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Evidence Against A 40°C Core Temperature Threshold For Fatigue In Humans

Brett R. Ely, Matthew R. Ely, Samuel N. Cheuvront, Robert W. Kenefick, David W. DeGroot, Scott J. Montain

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Evidence suggests that core temperatures of ∼40°C can induce fatigue, although this may be confounded by coincident elevations in skin temperatures and maximal cardiovascular strain. In an observational field study to examine core temperature threshold for fatigue, we investigated whether running performance is impaired when rectal temperature (Tre) is >40°C and skin temperature remains modest. Seventeen competitive runners (7/10 women/men: 8 km best 1,759 ± 78/1,531 ± 60 s) completed 8-km track time trials in cool (WBGT ∼13°C; n = 6), warm (WBGT ∼27°C; n = 4), or both ( n = 7) conditions. Tre, chest skin temperature, and heart rate were logged continuously; elapsed time was recorded every 200 m. Running velocity for Tre >40°C was compared with that for Tre <40°C for each runner. Changes in running velocity over the last 600 m were compared between runners with Tre >40°C and <40°C. Twelve runners achieved Tre >40.0°C with ≥600 m remaining (range 600–3,400 m). Average running velocity for Tre <40°C (282 ± 27 m/min) was not different from that for Tre >40°C (279 ± 28 m/min; P = 0.82). There were no differences in running velocity during the final 600 m between runners with final Tre >40°C or <40°C ( P = 0.16). Chest skin temperature ranged from 30 to 34°C, and heart rate was >95% of age-predicted maximum. Our observation that runners were able to sustain running velocity despite Tre >40°C is evidence against 40°C representing a “critical” core temperature limit to performance.