As some of you may know, I have a (now) 7.5-month-old Labrador Retriever puppy who is doing really great and I'm really enjoying him. I don't think, however, that I've been doing a very great job of giving him enough outlets for all his puppy energy. He is not 'hyper' - he just has normal adolescent lab energy. I've read about using dog backpacks (with water bottles or the like as weights) to get more out of walks and such, but I'm concerned about putting too much strain on his growing joints and ligaments. Does anyone know whether or not this would be a problem for a 7.5-month-old lab? He's about 50 lbs., and probably close to his adult height. I'm guessing he'll put on another twenty pounds or so, though - mom was 65 lbs. Any thoughts?