This article is from the Frisbee Dogs FAQ, by Kevin Robair with numerous contributions by others.
Care should be taken when training your dog to jump. When a dog leaps,
it should land flat, so that all four legs absorb part of the impact.
Many Frisbee dogs jump and land vertically, and this can stress the
rear legs and spine. The canine knee is not designed to absorb
vertical force, and a tear of one of the knee ligament is more likely
in a vertical landing, especially if a dog is twisting at the same
time. Try training your dog to jump by teaching them to jump through a
hula hoop, or some other device that will not hurt them if they crash
into it. This will force the dog the get their butts up in the air
during a leap and land flat. Remember that young dogs, under 14
months, should not be encouraged to jump.