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The butterfly stroke demands strong chest, back and shoulder muscles.
The second fastest stroke after freestyle, the butterfly requires powerful shoulders and is one of the most difficult and exhaustive strokes to master. Because it utilizes an undulating body movement -- as opposed to the body roll of freestyle -- swimmers need to execute back arches and abdominal contractions which require a particularly strong core musculature. Add a double kick to the torso undulation and the challenge of the butterfly becomes one of rhythm and timing.
Upper Body and Propulsion
The primary movers of the butterfly stroke are the pectoralis major in your chest and the latissimus dorsi in your back, which power upper arm adduction -- drawing your arms in toward your body -- during the push phase. The muscles at the top of your upper arms -- the biceps brachii and brachialis -- activate as your elbows move from full extension at the beginning of the catch to about 40 degrees of flexion midway through the pull. During the final stage of the pull phase, you use your triceps to explosively extend your arms. While your shoulders -- deltoids and rotator cuffs - position and move your arms during the recovery phase, your wrist flexors serve to keep your wrists in a slightly flexed position throughout the stroke.
Sweep and Speed of the Hands
Your hands enter the water at shoulder-width apart or slightly narrower and then trace a pattern through the water that resembles an hour glass. Once underwater, your hands sweep inward, moving under your body. To push against the water's resistance, you use the muscles in your hands, or thenars, as well as your forearm flexors -- the brachioradialis -- and extensors -- the extensor digitorum muscles. The degree to which you bend your elbows depends on how deep your hands are underwater. In turn, the depth of your hands is determined by the strength of your upper back and shoulders. During the final stage of propulsion, your hands move around and past your hips and then toward the water's surface for the recovery phase.
The Undulating Core
In contrast to the freestyle, the butterfly uses the undulating torso movement. The paraspinal muscles -- the muscles that run along your spine from your neck to your lower back -- contract to begin this undulation. First, the muscles in your back arch while your arms move into the recovery phase of the stroke. Then, the abdominal muscles -- or your rectus abdominis and obliques -- immediately contract. This contraction enables your upper body to follow your hands into the water and start the propulsive phase of your stroke.
The Dolphin Kick
The motion of the butterfly's dolphin kick -- a double kick -- also follows an undulating pattern of movement. Although you use the same leg muscles as you would in the freestyle stroke, your legs move together. The downbeat of the kick, which provides propulsive force, starts with a contraction of your hip flexors, or the rectus femoris and iliopsoas. You then extend your knees, which activates your quads. During the recovery phase of the kick, your glutes and hamstrings contract to extend your hips. Throughout the stroke, the plantarflexed position of your feet -- ankles extended and toes pointed -- involves the activation of your calf muscles, or gastrocnemius and soleus.