St Bernards require a lot of exercise and the correct diet due to their rapid growth rate and large size, due the origin of the breed - Working in mountains and cold temperatures, often as rescue dogs.
Yes, St Bernards have notoriously bad joints (Elbow and hip dysplasia, and Osteosarcoma is hereditary), but they will still require a lot of exercise. The exercise won't "hurt" or "damage" the joints, as the problems are already present due to the fact it is a pedigree breed.
