I know someone who bred and showed St Bernards for many many years, and I have mentioned this question to her today as a matter of fact, and she near fainted. As far as she is concerned there is not a doubt in her mind that Saints should not be subjected to any regime where their skeleton is put under undue stress. I knew what she would say actually because I remember her telling me about a couple who bought a St Bernard puppy and they took it on long walks, despite her speaking to them and imploring them not to. They ignored her advice and went their own way, because they knew better, the puppy was PTS before it was 2 years old due to it being crippled due to over exercise.

I agree that Saints are much heavier than Labs, and there may be a difference. The study I referred to did four breeds, including the Newfie, Leonberger, Irish Wolfhound, and Labrador, so would seem to apply to some giant breeds. The concept that bone and muscle should develop together seems to cross breed lines. There are old wives tales in many areas of life, including dog health.
Can you reference any study results to support the notion that juvenile exercise leads to later life bone problems in giant breeds?