Divers propel themselves by moving their legs (and the increased surface area of their fins) back and forth against the water with a flutter kick motion. This kicking results in a complex combination of propulsive forces that move the diver forward as well as resistance or drag forces that hinder that movement.
With other fins, virtually all of the propulsion comes from the downward phase of the kick (imagine you’re horizontal in the water with your stomach toward the bottom) and almost none on the up-kick. You can test this yourself in the water by trying to kick up, with or without fins. You’ll go nowhere. But simply extend the lower leg down and watch yourself move ahead.
One reason for this uneven propulsion is that humans naturally have more strength when kicking down. The power or downward phase of the kick emphasizes the powerful quadriceps muscle group at the front and side of the upper leg. The kick’s upward phase uses the weaker hamstrings at the back of the thigh and is mainly a recovery movement setting up the leg for the next power phase. The result is a natural two-stroke kicking cycle of power and recovery for each leg. It’s the same with other continuous activities like running, bicycling or swimming where a power/recovery cycle is built-in to conserve your energy.
While most other fins make you work just as hard on the recovery as on the power phase, Force Fins take advantage of this two-stroke cycle and work in harmony with the body’s natural strength. The flexible blade of our Force Fin opens out to its maximum surface area to move a high volume of water during the powerful downward portion of the kick.
During the recovery, the blade folds down, channeling water behind while limiting resistance to conserve your energy. (If you have to put the same amount of energy into both parts of the kick, you fatigue much sooner.)
As you kick down, the blade opens out to its maximum surface area, engaging the muscles of the whole leg to push water and provide initial forward thrust.
At the end of the downstroke, the fin recoils, setting up to rebound to its original position during the recovery or return kick stroke.
The fin then rebounds to help begin the upward recovery phase of the two-cycle kick stroke.
The fin’s tips fold inward to aid the upward recovery and to prepare your leg for the next downward power stroke.
Force Fins are the only fins that provide propulsion consistent with your natural two-stroke kick cycle to this degree. By providing for more of a separate power and recovery phase, oxygen depletion is reduced, and the onset of fatigue is delayed. The result is that divers can kick continuously for longer periods of time.