stiff arm

Question
My understanding is that it is legal for a receiver or running back with the ball to fend off a would be tackler by putting his hand in the face of the tackler. Are there any rules on how hard or how fast this stiff arm can be delivered? What is to stop someone from studying martial arts and just nailing the tackler. This would be especially effective with a 250lb running back nailing a 170lb corner back. Thank you for considering this, Rick

Answer
The rule simply states: "A runner may ward off opponents with his hands and arms," without any stipulation about how hard.  BTW, really good stiff-arm runners (Corey Dillon is one of the best for example) do nail DB, but it is with proper techinque, literally a stiff arm (not bent) and the force coming from their legs.  I don't think there is a martial art that teaches you how to do magic chi stiff-arms while running.  When you are running you can't get the kind of hip rotation that a boxer, fighter, golfer, batter, etc needs to produce a ton of force - it's a matter of kinetics.