Editing Breaststroke (section)

===Leg movement===

The leg movement, colloquially known as the "frog kick" or "whip kick",<ref name=Maglischo/> consists of two phases: bringing the feet into position for the thrust phase and the insweep phase. From the initial position with the legs stretched out backward, the feet are moved together towards the posterior, while the knees stay together. The knees should not sink too low, as this increases the drag. Then the feet point outward in preparation for the thrust phase. In the thrust phase, the legs are moved elliptically back to the initial position. During this movement, the knees are kept together. The legs move slower while bringing the legs into position for the thrust phase, and move very fast during the thrust phase. Again, the goal is to produce maximum thrust during the insweep phase, and minimise drag during the recovery phase. In the recovery phase the lower leg and the feet are in the wake of the upper leg, and the feet are pointed to the rear. In the thrust phase all three parts create their own wake, and the flat end of the feet acts like a hydrofoil aligned to give maximum forward thrust. The resulting [[drag coefficient]] (or more precisely the frontal area) is thus doubled in the thrust phase.

A fit adult creates a wake. Drag due to a wake is [[Aerodynamics|Newtonian drag]], increasing with the square of the velocity. For example, if the relative speed between the water and the leg is twice as high on the thrust phase than on the recovery phase, the thrust is four times as high as the drag. Assuming the legs are recovered with a relative speed between leg and body which amounts to the same as the relative speed between water and body, the legs must be kicked back with five times the mean velocity of the swimmer. This limits the top speed, with both effects combined together, velocity and frontal area, yield a thrust-to-drag ratio of 8 for the legs.

As a variant, some swimmers move the knees apart during the preparation phase and keep them apart until almost the end of the thrust phase. Moving both knee and foot outwards like a real frog avoids the extreme rotation in the lower leg.

All other variants fail to increase the frontal area, yet swimmers using them still generate some thrust by the velocity variation and do not drown. Another variant of the breaststroke kick is the scissor kick, however, this kick violates the rules of the [[FINA]] as it is no longer symmetrical. Swimming teachers put a great effort into steering the students away from the scissor kick. In the scissor kick, one leg moves as described above, but the other leg does not form an elliptical movement but merely an up-down movement similar to the [[flutter kick]] of [[front crawl]]. Some swimming teachers believe that learning the front crawl first gives a higher risk of an incorrect scissor kick when learning breaststroke afterwards.

Breaststroke can also be swum with the dolphin kick in butterfly, but this also violates the FINA rules. One kick is allowed, however, at the start and at the turn, providing that it is part of the body's natural movement.

Humans have strong muscles in the legs and would need [[swim fin]]s (like a frog) to bring all their power into the water and stand with the sole of the feet on the water. Rather the leg grabs almost as much water as the foot and a small amount of water is accelerated to high [[kinetic energy]], but not much [[impulse (physics)|impulse]] is transferred. The toes are bent, the feet point 45° outwards, the sole points backwards, to mimic a [[hydrofoil]]. While closing in a V shape to the rear a small "lifting" force can be felt. Unlike in the other kicks, the joints are moved into extrema. Before the kick the knee is maximally bent and the upper leg is rotating along its axis to its extreme outer position and the lower leg is twisted to extreme, at the end of the kick the ankles are maximally turned to the inside so that the soles clap together to achieve a nozzle effect like in a [[jelly fish]]. Therefore, training involves getting flexible in addition to fitness and precision. The sudden sideways stress on the knees at the kick can lead to uncomfortable noise and feeling for the beginner and to wear for the senior.