In my experience (pretty limited since I'm ROTC), some of the little skinny guys/girls have trouble carrying the heavy weight. The pack might be about a third of their weight whereas there are other big jacked guys weighing 200 lbs carrying the same as the 120 pounder ~ you can see where this may present problems. However, if your pft score is really good, especially the run, you should be fine because you'll probably have an aerobic advantage which is huge. I'm usually one of the smallest guys on the humps but I burn people, especially up hills simply because I have a much higher lung capacity and i'm not carrying around as much muscle mass or fat. Guess it depends on how underweight you are, your aerobic capacity, and if you're an underweight person who can handle some weight on your back. The 1/c mids that just got back from bulldog said the worst PT part was running with extra weight.