20 to 90 years. In this chart MVO2 is broken down according to age range, gender and percentile rank (10 to 90)

As a man advances from 20 to 80 years of age, his MVO2 drops precipitously from 46 to 17 ml/kg/minute leaving him with less than half of the cardio fitness he had when he graduated high school. Similarly, a woman drops from 37 to 15 ml/kg/minute.

But note that there are 10% of 70 year olds who have an MVO2 of 29, which is superior to 20% of 30 year-olds.

Regular exercise training is your major tool to make sure you have enough fitness to stay highly functional into very old age.

Perhaps a picture is worth more than the prior 1500 words.

This figure from a review article entitled “Survival of the fittest: VO2max, a key predictor of longevity?” plots physical fitness (as a combination of CRF and muscle strength) on the y-axis versus age on the x-axis.

Note the steady decline in physical fitness (both CRF and muscle strength) with aging.

To maximize your “healthy life years” and remain functionally independent you should aim to be as close as possible to the top (trained, aka excellent fitness) curve.

Your curve can be moved from the sedentary one which declines below the horizontal cross-hatched line of independence by the late 70s to the trained upper line which only crosses the line in the mid-90s.

Both lines converge on death at about the same time but the individual who regularly engages in aerobic exercise spends many more of their later years independent and functional with the physical capacity to do the things which make life enjoyable.

We don’t have a gold standard tool for the assessment of muscle strength (although we know poor hand-grip strength has been linked to premature mortality and is easy assessed) but regular resistance training is equally important to healthy aging as is CRF.

Exercise Dose to increase CRF

Much has been studied, written, and discussed on the optimal exercise for increasing CRF.

One of the most compelling studies showing improvement in CRF with aerobic exercise identified 61 sedentary men in their mid-fifties and randomly assigned them to either 2 years of exercise training or attention control (a combination of yoga, balance, and strength training 3 times per week for 2 years) and measured their left ventricular stiffness and max VO2 before and after the intervention.

Max VO2 increased by 18% and LV stiffness declined from .072 to .051 (i.e. improved) in the exercise group but did not change in the control group.

The exercise training arm of this study involved a mixture of continuous moderate-intensity aerobic exercise combined with high-intensity training. The high-intensity portion of the program involved exercising at 90-95% of HR maximum for 4 minutes followed by a 3 minute active recovery period, repeated 4 times.

This chart from recent European guidelines on lifestyle for prevention of disease describes different intensities of aerobic exercise:

These guidelines suggest that if you engage in vigorous exercise such as running or jogging, cycling fast or singles tennis, you only need to achieve 75 minutes per week.

Moderate exercise such as walking or elliptical workouts requires at least  150 minutes/week.

How do you know if the weekly exercise you are engaging in is enough?

By tracking your CRF, i.e. Max VO2!!

With knowledge of your MVO2, you now have a great tool to determine your CRF component of the “arc of healthy life”, the upper line in the graph above.

Independently Yours,

-ACP

(i) The 2016 AHA Scientific Statement defines and explains CRF as follows:

CRF reflects the integrated ability to transport oxygen from the atmosphere to the mitochondria to perform physical work. It, therefore, quantifies the functional capacity of an individual and is dependent on a linked chain of processes that include pulmonary ventilation and diffusion, right and left ventricular function (both systole and diastole), ventricular-arterial coupling, the ability of the vasculature to accommodate and efficiently transport blood from the heart to precisely match oxygen requirements, and the ability of the muscle cells to receive and use the oxygen and nutrients delivered by the blood, as well as to communicate these metabolic demands to the cardiovascular control center. Clearly, CRF is directly related to the integrated function of numerous systems, and it is thus considered a reflection of total body health. 

About half of the variance in CRF is considered to be attributable to heritable factors9; similarly, the contribution of inherited factors to the response of CRF to physical activity approximates 45% to 50%.10 It is noteworthy that these heritability estimates are similar in magnitude to other CVD risk factors, including, for example, insulin, glucose, lipoproteins, blood pressure, and high-sensitivity C-reactive protein.

(ii) 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. Another, more recent paper looks at ACSM and FRIEND equations for treadmill prediction of MV02