For novice swimmers, some middle distance and distance swimmers, and many triathletes, the freestyle kick is either a hindrance (if the kick is too big or a scissor kick) or it serves mainly to hold a better body position. It does not add much to forward motion - and that is OK for many swimmers. They don't need to spend the energy on the a kick that does not give a high return on that energy investment. But, for a sprinter, every bit of forward speed and power that can be found can be used, and the kick can come in handy (or footy?).
Part way through the conversation, it occurred to me that the swimmer may have plenty of fitness, power, and strength, but they are lacking in one or two things that are needed for a great freestyle kick. Ankle and foot flexibility, and / or big feet.
If a swimmer can sit on the ground, legs extended, no knee bend, heels on the ground, and can press his/her toes to the floor / toward the floor so those toes go below the level of the top of the foot - think straight leg, heel and toes both touching the floor - then flexibility is not an issue. For a decent kick, the swimmer needs to at least be able to get the top of the foot parallel to the ground. If they cannot do that, if the toes are higher than the top of the foot, I think of them as having a runner's kick. Stiff ankles, probably too much knee bend, kicking too much from the knee and not enough from the hips. Ankle and foot flexibility work may help. Wearing flippers can help with ankle and foot flexibility, too. That is what led me to this thought - the swimmer in question can kick the daylights out of her training group when flippers are part of the workout, but take them off and the swimmer is at the back of the group.
The other issue is small feet. If you think of a swimmer's feet as his / her fins, then small ones are not as good as big ones. There is no way to make a swimmer's feet bigger that I know of, so there is not a good way to overcome this in the leg department. The focus then would become upper body and faster rhythm, with enough kick to keep the body in a good position.
So what if the swimmer's feet and ankles are flexible, and the feet are more or less normal, or even big? Then the focus for a better kick might move to fitness, strength, and power. What kind of work will help the swimmer kick better? There are many things that could be done within a swimming workout. Here are a few ideas to try (kick with a kickboard or without - either is OK; with a board, the swimmer's position is not the same as swimming, but it allows for more breathing and might sink the legs deeper, increasing the workload; no board may put the swimmer in a position closer to normal swimming):
- Short sprint kicks on 1:2 to 1:~8 work:rest ratio. Very short, as in 10-20 seconds, all out max effort kick repeats. Push off the wall or do the repeat from a start. Legs only, no pulling, a maximum effort for a short distance with an easy, long recovery. Repeat. I'd do this 1-2 per week, and do a total of 4-8 repeats each time.
- Vertical kicking in the deep end of the pool. Vertical kicking is kicking as if the swimmer is standing. Head out of the water, good posture, the swimmer is trying to kick to the sky, straight up. This kind of kicking can be done with any kick, and with any type of repeat structure. Short sprints, longer durations, IMs, almost anything. It is done on time vs. distance, as in kick for 30-seconds, rest for 30-seconds. Want to make it tougher? Hold the hands out of the water, hold weights, or even wear a weight belt.
- Fins, fins, fins. Kicking with flippers on, swimming with flippers on, almost anytime in any set. Adding flippers can add an extra workload to the legs, which can lead to better strength / power. Do not go all flippers all of the time or the swimmer may become flipper-dependent, and don't throw a lot of flipper work at the swimmer all at one time. Build up to it a little at a time.
- Elastic tether kicking. Using a belt fastened to an elastic cord, attach the end of the cord to a diving board hand rail so it is up, out of the way of the feet, push off the wall and kick fast for 10-30 seconds, turn around and easy back to the wall and repeat. Be sure to inspect the cord for cuts or scrapes, a cord that snaps when fully stretched is dangerous.
- Speed assist kicking. Like tethered kicking, but with the belt stretched to start and kicking toward the anchor point, using the elastic to speed up the swimmer. Again, be sure to inspect the cord for cuts or scrapes, a cord that snaps when fully stretched is dangerous.