A movement made by an offensive player in which he extends his arm to ward off a defending player attempting to tackle him. A player can use a stiff arm to break a tackle or gain extra yardage. A particularly good stiff arm will actually knock over the defending player, allowing the ball carrier to extend the play considerably. Stronger players will be more effective at using the stiff arm.
The stiff arm is a classic part of a great running back's arsenal of tricks. Its goal is to ward off a defender, and it can be effective even if it does not knock the player over because it limits the defender's ability to get his hands on the ball carrier and make an effective tackle. Generally, the longer a player can keep the defender from wrapping him up, the more time he will have with his legs free to gain more yards. A highlight reel stiff arm will knock over the defender, allowing a free run down the field. Defensive players can counter the stiff arm by either fighting through it or grabbing the offensive player's arm and trying to swing him to the ground with it.