Breaststroke is a series of movements or the legs and arms, as well as butterfly. They both use multiple different muscles.
Butterfly uses your chest muscles to launch your body out of the water and your hamstrings and quads to mimic a dolphin quick, in of and on intervals. It also uses your neck muscles to lift your head out of the water to breathe.
Breaststroke uses your shoulder and back muscles by making a circular movement and up the middle of the circle. It also uses your thighs and quads to create a frog-like motion of making a circle and snapping your legs together.
(These answers come from me, a swimmer since the age of 3 just fyi)
