Anyone who's traveled to Nepal or Africa has probably been amazed at the loads that porters and villagers carry on their heads. Not only is the balance precarious, but I cringe at the loads carried with ease and grace on the head neck and spine. But it's not as hard as it looks. Really! 
Researchers studying the biomechanics of locomotion under load bearing find that people who routinely carry very heavy loads do so more efficiently by supporting the weight on their the head. In fact, only in the Europe and the U.S. do people predominantly carry the weight on their back.
A recent report studied women in Africa, and found that they can carry 20% of their body weight without increasing metabolic output, and up to 70% of their body weight while expending far less energy than Western European or Americans, for instance military personnel, would use to carry the same load on their back.
How? The researchers studying the African women found that they adjust their gait to accommodate the increased load without commensurate energy expenditure. Norman Heglund et al., explains the energetics of gait:
"When a person (or animal) walks, their body goes up and down, and goes faster and slower, within each step. The energy changes associated with these fluctuations in height and speed are out of phase and therefore tend to cancel each other, minimizing the energy required to keep the movements going, much like in a pendulum. But in walking the energy fluctuations are not completely cancelled (as would occur in a perfect pendulum); at most about 65% of the energy fluctuations are cancelled, leaving at least 35% of the energy fluctuations which must be supported by the muscles each step, requiring metabolic energy input."
As the loads increased, the African women in the study managed to cancel more of the energy fluctuations, therefore requiring relatively less muscular energy for each step with the heavier load, then would have been predicted if there was a linear relationship between increased load and effort to carry the load.
The same research group recently studied porters in Nepal, and found that these porters are even more biomechanically efficient then the African women. Scientists conducted the study by hanging out on the outskirts of Namche Bazaar, the small town which sits at about 3500 meters (~11,500) feet in Nepal, known as the gateway to Mount Everest.
The researchers counted 545 male and 97 female porters who trekked by them during the daylight hours one day before the weekly bazaar. The researchers averaged the distances the porters had walked and estimated that each porter had traveled on average 9 days to reach the market, over 100 horizontal kilometers (~62 miles), had ascended about 8000m (~26,250 feet) and descended about 6300m (~20,669 feet) en route. The authors estimate that more than 30 tons of products were carried by porters to the market that day.
Eight participants were randomly chosen to be in the study. These porters walked around a track carrying various loads at different speeds. The researchers measured the amount of energy they used by calculating the differential between oxygen intake and CO2 output.
They found that the Nepalese porters aged 11 to 67 can carry loads of up to 187% of their weight - 20% carried loads more then 125% of their body weight. For heavier loads, their metabolic efficiency increased, but at the "optimal" load weight they used less energy then either western backpackers or the African women. Which explains in part the guy able to carry tanks of fuel over the 18,000 foot pass.
Part of the efficiency in carrying these extreme weights over difficult terrain (11,000-18,000 feet) -- in bare feet -- is explained by the gait energetics. However the particular gait that the African women use, theorized to economize energy use is absent with the Nepalese porters, so part of the efficiency has yet to be explained. Perhaps it comes from cardiovascular strength, training, the mechanics of load placement during locomotion - or patience. With very very heavy loads porters walk very slowly and rest frequently.
