Swimming breaststroke might not be as fast as front crawl, but it still provides a good all-over workout for most of the major muscle groups in your body. In particular it tones the quadriceps, glutes, upper back, triceps, hamstrings and lower legs. It also helps to work and tone the chest muscles.
Above is a schematic showing some of the skeletal muscles of a Homo sapiens. The schematic was based on an image in the book “The human body” by Linda Gamlin.
(Licensed under Wikimedia Commons https://commons.wikimedia.org/wiki/File:Skeletal_muscles_homo_sapiens.JPG)
How breaststroke tones your upper body muscles
The catch phase is a crucial part of breaststroke and predominantly uses the pectoral muscles (which connect the chest to the bones of the upper arm and shoulder) and the latissimus dorsi (the largest muscle in the upper body) to sweep the arms inwards against the water. This movement is also assisted by the following arm muscles, the biceps brachii, brachialis and brachioradialis.
The deltoids, chest muscles and triceps help extend the arms after they’ve returned to the swimmer’s side following the catch phase to start the stroke again.
How breaststroke tones your leg muscles
When you extend the legs in the downward phase of the stroke, the quadriceps work with the glutes, while the adductor muscles are responsible for bringing the legs together once they’ve reached their widest point.
During the recovery phase the hamstring muscles (the biceps femoris, semimembranosus, and semitendinosus, which sit at the back of the thigh), the sartorius (the body’s longest muscle, which runs diagonally from the hip to the inside of the tibia bone on the lower leg), and gracilis (the thigh’s most superficial muscle) help the knee flex and bring the feet toward the swimmer’s bottom.