Then there’s the wear and tear that water polo players’ bodies endure. For starters, the repetitive motions of throwing the ball while treading water puts “a lot of stress” on one’s upper extremities, Dr. Wang wrote. That stress can lead to injuries such has “bursitis or tendinopathy of the rotator cuff muscles, tendonitis of the biceps, shoulder impingement, and elbow ligament, cartilage, and impingement injuries,” he added.
Meanwhile, the act of treading water impacts the muscles of the thigh and the knee, and this “egg-beater” kicking motion can lead to “muscle strains and tendon injuries, patellofemoral pain syndrome, and knee ligament and meniscus injuries,” Dr. Wang told readers.
Source : https://www.distractify.com/p/why-do-they-wear-hats-in-water-polo