Half the wrists in our studio wear a fitness watch, and we think that is mostly great. Anything that makes movement visible tends to create more of it. But a watch is a salesman as much as a scientist, and somewhere between the step count and the "body battery," it starts offering numbers it cannot actually back up.
Knowing which numbers deserve your trust, and which deserve a polite nod, saves you from steering your health by instruments that are guessing.
What does your watch measure well?
The trustworthy list is real and useful:
- Steps and general movement. Not perfect, but consistent, and consistency is what matters for a trend. If your watch says this week was 30 percent less active than last week, believe it.
- Heart rate at rest and during steady cardio. Wrist sensors have gotten genuinely good here, especially at lower intensities. A resting heart rate that drifts down over months of training is a real signal of improving fitness.
- Sleep duration. Watches are decent at knowing when you were asleep, so total sleep time trends are worth watching, even if the "sleep stage" breakdown is closer to astrology.
- Reminders and streaks. The psychology works. The nudge to stand, move, or close a ring creates real behavior, and real behavior compounds.
Where does it start making things up?
Three places, and they happen to be the three numbers people quote most confidently:
- Calories burned. This is the big one. Across validation studies, wrist devices routinely miss energy expenditure by 20 to 40 percent or more, and strength training confuses them completely: a hard session of heavy sets registers as barely more than a brisk walk, because your wrist was not moving much. Eating back the calories your watch says you earned is one of the most common ways tracked people gain fat.
- Heart rate during lifting and intervals. Grip pressure and wrist movement interfere with the sensor exactly when things get intense. That 112 reading mid-set is often fiction.
- Recovery and readiness scores. These blend estimates of estimates into a single confident number. Treat them as a conversation starter, not a verdict. You do not need a score to know whether you feel recovered, and the score regularly disagrees with reality anyway.
Why does this matter more after 40?
Because after 40 the goal quietly changes, and the watch does not know it. A fitness watch is fundamentally a cardio accountant: steps, minutes, beats, calories. But the project of this decade is body composition: keeping and building muscle while managing fat, something we laid out in our post on muscle mass after 40. Your watch cannot see muscle. It will cheerfully award a better score to a week of easy walking than to the three strength sessions that are actually changing your trajectory.
Steer by the watch alone and you will drift toward what it can count: more steps, more cardio minutes, more rings. Pleasant, healthy, and incomplete. The measurements that reflect the real mission live elsewhere: an InBody scan for what you are made of, and a training log for what you can do, and a VO2 max estimate taken with appropriate salt.
How should you actually use the watch?
Use it for trends, not verdicts. Direction of travel across weeks is where wearables shine; absolute numbers are where they wobble. Resting heart rate drifting down: real and good. Sleep duration drifting up: real and good. Calorie total on a Tuesday: decorative.
And let the humans handle what the sensors cannot. Your coach watches your actual bar speed, your actual form, your actual energy across a month of sessions. The monthly scan reports what the training built. Between the two, there is not much left for the wrist to adjudicate.
What should you measure instead?
Three things, none of which live on your wrist: your body composition, measured monthly; your strength, logged across sessions; and how you feel climbing stairs, carrying groceries, and getting off the floor. Those numbers move slowly and honestly, which makes them worth steering by.
If you want a baseline that actually means something, your first visit to our Ahwatukee Foothills studio includes exactly that: a Starting Point Session with an InBody scan, a movement assessment, and an introductory coached workout. Keep the watch; we like the reminders too. Book Your Complimentary VIP Consultation and come measure the things it cannot see.