A football player standing over a penalty in a hot, high-altitude stadium is dealing with more than pressure. His body is trying to keep cool. His heart and breathing may be working harder. Less oxygen is reaching his muscles. One poor decision can end his team’s World Cup.

The 2026 men’s World Cup has made fatigue harder to ignore. Some matches are being played in heat and humidity, while Mexico City’s Azteca Stadium sits more than 2,200 metres above sea level. Heat and altitude make sport uncomfortable, and they also change how the body and mind work under pressure.

Heat makes the body work harder to keep its core temperature stable. Humidity adds strain because sweat does not evaporate as easily, making it harder to cool down. At altitude, lower air pressure means less oxygen reaches the blood and muscles. Together, these conditions can affect endurance, recovery between sprints, concentration and decision-making.

Fatigue is not one state. Sport science is good at separating different kinds of fatigue because performance depends on knowing what is going wrong. Our research emphasises this point. Is the athlete slowing because muscles are tired, heart rate is high, body temperature is rising, sleep has been poor or concentration is slipping?

The answer changes the response. Heavy legs may call for pacing, which means slowing down or spreading effort so the body can cope. Fluid loss may call for cooling and replacing what has been lost through sweat. Slipping concentration may call for a mental reset, such as slowing breathing or........

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