Cardinals nose tackle Gabe Watson once seriously injured a knee falling off a treadmill.
It is possible, then, for NFL players to suffer freak injuries off the field.
But with so much focus on which defensive players chose not to attend the 49ers' voluntary organized team activities Monday -- cornerbacks Nate Clements and Shawntae Spencer led the way -- let's remember what happened when one seasoned vet graced the team with his presence last offseason.
Walt Harris, who didn't need May practice reps, suffered a knee injury that ended his season and possibly his career. Defensive end Kentwan Balmer and receiver Jason Hill also suffered injuries during voluntary practices last May.
Having veteran incumbent starters on the field in August and especially September means much more than having them on the field in May.