I can tell you from actual first hand experience (not quoting someone else
) that lack of exercise both physical and mental causes behavorial problems -- not only in dogs, but in children, cats, horses, and goats. Animals (human, canine, and otherwise) need both physical exercise and mental exercise to be healthy and well adjusted. I have raised 3 healthy well adjust children, scores of horses, cats and dogs plus a few goats. The more active and mentally stimulated my children and animal family were the better behaved they were and more receptive to training.
I have based all of my input on FIRST HAND EXPERIENCE -- not from quoting third party unknowns with letters behind their names that may or may not be valid.