"I don't know about you but when I walk across the room or up the passage at the extent of the stride my knee is straight."

How long are you going to go on ignoring this basic point of geometry?

high center: straight legs are easy

low center: straight legs are not possible, because they would put the feet impractically far apart.

If you walk, or even dance, with a high center you can have straight legs. If you lower as much as many leading dancers do today, you - like they - will be often unable to get the legs straight.

Simple geometry.