I would imagine it’s the same with all swim coaches. I’m always being asked by parents and swimmers why swimmers need incredibly strong cores in order to swim fast. Much stronger core strength than land-based sports. And it’s true and obvious from watching the sport. Swimmers need to have incredibly strong cores.
It took a while for me to come up with an analogy to explain this to new swim parents and inquisitive swimmers. In order to run fast, runners use their bodies to apply force to the ground through their feet. Anybody who has run on shifting sand can immediately tell you that it takes more effort and you run more slowly if you don’t have that stable ground to work with.
Ice skating is a little more complex, but any ice skater can tell you something similar about ice. As long as your skate doesn’t slip, you can apply maximum force through to your feet.
In other words, the most effective way to move forward is to use your body to apply force to a stable platform.
But swimmers don’t have an external and stable platform. We swim through the water, which is constantly moving. So in order to effectively use our strength to move forward, our core has to be our stable platform. When we grab a lot of water and pull/push that water behind us, the core of our body has to be able to handle those forces without losing its stiffness. And the more stiff and stable our core is, the more efficient our propulsive efforts are.
So the next time a parent asks about core, or the next time a swimmer complains about doing more core training, you can just tell them. Runners have the ground. Skaters have the ice. Swimmers have their core.