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After a month of running with the CORE 2 during a UK heatwave, Nick Harris-Fry found that its estimates offered useful training feedback—but they were not a clinical temperature measurement or a dependable standalone safety signal. His experience also showed why core and skin temperature readings can tell different stories, and why shade and pace mattered on hot runs.
What a month with the CORE 2 looked like
In a Tom’s Guide account published on August 20, 2026, runner Nick Harris-Fry described wearing the CORE 2 Thermal Sensor every day for a month during a UK heatwave. He attached it to a heart-rate chest strap and viewed readings in the app and through a Garmin Forerunner 970 data field. The observations below are his experience with that setup, not a controlled test. The device’s output is an estimate, not a direct clinical measurement.
Five things the sensor taught him
1. Core and skin readings can move in opposite directions
On one 15-mile run, Harris-Fry reported that his estimated core-temperature trace rose while the skin-temperature trace fell. He attributed the skin fluctuations to sweat and wind cooling. That is an observation from one run and one device setup, not a rule about how every wearable or runner will behave. A falling skin reading should not be taken to mean the body’s internal temperature is also falling.
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After an afternoon track workout, the sensor displayed a peak estimate of 101.8°F (38.7°C). Harris-Fry said it took 50 minutes after the workout for the reading to return to his usual level. On a separate easy run in weather of about 95°F (35°C), the display went above 100°F (37.7°C). These are his device readings; they are not clinical temperature measurements or a prediction of what another runner will experience.
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3. Shade coincided with a lower heat-strain reading
On a day around 95°F (35°C), Harris-Fry reported that the estimate rose on an exposed road, then plateaued after he entered forest shade—even though he felt he was working harder on undulating trails. In another personal route comparison, he remained in the device’s lowest heat-strain zone during an hour in forest conditions around 85°F (29.4°C). Because this was not a controlled comparison, it cannot isolate shade from changes in route, effort, wind, or other conditions.
4. Easy runs sometimes called for an easier pace
Harris-Fry found the display useful for noticing when easy runs moved above the lowest heat zone, particularly when air temperatures exceeded 90°F (32.2°C). He responded by slowing down. That describes his own training choice; it is not a pace prescription for other runners, and a device zone should not override how you feel or established heat-safety guidance.
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5. Daily running did not automatically raise the app’s acclimation score
Despite running every day, Harris-Fry said the CORE app continued to show “Thermal Rookie” and “4% acclimatized.” He favored shade and earlier runs, and reported that workouts directly in the sun moved the app’s acclimation metric more. The percentage is an app-reported value, not a validated measurement of physiological acclimation.
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How independent validation changes the interpretation
Studies comparing CORE with reference temperature methods have not established it as an interchangeable thermometer. Results differ by activity, conditions, and study, so a result from cycling or field hockey cannot simply be applied to a road run.
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| Study and protocol | What the comparison found | What it means for a runner |
|---|---|---|
| McLaughlin et al., 2025: 24 healthy adults cycled for 60 minutes at 60% of maximum aerobic power in 35.9°C and 20.7% relative humidity; CORE was compared with rectal temperature. | Across the full bout, the mean difference was a non-significant overestimate of 0.15 ± 0.43°C. The overestimate was 0.31 ± 0.43°C in the third quarter and 0.36 ± 0.45°C in the fourth. CORE also overestimated skin temperature by 1.28 ± 0.96°C versus a surface probe. | This is a small cycling study, not a running validation. Its authors noted that skin-temperature overestimation and local subcutaneous fat may contribute to error. |
| 2021 cycling validation study: duplicate exercise trials compared CORE with rectal temperature. | Readings showed acceptable repeatability across duplicate trials, but agreement with rectal temperature was poor; about half of paired measurements fell outside the study’s predefined ±0.3°C validity threshold. | Repeatability does not guarantee accuracy: a device can produce similar readings under repeated conditions without closely matching a reference temperature. |
| Goods et al., 2023: elite women’s field hockey players during a heat-training camp; CORE was compared with an ingestible telemetric temperature pill. | Agreement was poor, with 41–60% of samples exceeding ±0.3°C disagreement. The researchers did not support CORE as a valid alternative for core-temperature assessment during hot exercise. | This team-sport study raises a caution about using the estimate as a substitute for a reference method, but it does not establish the exact error for every runner or setup. |
A separate 2022 study tested a different upper-chest dual-heat-flux patch during low-intensity walking and arm-cranking in heat. It is not evidence for CORE or the CORE 2. A 2023 paper examined CORE against gastrointestinal temperature during cold-water ingestion and a 5 km running time trial in heat, but detailed results are not available here, so no numerical conclusion from that study is asserted.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What the readings are—and are not—useful for
Harris-Fry’s month suggests how a wearable estimate may prompt reflection on route, effort, and recovery. A trend can be one input to training decisions, especially when considered alongside pace, perceived effort, conditions, and how you feel. It cannot establish that a runner is safe to continue, diagnose heat illness, or replace a medical thermometer or appropriate clinical assessment.
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- Use it as contextual feedback: Treat the displayed number and heat-strain zone as estimates from a wearable, not confirmed body-temperature values.
- Keep the setup and conditions in mind: Device version, fit, activity, weather, and reference method all affect how validation findings apply. Results from cycling or team-sport protocols do not prove accuracy during your run.
- Do not rely on it as a safety device: Do not use a reassuring reading to dismiss concerning symptoms or to decide that it is safe to push on in dangerous heat.
- Interpret acclimation scores cautiously: The app’s percentage is a device metric; Harris-Fry’s 4% reading does not establish his physiological acclimation level.
Sources
- Nick Harris-Fry, Tom’s Guide, August 20, 2026
- McLaughlin et al., Journal of Thermal Biology, 2025
- 2021 CORE cycling validation study
- Goods et al., European Journal of Sport Science, 2023
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