When a puppy is born the ends of its fore limbs, pelvis and hind limb bones are soft and it is from these 'soft parts' that the bones continue to extend as the puppy grows. These areas are called growth plates. They are the softer part of the bone and are therefore a weaker point. They continue to be soft as the dog grows, but once the dog stops growing the calcium and minerals within the bones harden the softer areas. When the hardening process is complete, most growth stops and the growth plate closes.
It is during this growth period that the puppy is at risk from bone and joint disorders, especially if they are over exercised. Too much activity can damage the growth plate development and it is this damage that can prevent the bone from growing normally.
If this happens it can result in incorrect growth patterns and a common example of this is when a dog's wrist starts to turn to the inside of the leg instead of being straight. Another example, is in the case of it stopping growth completely - this could then result in the dog having severe problems, such as a shortening of a limb or even developing deformities.
Growth plate damage can be caused by mild trauma like a knock or a bang to the leg, which can result in injury even if it doesn't break the skin. It can also be caused by over exercising while the growth plates are still soft. Therefore, puppies should be protected against damaging growth plates by monitoring the type of exercise the dogs are participating in. This means they shouldn't get involved in any high impact and leg twisting activities until the growth plates are closed.
It is therefore necessary to eliminate or limit long walks, jogging on hard surfaces, agility activities, leaping and catching high balls and frisbees until the puppy matures. You should build up the exercise levels slowly as the puppy grows, but always maintain the socialisation, adventures and exploring aspect of a walk as this is great for the dogs mental and physical state.
One alternative to high level activity is to come up with games and low level play. Games like 'find the toy,' coming to you when you call their name, sitting and heal work is a really good way for you to bond with your puppy in the early months and it is also good for mental and physical stimulation.
The kennel Club suggests 5 minutes of exercise for every month of age, up to twice a day, so a three month old puppy can have 15 minutes of exercise twice a day. But it is worth reiterating that this does not include high and intense levels of activity. If you add 5 minutes for every month it won't be too long before you can increase the levels of exercise as your puppy matures and stops growing.