A polo player must have great control not only of his body, but also over his horse. The two must be able to turn together on a dime. In the ancient game of polo—one of the oldest-known team sports, originally crafted as a mock battle for training cavalry—the speed is fast, the strategy is sharp, and the maneuvers are precise. With horse power driving the action, the players’ lives are at stake. “People don’t understand … that we literally put our lives on the line every time we step out on the polo field,” said Nic Roldan, current captain of the U.S. national polo team. …