Trackers wildly overcount

A wrist tracker can count your pulse like a decent speedometer, but calorie burn is more like estimating gas used from the shake of the steering wheel. In a Stanford study of 7 popular devices, none measured energy expenditure accurately, even though 6 of 7 kept heart-rate error under 5 percent. (med.stanford.edu) The same Stanford team tested 60 volunteers on treadmills and stationary bikes while they wore an Apple Watch, Fitbit Surge, Samsung Gear S2, Microsoft Band, Basis Peak, Mio Alpha 2, and PulseOn. The best device still missed calorie burn by an average of 27 percent, and the worst missed by 93 percent. (med.stanford.edu) That gap exists because heart rate is one signal, not a fuel meter. Stanford engineers later noted that coffee can raise heart rate and wrist motion can look busy even when energy use is modest, which is why smartwatches often miss the real number. (news.stanford.edu) In 2021, another Stanford group built a different wearable with 2 small leg sensors because legs reveal work more directly than wrists during walking, running, stair climbing, and biking. That system cut cumulative error to 13 percent, versus 42 percent for a smartwatch and 44 percent for an activity-specific smartwatch in the same paper. (nature.com) The practical problem shows up when people “eat back” exercise calories. If your watch says 600 calories and the true number is closer to 400, a post-workout muffin can erase the deficit you thought you created. (med.stanford.edu) That does not make wearables useless. Mayo Clinic says the most trustworthy features are usually the simplest ones, and step counts tend to hold up better than the more ambitious estimates like calories, deep sleep, or training readiness. (mcpress.mayoclinic.org) This is why many coaches fall back on rate of perceived exertion, which means how hard the effort feels in your own body. The American College of Sports Medicine says the classic Borg scale runs from 6 to 20 and tracks exercise intensity without needing a watch at all. (acsm.org) A moderate session is supposed to feel moderate, not like a sprint hidden behind a reassuring dashboard. The American College of Sports Medicine’s public guidance also uses a 0 to 10 effort scale, with moderate work around 3 to 4 and vigorous work around 5 and up. (acsm.org) Public-health targets are already written in effort, not calories. The Centers for Disease Control and Prevention says adults should get 150 minutes of moderate activity a week or 75 minutes of vigorous activity, plus muscle-strengthening work on 2 days. (cdc.gov) So the safer use for a tracker is trend-spotting, not precision dieting. If your resting heart rate drops, your weekly minutes rise, and your step count climbs, the device is helping even if its calorie stamp is closer to a guess than a receipt. (med.stanford.edu)