Go to Nimbasa City's Pokemon Center entrance, then go to your right, five steps exactly, and then head up.
Once you've gone up there will be another street on your left, turn there and go to the final house right next to the water and light post.
In the house there will be two people sitting at a long table, and then a blue haired Ace Trainer sitting in the back. Go speak with him and he will give you the HM Strength
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you get hm strength and hm defog after defeating two gym leader.Fantina(defog) Byron(strength).
With the HM move strength.
If you have beaten Mauville Gym and have gotten HM ROCK SMASH, go to RUSTURF TUNNEL and clear out the rocks. The digger will grant you HM STRENGTH.
HM STRENGTH HM STRENGTH
HM04 Strength is found in Nimbasa City in the top-left building.