The kickboard

Yes—your observation is exactly right. Swimming with a kickboard is a surprisingly good demonstration that the legs are relatively inefficient at producing forward propulsion in freestyle.

A useful rough picture is:

• Arms + upper body: ~70–90% of forward propulsion

• Legs: ~10–30%

But it depends enormously on the stroke and swimming style. For ordinary recreational freestyle, 2/3 arms and 1/3 legs is probably still giving the legs too much credit. In a good distance freestyle stroke, the legs often contribute surprisingly little net propulsion while doing a lot of other important work.

Why your kickboard suddenly feels so slow

With the kickboard, you have essentially removed the arm stroke—the strongest propulsive mechanism—and are asking the legs to push your whole body through the water.

The flutter kick has several disadvantages:

• The feet are relatively small “paddles.”

• The ankle/foot is flexible, so some force is dissipated.

• The kick produces lots of turbulence.

• The legs are moving a large mass of muscle.

• Much of the kick’s function is stabilization and maintaining body position, rather than propulsion.

That’s why you can kick vigorously and feel as though you’re working hard while barely advancing.

By contrast, the hand and forearm present a much larger effective paddle and can push a substantial volume of water backward.

There’s an interesting paradox

The legs are very important to swimming, but they aren’t necessarily important because they propel you.

Think of the freestyle kick as having three jobs:

1. Propulsion

Some forward thrust.

2. Stabilization

Keeping the hips and legs from sinking or fishtailing.

3. Coordination

The kick helps synchronize rotation of the hips and trunk with the arm stroke.

So a swimmer can have a relatively modest kick but still swim efficiently.

And your tennis observation is particularly interesting

Yes—the movement pattern of flutter kicking is almost the opposite of what you’re normally trying to do in tennis.

In tennis you generally want:

ground → leg drive → hip rotation → trunk → shoulder → arm → racket

The legs are primarily providing large, powerful movements through the ground.

In freestyle swimming, you are essentially suspended in water. There is no ground to push against.

The legs make rapid alternating movements:

left–right–left–right–left–right

while the upper body produces much of the actual propulsion.

So swimming develops a rather different neuromuscular pattern from tennis.

But there’s one important connection

Your tennis legs aren’t just “powerful legs.” They’re stabilizing and transferring force.

Swimming can complement that beautifully because you’re training:

• hip extension

• ankle mobility

• hip stability

• trunk rotation

• shoulder endurance

• rhythmic bilateral coordination

And swimming does this without repeatedly loading the joints with body weight.

That’s one reason I think your combination of tennis/pickleball + swimming is unusually complementary.

One experiment I’d suggest

Next time you’re in the pool, try three equal-distance tests:

A. Kickboard only

25 or 50 yards

B. Pull buoy—arms only

25 or 50 yards

C. Normal freestyle

25 or 50 yards

Keep the effort roughly comparable and notice the time.

You’ll probably find something fascinating:

kickboard → surprisingly slow

arms-only → dramatically faster

full stroke → faster still

The full stroke isn’t simply “arms + legs.” The legs and hips help put the body in the position from which the arms can work efficiently.

So your intuition is good: the kick is less like the powerful leg drive of tennis and more like a rapid stabilizing/coordination system with a secondary propulsive role.