Breaststroke Muscles: The muscles used are about the same for breaststroke and butterfly as freestyle. It is just a slightly different group of primary muscles used. The primary muscles used in breaststroke are the hand, inner forearm, biceps, pecs, lats, groin, glutes, and calf.ll of the muscles that are circled in the picture above are muscles used when you swim freestyle. There are 24 different muscles circled in this diagram. The ones that are circle in blue are more primary muscles that are worked and the muscles circled in black are the more secondary muscles exercised.