MIAMI, June ⁠17 (Reuters) - Players arriving in Miami for World Cup matches may find themselves battling more than just opposing teams, as sweltering heat ⁠and humidity threaten to turn the tournament into a test of endurance as much as skill.

Teams from cooler climates face ‌particular risks if they have not had time to acclimatise to Miami's punishing conditions, warned Dr Jason Kostrna, an assistant professor and graduate kinesiology program leader at Florida International University. Kinesiology is the study of movement, exercise, and physical performance.

"Unfortunately here in Miami, it's very humid and so when you go and exercise, you start dripping sweat," Kostrna ​told Reuters.

"That means that you are not evaporating it. It's just rolling off you ⁠and you are not getting that heat loss."

The physiological battle ⁠is intense. As players' bodies struggle to cool down, their hearts work overtime pumping blood to the skin in a desperate attempt to ⁠release ‌heat through evaporation -- a process that fails in Miami's sticky air.

"Then when it can't, it starts to really try and increase that heart rate," Kostrna explained.

Kostrna said the heat stress compounds the already enormous pressure of representing one's country, creating an added burden that ⁠affects both physical performance and mental sharpness.