The brace position does not mean that you have to get your head between your knees. It is to get your head leaning on the seat in front, so the head is lower than the top of the seat. This is for 2 reasons......... 1 to protect the head from flying debris on impact i.e bags, books etc. and 2 to protect the head from the overhead lockers coming down and breaking your neck like what happened on the british midland crash on the motorway. The people killed in that were the ones who had their necks broken by the overhead lockers collapsing onto them, and in those days, the seat backs were lower than the head. Hence a longer seat back was implemented to prevent this incident occurring again in the future. There is no need for you to get your head down between your knees except for those in legroom seats where there is no seat in front to rest on.