One thing I've been told over and over is that it's necessary to have flexible ankles to be able to kick quickly in freestyle. Personally, I don't have great ankle flexibility and have never been able to kick 100m in much less than 2:00 (note this is with a board and snorkel so similar body position to kicking on back). However, if I kick on my back I can kick 100m in 1:50 comfortably and could probably go 1:45 or a touch less if I really put some effort into it. I was thinking this morning: perhaps a lack of ankle flexibility is less of a detriment in backstroke than it is in freestyle. Thoughts?