The st bernards need enough exercise to keep them lean and healthy, but not so much that their soft growing bones, joints, and ligaments become over-stressed and damaged.
The proper amount of exercise can be difficult to regulate in giant breeds like st bernards. The st bernards can be rambunctious. St Bernards will romp with uncoordinated gawkiness all over your house. Left alone, young st bernards become bored and destructive.