Go to Nimbasa City, go to the building above the battle house, which is next to the Pokemon Center, and ask the blue-haired person on the 1st floor. After hearing his talking, he'll give you HM Strength.
you get hm strength and hm defog after defeating two gym leader.Fantina(defog) Byron(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.
Using Rock Smash move, break the rocks inside Rusturf Tunnel and the guy there will give you HM Strength.
You can get the HM Strength at Nimbasa City. One of the buildings behind the Poke Center will have a man with blue hair and will talk to you about the HM Strength and will give you it.
its hm
I believe you need to have the HM Strength
once you earn a certain amount of gym badges you will be able to teach a special move to your Pokemon( which will show up in HMS and tms part of bag).
With the HM move strength.
A boy gives it to you in a building in Nimbasa City
a guy in nimbasa city
Somewhere on route 18
You receive HM 3 from the Gentleman in Ecruteak Dance Theater after you save the Kimono Girl from a Team Rocket Grunt.