I love the part in the linked article: "A maximum of 25 percent of body weight is a rough guideline."  Sometimes we try to load too much on our critters.  25% of 38lbs is 9.5.  That can be hit fairly quickly.  Also think about the environment that we taking our dogs with their packs on.  Hot, humid locations will trap body heat in and possibly overheat a dog.  Rest time without the pack in place is important to think of.