I ran track at
Eastern Illinois and we had several football players every year on our team and they were great sprinters and hurdlers. Many of them were national-caliber track athletes, and
at least one of them went on to play in the NFL. We also had a wide receiver who was one of the fastest sprinters in the world and played in the CFL and XFL. He was lightening-fast, but also very short, which kept him from making it in the NFL. He was a Florida high school 100-meter track champion, but didn't get recruited by major college football programs because of his size.
It's a great way for them to stay in shape in the off-season. It was always funny watching the football players the first week of track practice after switching over from football, lying like dead people on the side of the track, because "football-shape" and track shape are two completely different things