The flutter kick is a kicking movement used in both swimming and calisthenics.
Swimming
In swimming, the flutter kick refers to an alternating up and down movement of the legs. It is seen in front crawl and backstroke. The downbeat refers to the portion of each individual kick where the leg moves downwards, and the upbeat refers to the repositioning of the leg upwards. In his book Swimming Fastest, Ernest Maglischo says that the downbeat of the front crawl kick is "undoubtedly the most propulsive phase" and opines that the upbeat is probably not propulsive at all. This corresponds with the upbeat being the most propulsive phase in the backstroke kick.<!-- Add info about oxygen usage -->
Rotation and stabilisation
In Swimming Fastest, Maglischo writes that the lateral movements of the kick "probably" contribute to rotation and stabilisation in front crawl, aiding the swimmer in maintaining a straight course.
Foot angle
Research suggests that a greater pitch angle (angle between the horizontal and the axis of the foot) increases propulsion by allowing the foot to push back on the water more. Athletes can increase this angle by improving their plantar flexion flexibility.
Calisthenics
thumb|Military personnel doing flutter kicks on the edge of a pool
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.