It is the speed of the kick that most differentiates the greatest swimmers from the not-so-great ones.
Leg strength and good ankle plantar flexibility are required to develop a strong flutter kick, but more is needed. Fitness of the leg muscles used in the kicking motion must also be developed to an extraordinary level. If a swimmer is determined to use the kick for strong propulsion, the way that most great swimmers do, then a six-beat kick is necessary and the legs need to be relatively fitter than the arms. Consider the following.
If a swimmer’s freestyle pulling stroke rate is 100 per minute (50 right arm pulls and 50 left arm pulls) then the kicking stroke rate with a six-beat kick is 600 per minute (300 down kicks and 300 up kicks).