In the past couple decades, sports science has made giant strides on every front — conditioning, nutrition, recovery, equipment, surgery, injury prevention. But all those gains come with a big asterisk: a reminder that individual results may vary. Some athletes respond seemingly overnight to high-intensity interval training; some see middling results. Some enjoy amazing recoveries from knee-ligament reconstructions; others never really heal. For athletes who feel like they’re doing everything right, this seeming arbitrariness can be discouraging and infuriating.