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Endurance Hour: Physics of Swimming Propulsion

Swimmers propel themselves through the water by a combination of kicking the legs and pulling water with the arms. Their arms are used as levers to propel the swimmer through the water. This is made difficult by the drag force, which is opposite the direction of the pulling arm. 

The greater the flexion in the ankle and the knee joints and therefore the greater range of motion, gives swimmers a greater propulsion through the water. The ability of swimmers to hyperextend their knees and ankles provides swimmers with more undulation and a great propulsion through the water. 

The lift forces arise from a difference in pressure as the fluid travels further and faster around the more curved side of the foil than the less curved side. Thus, a swimmer’s hand could act as a foil because the back of the hand is more curved than the front. Therefore, swimmers should scull or cup their hands to move most efficiently. The curved motion of the hands means that the force from the arms can increase and therefore cause the swimmer to move further per stroke. Since swimming is so inefficient due to the resistance of the water, it is advantageous to increase the power per stroke instead of trying to maximize the number of strokes. To further maximize this propulsion force, the fingers should be together so that the surface area on the levers is maximized and the most water is pulled. 

While underneath the water, the most efficient motion is in a streamline fashion with the legs together. Together, the legs can create a greater magnitude of force against the water because of their greater surface area.

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