While it is common for vets, breeders, and trainers to caution against over-exercising your puppy, there is limited scientific evidence of what is the correct amount of exercise to give to build a strong and healthy dog.
There is some evidence that stair climbing before the age of 12 weeks is a risk factor for developing hip dysplasia and the effect of a slippery surface in whelping and rearing boxes is also under investigation. Otherwise, we are mostly advising based on common sense and an understanding of how bones, muscles, ligaments, tendons, and the nervous system develop.
Very young puppies have large amounts of cartilage at the ends of their bones which is why X-rays of baby puppies look so weird. The cartilage growth plates allow the bones to grow before finally mineralising into bone in young adult dogs. The rate of growth and the closure of the growth plates varies between bones and breeds, but larger dogs have open growth plates for longer.
Some stress is required for normal bones and soft tissue development so puppies should run and jump and fall over! What could be problematic is repeated stress in exactly the same place, or occasional severe impacts on a joint. Examples of things that could cause repetitive stress might include jumping agility jumps or treadmill running. Puppies are more likely to sprain ligaments if they do exercise they aren’t ready for and before they have built up proprioception (or body awareness) so again, activities that involve lots of tight turns and explosive movement such as chasing balls and catching frisbees should be limited in the growing dog.
Most owners want to exercise their puppies in order to tire them out, but training and mental stimulation will do this more safely and effectively than really long walks. There isn’t a hard and fast rule about walking puppies, but stop before they start to get tired and stumble. Remember, your puppy can stop if you are playing in the garden and he feels tired, but on a walk he will follow you even when exhausted. Short, positive walks with lots of interaction are better than miles of stomping round the pavements, but you can have several short walks or training sessions a day. Stop exercise if your puppy wants to lie down or sleep, if he is lagging behind, and if he starts to trip or stumble. If your puppy becomes lame, rest him and seek veterinary advice if soreness persists. Don’t forget that a puppy requires 18 – 20 hours sleep!