Misbehavior while playing Outside

Question
My male Lab is just over 6 months old, and 52 lbs.  I am the primary pet
feeder, walker and trainer.  He has begun some distressing behavior when off
the leash in the yard: running around me in circles, growling and jumping up,
then sometimes running up behind me to jump up and hit me with his paws
or bite me on the backs of the legs.  He does not do this in the house, and in
fact is getting good at sitting, going down, going in his crate without urging,
and walking on leash.  Today I tried turnbing around when he was running, so
that I was always facing him (so he couldn't sneak up on me.)  That seemed to
help, as does blowing a loud whistle or screaming suddenly.  Do you have any
other tips?

Answer
He clearly does not understand how he should fit in.  The key to most behavior problems is approaching things using the dog's natural instincts.  Dogs see all the people and dogs in the household as a pack with each having their own rank in the pack and a top dog.  Life is much easier if the 2 legged pack members outrank the 4 legged ones.  You can learn to play the role of top dog by reading some books or going to a good obedience class. A good obedience class or book is about you being top dog, not about rewarding standard commands with a treat. Start at http://www.dogsbestfriend.com/  For more on being top dog, see http://www.dogbreedinfo.com./topdogrules.htm