Football is a tough game that is extremely demanding on the body. Even when there is no contact, the sharp cuts and sudden starts and stops all take a toll. Look at David Boston. He is out for the season after a non-contact injury. Even though Boston has been the poster-boy for excessive muscles and possible steroid effect, he isn't the only one who has suffered a season ending injury already during the first week of camp. To prepare their bodies for the NFL season, players have to practice at full speed and push their bodies to do things that normal human beings just can't do.
All of that has an effect on the body.