And here’s the part that stung: I’m a trainer. I had been working out consistently. I thought I was doing the work. But my body was telling me something different — that despite years of experience and good intentions, I had drifted into training that was no longer creating the stimulus my muscles needed to stay strong and metabolically active.

If it can happen to me, it can happen to anyone.

Your doctor checks your cholesterol. Your blood pressure. Your blood glucose. Occasionally, your resting heart rate.

Almost certainly not your VO2 max.

And yet, study after study has ranked cardiorespiratory fitness — measured by VO2 max — as the single strongest predictor of all-cause mortality. See the Companion Video.

How hard you can squeeze your hand predicts your risk of cardiovascular disease, cognitive decline, fall injury, surgical outcomes, hospital readmission, and all-cause mortality.

Here’s something that may surprise you: if you wear a Garmin, an Apple Watch, a Whoop band, or an Oura Ring to bed at night, there is a very good chance your device has been quietly measuring this metric for months — possibly years — without you paying much attention to it.

It’s called Heart Rate Variability. HRV.

According to the National Osteoporosis Foundation, osteoporotic fractures in men over 50 are more common than heart attack, stroke, and prostate cancer combined.

Read that again.

More common than prostate cancer. And yet men are almost never screened for bone density the way they are screened for prostate cancer.

Resting heart rate — RHR — is simply the number of times your heart beats per minute when your body is at complete rest. Not during exercise. Not after climbing a flight of stairs. At rest — ideally first thing in the morning, before you get out of bed, before coffee, before the day begins.

It is the baseline rhythm of your cardiovascular system. Like all baselines, it tells you far more when you track it over time than when you check it once.