As suggested at this link I quote: "Canine hip dysplasia is a complex disease. It is a concentration of factors from a pool of genetic weaknesses and environmental stresses that fall into a programmed pattern of progressive remodeling and degenerative joint disease. The degree of involvement varies from minute changes in bone structure to total destruction of the hip joint. Investigators have searched intensively for genetic, chemical, and metabolic defects, but the cause has remained obscure. No specific genetic pattern of inheritance has been demonstrated in this variable disease. It has been demonstrated that both genetic and environmental influences contribute to development, regardless of the species affected.(15,31, 32,40,74,76) Consequently, the disease has been designated as polygenic or multigenic.(28) As in most polygenic diseases, there are both major and minor causative factors. There is no evidence that a primary defect of bone exists but rather the disease is a failure of the muscles and other soft tissues to hold the hip joint in full congruity.(31,32) This is further supported by the fact that bony dysplasia can be increased, decreased, or prevented by controlling the degree of joint instability and incongruity.(53) No other malformations are associated with the disease.(79) A causal relationship between muscles and soft tissue defects or pathologic changes other than lack of muscle mass or strength has not been established.(40,41) The general cause of hip dysplasia, when defined, must be broad enough to explain its development, not only in dogs, but also in all other affected animals. Many genetic and environmental factors can trigger events that bring about the condition secondarily.(74,77,79,88) Hip dysplasia, therefore, is not one disease but many diseases that result in common degenerative lesions of the hip joints.(77)"
At this link you'll find studies that suggests that "puppies born in the spring or summer, and to breeders who lived on a farm or a similar area, had a lower risk of developing hip dysplasia." Through observations, Krontveit confirmed that varied exercise had a positive effect and dogs that exercised on a daily basis on a leash as well as running free in different types of terrain were free of symptoms longer than the dogs that were less active. She adds that canine hip dysplasia in its most serious forms can be prevented, and that the life quality of dogs improves if preventative measures related to early canine life is introduced. "I also will highlight that the breeds included in our research are large breeds and that the results might not be directly transferable to small breeds," Krontveit says.
Unfortunately, older puppies do not benefit in the same way from free exercise. Krontveit says, "Based on our research, I can say that none of the exercise parameters we registered seemed to have any effect on hip dysplasia risk after approximately three months of age. [This means] that preventive efforts probably are most effective before this age. Having said that, I would still recommend that genetically prone pups should be exercised regularly to strengthen musculature, but extensive jumping/stair use, etcetera should be avoided until growth is completed (at approximately 12 months)."
Living and growing up on a farm in the Pineywoods of East Texas, my first exposure to the German Shepherd breed was a neighbor's dog. Interestingly, as a child, I admired his calm demeanor and air of intelligence, much like the coonhound's my father raised and hunted. Being an avid horse rider, these dogs were ever at my side with energy to spare. I was one of those wild children, ever with animals. I once enticed a coyote to follow me home. I tamed him without ever putting him in a cage, to me, he was my German Shepherd dog. this horrified my Grandmother who was certain I would be attacked, this never happened, we just started losing chickens and cats at an alarming rate. My Father hauled him away for his nefarious ways. But until that day, I would go out and he would appear for his daily scratching and followed me around, like any dog would. He loved to lead me by my hand in his mouth.