No, I don't think it's the "#1 avenue to lamenss." And neither stifle or hock injuries are particulary related to jumping.
Jumping is harder on the horse's front end than the back - injuries I might associate with a horse being jumped too often or too much would include all the concussion related ones - navicular, ringbone, sidebone and soft tissue strains of the front end - pulled suspensory, check ligament, DDFT, etc.
I'll also point that endurance horses are also prone to the former, and race horses prone to the later.
Hock and stifle lameness are more common in horses that really work of their hind ends - dressage and cutting horses.
As others have said, best way to keep your horse sound is with good care and careful conditioning. There's no reason to avoid jumping or any other activity in moderation. If you wrapped the horse in bubble wrap and cotton wool, and kept him in a padded stall 24/7, he might still find away to injure himself and go lame.