The CORE Performance Team Battles the Heat of Kona

This increased thermal load is clear when analyzing the data from 21 CORE athletes who participated in this year’s race. On average, the athletes spent 91% of the race in Heat Zones 2 and 3, indicating sustained moderate-to-high thermal strain throughout the day. Nearly half of race time was spent in Heat Zone 3, where performance impacts become increasingly likely and athletes often begin to experience greater cardiovascular and thermal stress. 

In a hot, long-distance course like this, athletes want to execute responsible thermal pacing by avoiding early peaks in Heat Strain Index (HSI). The athletes executed this strategy exceptionally well, showing a gradual and controlled rise in HSI throughout the race. This pattern reflects disciplined thermal pacing and helped prevent excessive heat accumulation early in the day. Even so, the marathon ultimately became a battle against rising thermal strain. While HSI continued to increase, average heart rate gradually declined during the later stages of the race. This suggests that many athletes reduced intensity, incorporated cooling breaks, or adjusted their pacing as heat stress and fatigue accumulated. The fluctuations in skin temperature observed during the run further highlight the effectiveness of cooling.

The group’s data show just how gruesome the Hawaiian island’s conditions can be. Even with extensive heat preparation and textbook thermal pacing, athletes spent the vast majority of the race under meaningful thermal strain. Even among the world’s strongest athletes, Kona continues to push thermoregulation and performance close to their limits.

Even with excellent preparation, athletes cannot eliminate the effects of Kona's heat. But by monitoring core temperature and thermal strain, they can make smarter pacing and cooling decisions that help them stay in control for longer and perform at their best when conditions are at their toughest.