Develop Leg Strength
Kicking with fins is like lifting weights: the added resistance of the water on the blade of the fin increases the workload on your leg muscles. Your body adapts by increasing the strength and endurance of the muscles involved. Stronger muscles move more water making you swim faster, all other things being equal.
A word about specificity: It’s important to realize that muscular strength for swimming needs to be “specific.” Good runners, cyclists, roller bladers, etc. can have very strong leg muscles, but the muscles have developed for running, cycling or roller blading, not for swimming. Fins develop leg strength specifically for swimming, and in a way that few other activities can.
“Force Fin swimming fins utilize the body’s strength and put it where it’s needed. The human body is built to have more strength when kicking down (during freestyle), than when kicking up. The downward or power phase of the kick emphasizes the powerful quadriceps muscle group at the front and side of the upper leg. The upward, recovery phase uses the weaker hamstrings at the back of the thigh. While still developing both sets of muscles, our blade design assists — or helps take the load off — the upward recovery phase that uses the weaker muscles, maximizing energy efficiency.”