Whenever two or more dogs are playing in a yard, always bend your knees because if they lose control and bang into you then your knees can get dislocated or get hurt really bad.
Within my 21 years of working with dogs, this is happened plenty of times and especially a medium to large dog, they can bring you to your knees and make you fall and it HURTS! It happens in a split second. Any aged dog can loose control and bang in to you but it happens more with puppies. They are more clumsy and do not have much spatial boundaries. Do not put your guard down especially if you are outside with your pups in the morning and you are really not fully awake.
Imagine walking straight, and Then getting plowed from the side from your pup and get thrown to the ground on all fours. THAT WILL WAKE YOU UP!! TRUST ME.