The inventor of the modern swim fin, Owen Churchill, won a gold medal as a yachtsman. Churchill wasn't a strong swimmer, however, according to the Smithsonian website. When he saw swimmers in Tahiti using woven leaves attached to their feet to help propel them through the ocean, Churchill had the idea of making a swim fin out of rubber. Almost identical in appearance to a modern swim fin, it was patented in 1940 and continues to be an essential swim tool. Swim fins not only propel you through the water more quickly and effortlessly, they offer a number of fitness benefits as well.
Strength Training
Swim fins make you go faster by increasing the surface area of your feet in the water. In some respects, it is much easier to swim with fins, since you cover more ground in less time. However, Swim Fins101 explains that you're actually working harder with fins in order to maintain the increased speed. As a result, your leg muscles become stronger.
Cardio Training
Swimming with or without fins is an excellent aerobic exercise. Adding fins, however, enables you to work your heart muscle even more efficiently. The best cardio workouts require you to use a large percentage of your overall muscle mass, and 60 percent of your muscle mass is in your legs. You use your upper body much more than your legs when you swim without fins. Since fins increase the surface area of your feet, you'll feel more resistance when you kick, using your legs more and working your heart harder. As an added bonus, fins enable you work out longer, so you'll burn more oxygen and calories, which can aid in losing weight.
Improved Technique
If you use fins with enough buoyancy to float, the fins will keep your legs higher and give you an overall higher body position -- a better posture for optimal swimming. Plus, swimming with fins makes your feet extend beyond their normal range of motion. This increases the flexibility of your ankles and feet. When you take the fins off, your increased range of motion leads to a more efficient kick with added power.
Considerations
The longest swim fins are used by scuba divers and snorkelers, who are involved in water activities that make much less use of the arms for propulsion. U.S. Masters Swimming recommends intermediate fins for beginners and for people who want to increase their ankle flexibility. However, longer fins can be especially hard on your legs and ankles until you get used to the extra weight, so start with short sessions until you become accustomed to using fins. Shorter fins are useful for increasing your swimming speed, and they are less likely to overexert your leg muscles
