The breaststroke is a type of swimming style. The style requires coordination and timing between the muscles of the upper body and lower body. Most of the major muscle groups in the body are involved in performing the breaststroke, but specific muscles are primarily responsible for producing the force to drive the body forward through the water.
To complete the breaststroke, you start in a fully extended position just below the water. Your legs are straight and arms are fully extended in front of you. Raise your hips up slightly and bend your knees to load up your legs, while simultaneously driving your arms back and to the side, pushing through the water to propel your body forward until your arms finish next to your torso. Bend your elbows to bring your arms underneath you, then straighten the arms back out as you extend your hips and knees to move yourself forward, using the power in your lower body in what is called the frog kick, finishing back at a fully extended position.
Most of the propelling power in the breaststroke by the upper body comes from the latissimus dorsi muscles in your back as you adduct your arms and push against the water, the "National Strength & Conditioning Association Journal" explains. During this phase, the pectoralis major muscle, in the chest, and the biceps brachii, brachialis and brachioradialis muscles in the arms also assist in the movement. Returning your arms back to the extended position to load up so they can drive back again works the deltoids, chest and triceps. Smaller shoulder muscles like the rotator cuff collection work to stabilize the shoulder joint throughout the breaststroke movement.
The power from your lower body that propels your body forward during the frog kick as your hips and knees extend is completed by contraction of the glutes, or butt muscles, and the quadriceps, at the front of your thighs. The glutes are the most powerful muscle in the body, and they work to extend your hips while the quadriceps extend your knees. At the very end of the frog kick, the calves plantar-flex your ankle joint, or cause you to point your toes. Your hip flexors and hamstrings bring your legs back to load them up to extend again.
Your rectus abdominis, obliques and lower back muscles also contract throughout the breaststroke movement to keep your torso taut. Because of the frequent movement around the shoulder during the breaststroke, it’s not abnormal to have discomfort or problems at the shoulder. Dr. Scott Riewald of the National Strength and Conditioning Association explains that muscle-strength imbalances in the internal and external rotators of the shoulders, as well as weakness in the muscles that stabilize the scapulae, are common in swimmers.