Here's an interesting look at it...
Middies/attack = lots of running and quick turns which can lead to rolled ankles and acl problems if hit wrong. Also with slashes, checks, etc can lead to broken ribs/bones (we had a middie with a shattered knee cap after a hit as by some random event the stick manage to hit him just so on his knee cap)
Goalies = wear more padding, fewer oportunities to get hurt (depending on how crappy your defence is/how fast your middies come back), sure you will get a few bruises, but so do field players. You can minimise you cardiac arrest issues with proper pads, and if they are really really worried you can where underpads also.
Just tell your folks your doing it to get the girls...
(broken bones heal and chicks dig scars
)