A few weeks ago my Garmin watch quietly informed me that I wasn’t really 78. At least not in one important respect.

The watch displayed my “Fitness Age,” and I was intrigued. Until then, I hadn’t even known the concept existed, much less that my Garmin was calculating it.

Chronological age addresses the question: “How old are you?”  One thing about chronological age – it is fixed at any point in time and we cannot change it!

Increasingly, scientists, medical professionals, and technology companies are now asking a different question: “How old is your body?”

Background

I’ve been using wearable fitness devices since buying one of the original Fitbit clip-on trackers shortly after it was introduced in late 2009. Over the years I’ve watched these devices evolve from simple step counters into sophisticated health monitors measuring heart rate, sleep, blood oxygen, stress, recovery, and much more.

Today, a few major concepts addressing aspects of aging are available from the major wearable device manufacturers.  They are concluding that “age” is a more meaningful way to communicate health than raw physiological measurements.

The trend is unmistakable: wearables are moving from measuring what you did today to estimating how well your body is aging (or living).

Current products and concepts (as inclusive as I know) are shown in the figure below for perspective and not discussion.  Garmin is first because that is my watch.  The rest are in alphabetical order.

Below the table, I’ll discuss Garmin’s age measure concept and how it relates to me (and you).

Fitness Age

Garmin describes Fitness Age as an estimate of how your cardiorespiratory fitness compares with that of other adults. The concept is based primarily on your estimated VO₂ max – the maximum amount of oxygen your body can use during exercise – which is widely recognized as one of the best measures of aerobic fitness and an important predictor of long-term health (see the table above).

Garmin’s calculation also considers factors such as your age, sex, resting heart rate, body composition (or BMI if body fat percentage isn’t available), and your recent activity levels. The result is a number expressed in years – and it relates to you (or me).

Unlike your chronological age, your Fitness Age can move in either direction. As your aerobic fitness improves, your resting heart rate declines, your body composition improves, and you maintain an active lifestyle, your metric may decrease. Conversely, inactivity, declining fitness, or weight gain can cause it to rise.

Of course, Garmin’s measure is only an estimate. It isn’t a medical diagnosis, nor is it intended to predict how long you will live. Rather, it is a practical way to summarize several important measures of health in a single number that most of us can understand.

We may have little intuition about whether a VO₂ max of 38 or 42 is “good.” But we immediately understand the idea of having the fitness of someone younger—or older—than our chronological age.

That simple translation is what captured my attention. For years I’ve tracked my daily steps, walking distance, resting heart rate, sleep, weight, and more. Fitness Age doesn’t replace any of those measures.

Instead, it pulls several of them together into a single, easy-to-understand indicator of how well my body (or yours) is aging.

My Fitness Age

When I first noticed Fitness Age on my watch, it informed me that my metric had been lowered from 68 to 67.5.

That got my attention because my chronological age is 78.

I wasn’t looking for another number to track. I was intrigued because this one attempted to summarize several aspects of health into a single measure I could actually understand and relate to intuitively.

Based on the fitness measures Garmin uses, my cardiovascular fitness resembles that of the average healthy 67½-year-old rather than the average 78-year-old man.

Before I put too much of a new spring in my step, I looked at what Fitness Age does not mean.  Since we are on it, I’ll personalize the response.

It does not mean:

• My entire body is biologically 67½ years old.
• I have added 10½ years to my life expectancy.
• I have the athletic ability of every 67-year-old.
• I am protected from age-related disease or injury.

Garmin is estimating one important dimension of health: cardiorespiratory fitness. They are not measuring memory, bone density, joint health, vision, hearing, muscle strength (except indirectly), or many other aspects of aging.

The fact that Garmin lowered my metric suggests that it is a range concept.  One’s Fitness Age can be lowered or it can be increased.

I do not know the algorithm Garmin uses to calculate the metric, but they leave clues with their “recommendations” when providing my (or your) Fitness Age.

Per my Garmin Connect app, my metric is 67.5 years.  The chart shows my chronological age on the right as 78, which it is.  It also provides a Target age of 61 years on the left.  How would that be possible?  Garmin provides me with four “Recommendations” to help lower my Fitness Age:

1. Increase Vigorous Minutes. 6w average 42.6 min per week. Higher Priority

2. Increase Vigorous Days. 6w average 0.5 days per week.

3. Reduce Body Fat.  Weekly avg 28.7%

4. Resting Heart Rate.  Maintain 59bpm (beats per minute). On Target

When I saw these recommendations, I remembered that my recent activity has been unusual. During a trip to Peru with my son in May, we spent much of our time at elevations between 4,500 and 7,000 feet, walking only when necessary. I also picked up a lower GI illness that slowed me down for several weeks afterward. My vigorous activity—and pickleball—had dropped considerably.

So I understood recommendation #1 of Increasing Vigorous Minutes.  If I accomplish it reasonably during the coming days and weeks, I’ll also be focusing on recommendation #2, that of Increasing Vigorous Days.

To reduce Body Fat, or lower my Body Mass Index, I’ll need to lose some weight – or increase muscle mass.  We will see.

Finally, it says to maintain my Resting Heart Rate.  If I do #1 and #2, that is likely.

Final Thoughts

Reviewing my Fitness Age information led me to think about aging more generally.

Chronological age is something we receive or achieve as we live each day and year.  It is fixed at any point in time and cannot be changed.

Fitness Age, on the other hand, is something we can influence.

Our measures can improve—or worsen—depending on the choices we make over time. That makes it different from our birth certificates. It reflects the cumulative effect of thousands of ordinary decisions over time: whether we walk today, whether we play pickleball, whether we do strength and balance exercises, whether we maintain a healthy weight, and more.

We cannot determine how long we will live.  But, we can influence how well we live and the odds of avoiding the bad things that can happen to any of us – or the odds of surviving if they happen to us.

Chronological age tells us how long we’ve been alive. Fitness Age asks how well we’re living.

None of us can change our chronological age. Every birthday moves us forward another year. But Fitness Age reminds us that many of the choices we make every day still matter. We may not control how many years we are given, but we can influence how well we live those years. To me, that’s a hopeful message—and one worth paying attention to.

Until the next post, be well, and age gratefully,

Chris