Why do baseball players wear that one sleeve under their jersey?
A lot of players now wear “compression” sleeves. They’re similar to the tight-fitting undershirts made by companies like “Under Armour,” but give you the option of just wearing it where you feel you need it. They replace the bulky old Ace bandages that players used to wear on their joints and muscles to give a little added support and warmth. The theory is that the light, evenly distributed pressure applied by these sleeves helps sore joints and muscles. Many players today also wear compression sleeves that are impregnated with ionized metals (the most popular ones are made by Phiten, the guys who make those cloth necklaces worn by Beckett and all the Japanese players). In addition to pressure support, these sleeves supposedly improve blood circulation by affecting the body’s natural electrical field.