Strength is a Normal type move and in all the games it is HM04.
D/P: The foe is slugged with a punch thrown at maximum power. It can also be used to move boulders. In Pokemon Black/White, it is located in Nimbasa City two houses to the west of the Battle Subway.
Nimbasa City, go to the buildings behind the Pokemon center, and in the house to the far left, on the first floor the guy in the back left hand corner has it.
Strength being used in the field