Swimmers are known for their strong shoulders and V-shaped bodies tapering to slim waists and hips. The butterfly stroke is a swimming stroke that involves the simultaneous use of the arms while the legs closely resemble the motion made by a dolphin. Considered an advanced swimming stroke, the butterfly stroke requires a significant amount of power and technique.
Arm and Shoulders
The butterfly stroke relies heavily on the arm and shoulder muscles to propel the body forward in the water. When the arms make a rotating motion through the water, this builds the deltoid muscles, which are on the front and back of the shoulders. This action also builds the trapezius muscles, which are situated beside the shoulders on either side of and behind the neck. The biceps and triceps muscles play a smaller role in the butterfly stroke. The arms should be kept straight because these muscles are used to slice through the water.
The core muscles -- those muscles that make up the abdomen and the back -- are of vital importance to a swimmer's power, particularly in the butterfly stroke. The abdominal muscles provide the strength to lift out of the water, then curve in to return. The latissimus dorsi muscles of the back, which give swimmers their V-shape torso appearance, are also relied upon to provide stability to the motion and to push against the water when the arms are below the surface. Swimmers with well-developed core muscles are better protected against injury because the core muscles protect the back from injury. Swimmers who swim the butterfly stroke are more susceptible to lower back pain.
Legs and Buttocks
Other swimming strokes require the legs to kick individually to move the body forward. Instead, the butterfly stroke requires the legs to move as one. This requires special strength in the buttocks muscles, also known as the gluteus maximus. The hamstring muscles, situated on the backs of the legs, also are extremely important because the legs must kick in a backward motion instead of the scissoring motion used in most other swimming strokes.