we use a backpack on axle (except i pack it with canned foods so i know the weight is exact, and HEY if i ever get lost, i always have something to eat ha ha ha) and he really likes it. ours sits on his front shoulders, and we don't use it very often b/c he has hip and joint problems that sometimes flare up, BUT i can say that it does wear him out easier on our walks, which is why we originally got it also. i was preggo at the time and couldn't walk as far, and he was BOOMING with energy. it does build their muscles up, and make them stronger, though. when we got the back pack this is also the time we started using our first halti lead. it REALLY helped us out, and the benefits were complete once we used it. i got the shorter walk i needed then, axle got his exercise and stayed healthy as could be, and axle couldn't pull me. oh, another note, axle loves having his job especially when we put diapers, wipes, and sippy cups in it. you can see the happiness in his face when he finds out he's doing something useful!