The backstroke kick is a flutter kick. Its technique is similar to the kick used in the freestyle stroke, with the difference that you are swimming on the back.
The legs kick in an alternating up-down manner, where the kick is driven by the hip. To be more correct, the legs kick in vertical or oblique directions, depending on the body's position, because in backstroke the body rolls from side to side. The feet are pointed. Here's a demonstration of the backstroke kick:
The backstroke kick should use quick and compact movements for best efficiency. The feet should stay in the shadow of the swimmer's body and the knees should not break the water surface. The kick will create drag and slow the swimmer down if it is too wide.
The upbeat starts when the lower leg is still moving downward from the previous kick. The hip and leg flex and the upper leg starts moving upward. Shortly thereafter, the lower leg is dragged along upward and the pressure of the water pushes the foot in plantar flexion (foot pointed).
This is the beginning of the propulsive phase and water is pushed backward by the lower leg and the foot. Swimmers with flexible ankles are an advantage here, because they can keep the top of the foot facing backward for a longer time and therefore have better propulsion.
The downbeat starts while the lower leg is still moving upward. The hip and knee extend and the upper leg starts to move downward.
The lower leg is now facing upward rather than upward and backward and doesn't provide propulsion anymore. If the swimmer has flexible ankles, the foot is still facing a bit backward and pushing water backward.
The propulsive phase of the kick ends when the leg is fully extended. Shortly thereafter the whole leg is moving downward. The pressure of the water against the back of the leg will keep it extended until the next upbeat. The pressure of the water against the foot moves it in a neutral position.
The downward movement of the leg should be more relaxed than the upward movement so the leg muscles can rest.
When swimming backstroke, most swimmers use a six-beat kicking pattern. This pattern means each foot kicks three times per backstroke cycle, for a total of six kicks. This six-beat kick produces the best propulsion.
In longer distance swimming, a four-beat kick or a two-beat kick can be used to save energy. The swimmer must have good balance to use these slower kicking patterns. A good drill which lets you practice balance is balance on the back.
Synchronization Between Arms and Legs
The timing between arms and legs in the backstroke is similar to the timing in front crawl when a six-beat kick is used. The synchronization between the left arm and left leg is as follows:
- The upbeat of the first kick occurs during the extension forward and first downsweep of the arm.
- The downbeat of the first kick occurs during the first upsweep of the arm.
- The upbeat of the second kick occurs during the second downsweep and second upsweep of the arm.
- The downbeat of the second kick occurs during the release and beginning of the recovery of the arm.
- The upbeat of the third kick occurs during the middle part of the arm recovery.
- The downbeat of the third kick occurs during the end of the arm recovery.
The timing between the right arm and the right leg is of course similar.
The backstroke kick has two purposes:
- It contributes to the swimmer's propulsion. However, most propulsion comes from the arm stroke.
- It stabilizes the swimmer's body in the water because the leg movements balance out the arm movements.
If the swimmer has poor ankle flexibility, he will not be able to point his feet and the propulsion of the legs will be lacking.
I have even seen cases where the flutter kick used without arm strokes was moving the swimmer backward instead of forward, because the ankles were so stiff.
Regular use of swimming fins will improve the ankles' flexibility over time if they are stiff.