Same Effort, Very Different Thermal Profiles

Despite riding the same route under the same environmental conditions, the three riders exhibited substantially different core temperature, skin temperature, and heat strain responses. While the terrain largely dictated when thermal strain developed, individual physiology appeared to influence how much thermal strain accumulated.

The elevation profile helps explain many of the fluctuations in physiological strain. Across all three riders, core temperature generally increased during the major climbs and stabilized or decreased during descents. This consistent pattern suggests that changes in climbing intensity and opportunities for convective cooling were important drivers of thermal strain.

The first major rise in core temperature occurred on the Col de la Faucille (11.6 km at 6.3%), where Riders 1 and 3 peaked at approximately 39.5°C, compared with 38.9°C for Rider 2. Skin temperature also differed substantially, ranging from 32°C for Rider 2 to 35°C for Rider 1. This translated into a peak HSI of 5.3 for Rider 1 versus just 2.3 for Rider 2, despite similar heart rate responses.

The descent produced a sharp drop in skin temperature across all three riders, but core temperature responses varied. Rider 1 cooled by approximately 0.6°C, while Rider 3 showed little reduction, highlighting that the same cooling stimulus can produce very different physiological responses.

On the Côte de Lajoux, core temperature changed only modestly, yet HSI increased across all three riders as lower speeds reduced convective cooling and skin temperatures rose. The Col de la Savine produced another increase in core temperature, but Rider 1 again recorded a higher HSI than Rider 3 despite a lower core temperature, driven by higher skin temperature.

During the final 60 km, Riders 1 and 3 progressively accumulated thermal strain while Rider 2 remained comparatively cooler. Overall, the pattern was consistent across riders: climbs increased thermal strain and descents provided cooling opportunities. What differed was the magnitude of the response, suggesting that terrain largely determined when thermal strain developed, while individual physiology influenced how much accumulated.

The thermal zone data further illustrate this variability. Perhaps the most striking comparison is between Riders 1 and 3. Although they finished in the same group, Rider 3 accumulated 118 minutes in Zone 3, compared with 72 minutes for Rider 1. Rider 2, meanwhile, spent the entire stage in Zones 1 and 2 and accumulated no time in Zone 3. 

The key takeaway is not that some riders worked harder than others. Rather, these data demonstrate that athletes exposed to the same course, weather conditions, and race dynamics can experience markedly different thermal loads. 

The climbs largely dictated when thermal strain developed, but individual physiology appeared to influence how much thermal strain accumulated. Factors such as metabolic heat production, sweat rate, skin blood flow, body size, cooling efficiency, race positioning, and pacing strategy may all contribute to these differences. Furthermore, individuality is even more exaggerated among women, who experience more variability in their physiology throughout the month.

An important implication is that heat strain should be interpreted primarily within the context of the individual athlete. Some riders may tolerate and perform well under thermal conditions that would represent considerably greater strain for another athlete. For this reason, heat monitoring is often most valuable when tracking changes within an athlete over time rather than directly comparing athletes against one another.

Stage 3 of the Tour de France Femmes provides a compelling example of why heat strain should be viewed as an individual response rather than simply an environmental condition. Three riders raced the same 156.5 km course in temperatures averaging 32°C, and Riders 1 and 3 even finished in the same group. Yet one rider spent almost two hours in Heat Zone 3, while another never entered it at all. 

The lesson is clear: weather and terrain influence thermal strain, but they do not determine it. The same race can create very different physiological challenges for different athletes. Understanding how each athlete responds to the heat may therefore be more valuable than relying on environmental conditions alone. Same race. Similar demands. Very different thermal profiles.