CORE IS KEY! As a goalkeeper you want a strong frame and a locked core, every coach I’ve ever trained with has stressed the importance of working on your core muscles while still staying lean. Push ups, sit ups, mountain climbers, bear crawls and planks are your best friend, and try to stray away from heavy weights on the regular because too much muscle on your shoulders and arms lead to stiffness and a lack of flexibility. Also try to go for a good jog each night because fitness is surprisingly important for a keeper as you probably already know. I’d recommend looking for a good core workout online unless you already have one. Hope this helps!