My experience in the army was that the guys that just seemed moderately strong and were fairly skinny were the ones that could haul the most gear over distances. Ruck, m60, etc. I think most of the armed forces are trying to go for rounded performance, but they have different ways of trying to get there. It was almost never some 225lb muscle bound 5'11" guy that was killing the obstacle course, carrying the gear, etc. The special forces guys I knew were tough as nails, but so much of it was mental. The AF should have some systems like the MC or Army for determining this BF thing, but you have to be careful it doesn't promote more unfit airmen, which really just depends on the exact program specifics.