In the water, legs don't workas well. Even the best swimmers are able to transfer only a small percentageof their leg power to the water efficiently.
Slower swimmers transfer evenless.
Kicking involves pressing surfaces of the feet against the water at anglesthat will push us forward. Swimmers who have excellent joint range of motionare able to orient the surfaces of their feet so their kicking movementsprovide good propulsion. Swimmers with less joint flexibility produce lesspropulsion from similar movements while exerting equal force. They go slower. Increase the safety of the movement so you can use higher muscle forcewithout worrying about causing groin or knee damage.