A stifff arm is the name of a move a football player does to defend the football (it's always the player that's running with the ball). While he has the ball in one hand, the other arm sticks out and stiffens, hence the name stiff arm. By doing so, it appears as if he is commanding the person to stop, and this creates distance between the ball and the defender, making it harder for the defender to get the ball.
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A stiffarm would involve forearm, triceps (obviously), shoulders, sometimes the hips and chest.
Also, astiff arm can have more force when running at full speed.