U.S. NATIONAL TEAM TRAINING CENTER (Fayetteville, Ga.) – The epiphany came to Emma Hayes about a decade ago. She was well into her coaching tenure at Chelsea and her team had just lost to rival Arsenal, 1-0, in the 2016 FA Cup final at Wembley Stadium. When Hayes talks about this moment, she explains the Gunners deserved to win that game. What she couldn’t see at the time – but very clearly understands now – is why her players were so off that day. Their reaction times were slower than normal and their coordination was like "watching a group of people that looked separated from their bodies,"...