According to Academy.com, internal frame backpacks were designed by rock climbers who wanted a pack that would not hinder arm movement while they were climbing. The pack sits snugly onto the back so it moves easily with the body as opposed to its counterpart, the external frame backpack.
The internal frame back does sit closely to the body, which makes not only arm movement manageable but also moving in general. According to Georgia State University's Touch the Earth Outdoor Program, the pack does not bounce on the back, which helps keep a person balanced. The pack is more comfortable than the counterpart, and because it's narrow, it fits into smaller spaces without getting caught on things, according to Academy.com
Because internal frame backpacks sit closely to the back, they are hotter than external backpacks, and because most internal frame backpacks have only two sections, they are harder to organize and pack, according to GSU. According to Academy.com, internal packs have minimal external pockets so all of the gear has to be packed inside, which can be a hindrance and time consuming.
