Flutter kick

The flutter kick is a kicking movement used in both swimming and calisthenics.

Swimming

The flutter kick in a front crawl

In swimming strokes such as the front crawl or backstroke, the primary purpose of the flutter kick in beginner and intermediate swimmers is not propulsion but keeping the legs up and in the shadow for the upper body and assisting body rotation for arm strokes.

In experienced competitive swimmers, the flutter kick, also known as the freestyle kick, can account for approximately 10-30% of propulsion during the front crawl, with the rest being generated by the arm stroke.

The legs are extended straight backwards in line with the body. They are moved up and down, one leg kicking downwards (relative to the front of the swimmer's body) as the other leg moves up. The knees are slightly bent to facilitate the kicking action, but not too much in order to minimize drag created by the thighs as they move out of the shadow of the swimmer's body. Similarly, toes are pointed to minimize drag.

The downward moving leg provides a majority of the thrust by creating vortices which swimmers use to increase velocity. An integral part of the kick is ankle dorsiflexion; it is this flexing of the ankle that catches and pushes water, promoting the importance of optimal ankle flexibility. The knees are not kept rigid when kicking but are allowed to flex slightly to allow the required "snapping" action through the end of the toes.

Underwater diving

The flutter kick used with swimfins can be a powerful propulsion technique, and is used by scuba divers and freedivers underwater and at the surface, but there are other finning techniques more appropriate to some underwater environments and some types of fin. Divers in a confined environment or where silting may be a problem may use a modified flutter kick or frog kick, done entirely with bent knees, pushing water up and behind the diver to avoid stirring up sediment on the bottom.

Calisthenics

The calisthenics version of the flutter kick is often used as an intensive training tool in the military. They help to develop the hip flexors, abdominal muscles and leg muscles. Flutter kicks are a four-count exercise. Starting position is lying flat on the back with the feet and head approximately 6 inches (15 cm) off the ground. Hands are under the buttocks to support the lower back. Count one: raise the left leg to a 45-degree angle, keeping the right leg stationary. Count two: raise the right leg off the ground to a 45-degree angle while, at the same time, moving the left leg to the starting position. Counts three and four are repetitions of the same movements. Legs must be locked, with toes pointing away from the body.