In Madden 21, there are a few different ways to stiff arm your opponent. One way is to use your left hand while you hold your right hand out in front of you. This will cause the opponent to think you are trying to block them. Another way is to use your right hand while you hold your left hand out in front of you. This will cause the opponent to think you are trying to pass them. Finally, one can stiff arm by using both hands and then using the elbow and shoulder as well as the palm of their hand.
