Just to warn you, when I went to Uganda on medical missions, we treated untold numbers for neck pain, back pain, and cervical neuropathy for the years and years of carrying things via tumpline and/or just balancing objects on their head. Of course, we are talking about 20-60 lbs for 20 or so years, but I can't imagine it is good at all. Even on rare occasion it is still a bad idea. As others have said, your muscles are not conditioned to carry objects in that manner.
Just my 2 cents.