With any given players anatomy being capable of handling only so much, being that the skeleton tendons and ligaments load bearing capacity is not infinite, muscle strength loading/forces while impacting the playing surface or other players will at some point be greater than the structure can endure, and then something will give out, Only meticulous training methods can mitigate the eventual injury, be it minor or major, the NFL is an example of extremes in human physiology, with players pushing the proverbial envelope with strength training we will continue to see a fair number of injuries, is what it is.