When the Heat Rises, So Does the Load, And It’s Different for Every Player

Every coach knows a hot session feels harder. What’s less obvious is that the same session in the same heat puts wildly different strain on different players.

That’s the case for physiological load monitoring, and it’s the reason that relying on external metrics alone can leave your most at-risk athletes invisible.

The Trap: When External Load Falls, Internal Load Climbs

During the 2026 World Cup, nearly one in five of the tournament’s 100-plus matches took place in levels of heat and humidity that a football players’ union has previously said should trigger delays or postponement.

As climate change continues to warm the planet, extreme heat will have a greater impact on tournaments and leagues, and as temperatures climb on the training ground or in the stadium, players slow down.

Looking at GPS metrics alone, a hot session could appear easier than a cool one. Distance drops, and high-intensity running fades, particularly in the closing stages.

However, internally, the opposite is happening. Players downshift precisely to protect their bodies.

Recent match research shows that players reduce running specifically to lower core temperature [3]. Heart rate rises, thermal strain builds, and perceived effort increases even at lower speeds. The load hasn’t disappeared; it has simply moved somewhere your movement-tracking can’t see it.

This is the single most important finding for anyone monitoring training in the heat: reduced external load does not mean reduced physiological strain [2]. A falling distance number is often the symptom of heat strain, not evidence that a player is coasting.

What the Research Shows

Heat’s effect on the body is well documented in elite football:

In experimental matches at roughly 43 °C versus 21 °C, players’ core and muscle temperatures ran about 1 °C higher, and muscle temperature in some individuals exceeded 41 °C by the final whistle [1].

Even in moderately hot conditions around 30 °C, high-intensity running dropped sharply in the last 15 minutes of a match as fatigue set in [1]. Core body temperature routinely reaches 39–40 °C during soccer, and heat exposure has been shown to impair decision-making, cut running distance, and raise physiological strain such as heart rate and core temperature [4].

These aren’t marginal effects. They change how a session should be programmed, who should be rotated, and when recovery needs to be triggered.

Why an Individualized Approach is Essential

A 2025 study of match play in the heat found that heat strain is largely a function of individual metabolic demand, so players carrying higher running loads experience greater strain and may need individualized cooling and hydration strategies [3].

Two players in the same drill, in the same weather, can sit in completely different physiological zones. Fitness, acclimatization, body composition, playing position, and role in the session all have an influence on an individual’s response.

The players under the greatest physiological strain are rarely the easiest to spot. Group averages and GPS metrics can hide them, but individual heart rate monitoring reveals who is working hardest internally.

Why Monitor Internal Responses? The Practical Case

Tracking internal, heart-rate-based physiological load in the heat allows coaches and performance staff to:

1. See the load that GPS hides when players downshift; internal load captures the strain that distance metrics alone miss. Coaches can monitor athletes’ internal load in real time with Firstbeat, making it easier to identify individuals who are working significantly harder than teammates completing the same drill.
2. Manage per player, not per squad, and spot the individual who’s redlining while the group average still looks comfortable. Using historical athlete comparisons allows coaches to see how today’s physiological response compares with previous sessions, making it easier to identify athletes whose response is significantly higher than normal despite completing similar work.
3. Make live decisions based on objective data, adjust intensity, rotate, or trigger recovery based on what the heat is actually doing to each body, rather than guesswork.
4. Protect health, not just performance. The gap between “hard session” and “heat illness risk” is individual, and physiology is where that line shows up first.

Monitoring athletes shouldn’t stop when the session ends. Heart Rate Recovery provides coaches with another valuable indicator of how athletes are coping with heat stress. Following demanding sessions in hot conditions, a slower heart rate recovery may indicate elevated physiological strain, helping coaches identify players who could benefit from additional cooling, recovery strategies, or a modified workload in the following sessions.

Heart Rate Recovery in Firstbeat Sports Cloud: First values are recorded for each athlete for the entire session, followed by average values for each period, and finally, averages for the last 10 minutes of each period. Additionally, it’s possible to analyze the individual data of each athlete for each period.

The effects of heat can also carry over into the next day. Using a tool such as the Firstbeat Quick Recovery Test, coaches can assess each athlete’s recovery status before training, making it easier to identify players who may still be experiencing elevated physiological stress and adjust training plans accordingly.

QRT results give you immediate on-screen feedback on athlete readiness for more informed coaching decisions and training plan validation. There are four categories for the results: 0-30 Poor: Recommended to conduct an Overnight Recovery Test to confirm the athlete’s recovery status and to make adjustments to their training program. 30-70 Moderate: For moderate results, also check historic recovery progress data in the Firstbeat Sports Cloud. Athletes with decreasing recovery trends should be observed. 70-90 Good: Training can continue as normal. 90-100 Excellent: Training can continue as normal.

Turning Insight into Action

The challenge isn’t simply recognising that heat increases physiological strain. It’s identifying which players are experiencing the greatest strain before performance drops or heat fatigue becomes a concern.

That’s why combining external and internal load is so valuable. GPS tells you what the player did. Physiological monitoring tells you what it cost them.

During hot training sessions or matches, objective heart rate-based metrics can help coaches identify athletes whose bodies are working disproportionately hard, even when their running output appears to be falling. Those insights can support decisions around rotation, hydration, cooling strategies, recovery, or simply adjusting an individual’s workload before fatigue accumulates.

As hotter playing conditions become increasingly common across elite football, understanding the physiological cost of training and matches is just as important as measuring the physical work itself.

Understand How Every Player Responds Differently to the Heat

Monitor individual physiological responses, identify athletes under the greatest strain, and adjust training with confidence using Firstbeat Sports.

Learn More About Firstbeat Sports

References

1. Mohr M, Nybo L, Grantham J, Racinais S. Physiological Responses and Physical

Performance during Football in the Heat. PLoS One. 2012;7(6):e39202.

https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0039202

2. Monitoring external and internal training and match loads in professional soccer players

during excessive heat stress. Human Movement. 2025.

https://hummov.awf.wroc.pl/Monitoring-external-and-internal-training-and-match

loads-in-professional-soccer,199361,0,2.html

3. Core temperature responses of male football players during matches in the heat —

Associations with physiological and running demands. Journal of Science and Medicine

in Sport. 2025.

https://www.sciencedirect.com/science/article/pii/S1440244025005377

4. The Impact of Heat Exposure on the Health and Performance of Soccer Players: A

Narrative Review and Bibliometric Analysis. Sports (Basel). 2024;12(9):249.

https://www.mdpi.com/2075-4663/12/9/249

5. Extreme heat at the World Cup: are FIFA’s safeguards enough? The Conversation. 2026.

https://theconversation.com/extreme-heat-at-the-world-cup-are-fifas-safeguards

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