Quick gut check: when a client says,

"My watch says I burned 600 calories, yay for me!"

Do you believe it?

I hope you do not!

That little wrist computer may be useful and even helpful in the right context…

…but the calorie number is where the wearable industry puts on a lab coat, grabs a fog machine, and starts doing magic tricks.

I just pulled the research together for my ISSN wearable tech talk, so the numbers are fresh in my head. Here is the uncomfortable part:

The metric clients obsess over most is the least trustworthy metric.

I am looking at you, Energy Expenditure, aka calories. That is a dumpster fire metric.

Don't believe me? I have receipts up the a$$ on it, and you better grab it with both hands, Senior.

Commercial wearables are generally poor for estimating energy expenditure, especially at the individual-client level (Bunn et al., 2018; Doherty et al., 2024; Fuller et al., 2020; Shei et al., 2022).

Errors commonly run 10-20% or more, and some models can miss daily energy expenditure by several hundreds of calories.

In some validations, daily errors land around 400 to more than 1,100 kcal depending on the device, context, and activity pattern (Chowdhury et al., 2017; O'Driscoll et al., 2020).

Yes, over 1,100 kcal. You are not even in the same state, much less the ballpark. That is the difference between "nice deficit" and "congrats, you accidentally did a bulk you crazy meat-stick."

Garmin has been measured underestimating energy expenditure in a big chunk of comparisons, and one Garmin Vivofit validation showed a mean absolute percentage error around 93% for activity energy expenditure versus a metabolic chamber (Murakami et al., 2016; Tedesco et al., 2019).

When researchers tested 12 devices against the real-deal methods — metabolic chamber plus doubly labeled water — not one device was equivalent for physical activity energy expenditure (Murakami et al., 2019).

Here is the extra kick in the teeth: error tends to get worse as activity level goes up.

Ah crap in a hole and call me Karl.

The harder your client trains, the more confident the device can look while being absolutely piss wrong.

Flip It

The boring metrics, the ones nobody flexes on Instagram, are often the ones with more coaching value.

Heart rate variability, measured overnight, can be useful. Oura sleep HRV has shown about 6-7% error against ECG with strong agreement, and WHOOP sleep HRV has also shown reasonable agreement in validation work (Dial et al., 2025; Stone et al., 2021). There are a few issues / context here that I go ballz deep in the HRV Education Course, but it is useful.

Context Wins Again

Random daytime HRV grabs are not the same thing. Apple Watch daytime HRV has been reported to underestimate HRV substantially in one validation (O'Grady et al., 2024). Overnight, stable window: useful. Random moment after coffee, email, traffic, and existential dread: worthless and often induces more stress.

Chest Strap For The Win

A Polar H10 is close enough to ECG for many HRV applications that I will gladly take it over wrist optical wizardry during movement (Schaffarczyk et al., 2022).

Respiratory Rate = The Sleeper Metric

It can respond quickly to metabolic, thermal, emotional, and cognitive stressors, and sleep respiratory rate from wearables like WHOOP has been validated against polysomnography with roughly breath-per-minute-scale error (Chinoy et al., 2021; Nicolò et al., 2020). I went super deep on RR in my private presentation to the coaches for F1 racing this past November. Yes, the F1 on Drive to Survive.

Take Home Points

Stop building nutrition decisions off the calorie readout. Use that number as a rough directional cartoon at best.

For recovery and readiness, watch overnight HRV trends, resting heart rate, respiratory rate, performance, mood, soreness, and what the athlete tells you with their actual human mouth.

Wearables are decision-support tools. Not tiny wrist prophets.

Bad inputs wreck good coaching.

Great dashboards still produce garbage decisions when the coach worships the wrong number.

Trust the boring metrics.

Question the flashy one.

I am working on something pretty cool related to this. More soon.

Much love,
Dr Mike

PS- The Flex Diet Cert opens Monday AM. The wearable stuff connects to it more than you might think: better coaching is not just knowing more physiology. It is knowing which inputs deserve your attention, which ones deserve the trash can, and what to do next.

References (aka Receipts Beeeotches)

Álvarez-García, J. A., Cvetkovic, B., & Luštrek, M. (2020). A survey on energy expenditure estimation using wearable devices. ACM Computing Surveys, 53, 1–35.

Bent, B., Goldstein, B. A., Kibbe, W., & Dunn, J. (2020). Investigating sources of inaccuracy in wearable optical heart rate sensors. NPJ Digital Medicine, 3.

