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 unco