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 angles that will push us forward. Swimmers who have excellent joint range of motion are able to orient the surfaces of their feet so their kicking movements provide good propulsion. Swimmers with less joint flexibility produce less propulsion from similar movements while exerting equal force. They go slower. Increase the safety of the movement so you can use higher muscle force without worrying about causing groin or knee damage.