Berryhill, S., Morton, C. J., Dean, A., Berryhill, A., Provencio-Dean, N., Patel, S. I., Estep, L., Combs, D., Mashaqi, S., Gerald, L. B., Krishnan, J. A., Parthasarathy, S., & Quan, S. F. (2020). Effect of wearables on sleep in healthy individuals: A randomized crossover trial and validation study. Journal of Clinical Sleep Medicine, 16(5), 775–783.

Buchheit, M. (2014). Monitoring training status with HR measures: Do all roads lead to Rome? Frontiers in Physiology, 5, Article 73.

Bunn, J., Navalta, J., Fountaine, C., & Reece, J. D. (2018). Current state of commercial wearable technology in physical activity monitoring 2015–2017. International Journal of Exercise Science, 11, 503–515.

Carrier, B., Chaves, S. M., & Navalta, J. W. (2025). Validation of aerobic capacity (VO₂max) and pulse oximetry in wearable technology. Sensors, 25(1), 275.

Carrier, B., Creer, A., Williams, L. R., Holmes, T. M., Jolley, B. D., Dahl, S., Weber, E., & Standifird, T. (2020). Validation of Garmin and Polar devices for continuous heart rate monitoring during common training movements in tactical populations. Journal for the Measurement of Physical Behaviour, 3(4), 331–337.

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Chowdhury, E. A., Western, M., Nightingale, T., Peacock, O., & Thompson, D. (2017). Assessment of laboratory and daily energy expenditure estimates from consumer multi-sensor physical activity monitors. PLOS ONE, 12.

Dial, M. B., Hollander, M. E., Vatne, E., Emerson, A. M., Edwards, N. A., & Hagen, J. A. (2025). Validation of nocturnal resting heart rate and heart rate variability in consumer wearables. Physiological Reports, 13.

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Murakami, H., Kawakami, R., Nakae, S., Nakata, Y., Ishikawa-Takata, K., Tanaka, S., & Miyachi, M. (2016). Accuracy of wearable devices for estimating total energy expenditure: Comparison with metabolic chamber and doubly labeled water method. JAMA Internal Medicine, 176(5), 702–703.

Murakami, H., Kawakami, R., Nakae, S., Yamada, Y., Nakata, Y., Ohkawara, K., Sasai, H., Ishikawa-Takata, K., Tanaka, S., & Miyachi, M. (2019). Accuracy of 12 wearable devices for estimating physical activity energy expenditure using a metabolic chamber and the doubly labeled water method: Validation study. JMIR mHealth and uHealth, 7.

Navalta, J. W., Carrier, B., Blank, M., Zarei, S., Davis, D. W., Craig, M., Perez, O., Baca, J., Sweder, T., Carballo, T., & Bovell, J. (2024). Validity and reliability of wearable technology devices during pickleball activity. Sports, 12(9), 234.

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Shei, R.-J., Holder, I., Oumsang, A. S., Paris, B. A., & Paris, H. L. (2022). Wearable activity trackers—advanced technology or advanced marketing? European Journal of Applied Physiology, 122, 1975–1990.

Stone, J., Ulman, H. K., Tran, K., Thompson, A. G., Halter, M. D., Ramadan, J., Stephenson, M., Finomore, V., Galster, S., Rezai, A., & Hagen, J. A. (2021). Assessing the accuracy of popular commercial technologies that measure resting heart rate and heart rate variability. Frontiers in Sports and Active Living, 3.

Tedesco, S., Sica, M., Ancillao, A., Timmons, S., Barton, J., & O'Flynn, B. (2019). Validity evaluation of the Fitbit Charge2 and the Garmin vivosmart HR+ in free-living environments in an older adult cohort. JMIR mHealth and uHealth, 7, e13084.

Van Oost, C. N., Masci, F., Malisse, A., Schyvens, A.-M., Peters, B., Dirix, H., Ross, V., Wets, G., Neven, A., Vandenbosch, J., & Aerts, J.-M. (2025). Accuracy of heart rate measurement under transient states: A validation study of wearables for real-life monitoring. Sensors, 25(20), 6319.

Mike T Nelson CISSN, CSCS, MSME, PhD
Associate Professor, Carrick Institute
Owner, Extreme Human Performance, LLC
Editorial Board Member, STRONG Fitness Mag

Mike T Nelson is a Ph.D. and not a physician or registered dietitian. The contents of this email should not be taken as medical advice. It is not intended to diagnose, treat, cure, or prevent any health problem - nor is it intended to replace the advice of a physician. Always consult your physician or qualified health professional on any matters regarding your health.

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