Where to find the hm strength in black and white?

In Pokémon Black and White, the HM Strength can be found in Route 12, right above Lentimas Town. When you use the Move Tutor option at Pokémon Centers, it will always be the sixth option.

To receive it, you must show the Strong Fist to the man leaning on the boulder that’s blocking the path. You can obtain this Fist by defeating the Gym Leader Elesa in Nimbasa City. Once he moves aside, you’ll be able to pick up a hidden TM containing Strength within the grass.