There are three different forms of exercise - Free, Forced and Impact.  All can be strenuous but only one is beneficial to a young growing puppy while the other two need to be  limited until the baby's joints ligaments and tendons are fully developed.

FREE EXERCISE -  This is the exercise a puppy gets when he/she is off leash and free to play, stop, change direction, flop down, get up again, turn and twist and 'do what comes naturally'.  Your puppy will regulate his or her own amount of exercise this way and will lie down and rest when it needs to.

FORCED EXERCISE is when a puppy is walked or trotted on leash.  During this kind of rhythmic exercise, the young animal is using the same sets of muscles, tendons and ligaments in  repetitive continuous movement.    Being forced to exercise this way for lengthy periods has much the same affect on the puppy as a repetitive motion for extended periods can cause 'repetitive strain injury' in human beings.  Immature joints can also suffer irreparable damage.

IMPACT EXERCISE is when the puppy is allowed to jump down off a higher place (not recommended until after 12 months of age)  such as furniture or down out of the car.   A dog's joints muscles tendons and ligaments are not mature until he/she is over twelve months old and in very large breeds often older so exercise needs to be carefully monitored.

Walking On Leash and Swimming

Three  to Four Months of Age                         No more than ten minutes  in one session

Four to Six months of Age                                   Fifteen minutes maximum in one session

Six Months to Twelve Months                       Gradually working up to half an hour in one session

Twelve Months and Over                                     Develop stamina for long walks or jogging with 'interval training'. 

For a puppy, Interval Training means varying the pace in intensity over short bursts of timeframes, mixing up a minute or two of galloping, with a few minutes of walking, slow trotting and fast trotting.

Free (off leash) exercise can be interspersed between the regulated leash walking above.  Puppies instinctively will flop down and rest intermittently before leaping up and running some more.

Click to Read about Interval Training

 Puppies under twelve months should never be allowed to play on stairs, nor be allowed to run up and down stairs.  Dogs are quadrupeds and are anatomically designed to run on relatively level surfaces. Running up or down stairs before maturity can cause irreparable damage  by way of OCD's i.e. Hip Dysplasia or Elbow Dysfunction.