I can’t see your son’s condition, so I can only provide some experience in learning freestyle kicking. The main function of the freestyle kick is its balancing function, and the function of providing forward momentum accounts for a small part. This is why your son can swim faster after adding hands. Or, as someone said above, if the frequency of kicking is slower, the speed will be slower.
Learn the basic skills of leg movements in freestyle (whip-like leg kick):
1. The kicking action starts from the hip, and the thigh drives the calf to make a whip The action of fetching water.
2. Kick your leg upward from straight to the bend. Start kicking upwards with straight legs. When your feet are close to the water, bend your knees and lift your calves up so that your feet are above the water and then push downwards. You can start to draw water with your legs straight, but relax your legs slightly and don't stiffen them. Your legs will naturally bend under the pressure of the water. Before pumping water downward, the knee joint bends at an angle of about 130-160 degrees, and the water pumping range is about 30-40 cm. When fetching water, you should stretch your feet (ballet feet) and don't hook your feet.
Freestyle learning leg movement exercises:?
1. ?Land imitation exercises?
(1)?Seated water fetching: Sit on the edge of the pool or on the ground, support your hands behind your back, straighten your legs, rotate your legs internally so that your toes face each other, and separate your heels into a figure of eight. Relax, use your hips as the axis, your thighs drive your calves, and pump water alternately up and down. ?
(2) Recumbent position to draw water: Lie prone on a stool, and alternately draw water with your legs up and down. The requirements are the same as above. ?
2. ?Exercises in water?
(1)?Prone water kick: Hold the tank in hand, or have a partner support your abdomen, get into a horizontal position, straighten your legs, and do straight-leg or bent-leg kick. ?
(2) Supine drawing: Lie on your back, hold the trough in your hands, or have a partner help support your back, and draw water alternately with your legs. Be careful not to expose your knees to the water. ?
(3) Sliding kick: During the practice, you are required to hold your breath, straighten your arms and put your head between your arms. ?
(4) Pushing water on the support board: During practice, straighten your arms, relax the support board, immerse your shoulders in the water, do not press the board hard with your hands, and breathe naturally.
After mastering the correct movements, strength training is also necessary to improve the efficiency of kicking. It can be divided into land-based strength training such as squat exercises, squat weight-bearing exercises and water training such as calf water weight-bearing exercises, long-distance endurance exercises, etc. Choose the appropriate exercise method according to your own conditions.
Attached is a freestyle kick action rendering for your reference.