It’s the literal Achilles heel of football players. Called “the new ACL” by one former team doctor, a ruptured or torn Achilles tendon is becoming the most dreaded injury for the pro athlete. In football, it can be career-ending. From 2011-2015, 46 players reported a torn Achilles and 24 of them retired shortly thereafter or never played professionally again. Doctors say it’s a matter of scale: As athletes get larger and stronger, their tendons and ligaments stay about the same, becoming physiological weak points — and disasters waiting to happen.