This section is from the book "The Dogs Of Great Britain, America, And Other Countries. Their Breeding, Training, and Management in Health and Disease", by John Henry Walsh (Stonehenge). Also available from Amazon: The Dogs Of Great Britain, America And Other Countries.
Exercise is necessary at all ages, but the fully developed dog may be confined for some little time without permanent injury, the formation of his feet and the texture of his bones and muscles being then finally settled. On the other hand, the puppy will grow according to the demand made upon his mechanism, and if the muscles are left idle they do not enlarge; while the feet remain thin and weak, with the tendons and ligaments relaxed, so that they spread out like a human hand. Growing puppies should be provided with an area sufficiently large for them to play in, according to their size, and under cover up to the end of the third month; after which, if they have a sheltered sleeping-place to run into, they will generally avoid heavy rain. Young puppies play sufficiently in a loose box or similar enclosure; but, after the time specified above, they must either have their entire liberty, or be allowed the run of a larger space, the alternative being bad feet, defective development, and weak joints.