Nepal's mountain porters, who climb steep Himalayan slopes carrying loads up to 10 times their bodyweight, have been found to be the fittest and most efficient load-lifters in the world.

A study by Belgian researchers, which was quoted by Nepali media Friday, has for the first time quantified the remarkable efficiency of Nepal's porters, most of whom are from the Sherpa, Rai or Tamang ethnic groups.

The porters carry huge loads in a basket known as "doko," which is supported by a strap looping around the top of the head.

The study found their combination of technique and physical ability makes their performance far more effective than that of Western soldiers marching with backpacks, reports Xinhua.

The research credited the ability to lift huge loads to three factors - physiology - the combination of a short but powerful stature and a high red blood cell count evolved as a result of living at high altitude.

Also critical was their carrying technique, by which a strap around the head bears the majority of the load. The third element seems to be the regular rests they take during their climbs.

The Nepali technique even surpasses the most efficient carrying methods studied to date - those of African women whose loads are balanced on or suspended from the head, said the study.

A team led by Norman Heglund of the Catholic University of Louvain, in Brussels, conducted tests on eight porters travelling to a market in Nepal's northeastern town of Namche, which lies 3,500 meters above sea level and close to Mount Everest. The youngest porter was 11 and the oldest 68.

The dirt-track route from the Kathmandu Valley to Namche covers 100 km, with combined ascents of about 8,000 metres and descents of about 6,300 metres, and takes seasoned porters between seven and nine days to complete.

Hundreds of porters make the trek every week. On a single day, the researchers counted 545 men and 97 women, along with 32 yaks, on the route with many more people passing earlier and later in the darkness.

All carried loads that seemed unfeasibly heavy to Western observers. The men bore an average of 93 percent of their bodyweight and the women an average of 66 percent. A fifth of the men were carrying 125 percent of their bodyweight and one managed an astonishing 183 percent.

By contrast, the greatest loads carried by African women, such as those of the Kikuyu tribe in Kenya, amount to 60 percent of bodyweight, and the loads typically included in military backpacks are lower still.

In the study, the porters were asked to walk along a 51-metre flat track at five different speeds, carrying six or seven different loads, while their oxygen intake and carbon dioxide output was measured.

The tests revealed that loads of up to 20 percent of bodyweight were carried "for free," meaning that the porters' metabolic rate did not increase at all compared with an un-laden walk.

With higher proportional loads, their energy efficiency was far greater than seen with the most efficient head-based carrying techniques used in Africa.