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Deals damage and lowers the user's Speed by one stage. Damage is boosted to 1.2x by the ability Iron Fist.
While Hammer Arm may have a debilitating side effect and poor distribution, it is a great move for wallbreaking. Common walls such as Ferrothorn and Blissey are hit super effectively by Hammer Arm, 2HKOing them in most cases. Pokemon that usually run special attacks are great choices for the move. With this move, Tornadus can lure in Blissey and Ferrothorn and come out on top. Conkeldurr and Emboar can use it as a reliable STAB move, as they don't care too much about the Speed drop. Rhyperior and Metagross can use it for coverage, though they usually prefer Earthquake.
Brick Break is more accurate and does not lower Speed, but is significantly weaker.
Drain Punch is more accurate, does not lower Speed, and heals the user, but is significantly weaker.
Superpower is more accurate and stronger, but lowers Attack and Defense instead of Speed.
Close Combat is more accurate and stronger, but lowers Defense and Special Defense instead of Speed.
Hi Jump Kick is more accurate and much stronger, but causes recoil damage if it misses.