Thursday, January 10, 2008
In praise of the tump line
[a villager uses a tump line to lift a load of elephant grass in Chitwan National Park, Nepal]
There is something to be said for simple inventions that work well. Like the tump line.
For me the tump line was a curiousity of remote outposts until 1985 during a trek in the midland hills of Nepal. That was when Mrs Galloway broke a metatarsal bone, and Mrs Joslyn's knees went on strike.
"We will carry them", announced Sherap Jangbo, our Sherpa leader. He wasn't kidding. He consulted with the porters, and the youngest among them stepped forward to "do the needful".
At first the lad tried to fashion a tump line from a used lungi that resembled an oversize dishrag, but then Jangbo handed him a cinch belt from a horse. This ancient piece of leather had snapped the day before, and as a result a retired school teacher from Virginia learned that she had a talent for tumbling. Equally amazing were the repairs to the belt which was about to be used as a tump line.
With mime and broken English our Sherpa informed the ladies how to sit in the sling. We watched dubiously as Mrs Joslyn assumed the take off position and wonderboy squatted. Then the moment of suspense. . . wonderboy slowly straightened his legs. Someone whispered "hernia" and then the tumpline's mechanical advantage became evident. Mrs Joslyn piggybacked up the trail on two adolescent legs in slow motion. For the rest of the trek the two ladies took turns riding the remaining horse with a saddle and wonderboy, who became a celebrity among his fellow porters.
This was the first and last tour I ever guided, but I was much taken with the tump line, and I commend it to any game readers who have hauled logs in a one-handed firewood carrier, and now suffer from one elongated arm, a lateral sigmoid deflection of the spine, a wrenched neck, or other orthopedic quirks.
Our ancestors invented several ways of hauling heavy loads, and probably long before beasts of burden were harnessed to the travois or fitted with panniers. You can balance the load on your head, suspend the load from your head using a head band and tump line, or hang the load from your shoulders using a back pack. You can also suspend two loads from a spring pole on the shoulder or on a yoke. Each method has its own mechanical stresses and energetic costs.
Recently a team of Belgian physiologists shed light on the energetics of hauling loads with tump line. Their subjects were Sherpa, Rai, and Tamang porters who trek weekly from Kathmandu to Namche, a distance of about 100 km. To reach Namche at an elevation of 3500 m requires 8000 m of ascent and 6300 m of descent. The porters are men and women aged 11 to 68 years, and they do it all in 7-9 days, barefoot.
Now get this. Men haul an average load that is 93% of their body weight, and women carry 66% of theirs'. My hat is off to the porter whose load was 183% of his body weight.
The eight porters who were the subjects of the energetics study were most accommodating. Not only did they agree to wear the 3 lb face mask and analyzer that measures oxygen consumption and carbon dioxide production, but they packed various loads up to their own body weight at different speeds (0.5 to 1.5 m/sec) on a 51 m flat track. A drillmaster kept them in step at the various assigned velocities.
For the sake of comparison the investigators used control data from previous studies in which Europeans backpacked, and African women head-balanced similar loads at different walking speeds.
The results showed that Nepalese porters were "far more economical energetically than the controls at all loads and more economical than African women at all except the lightest loads." Loads that were lighter than 20% of the porter's body weight had a negligible energetic cost, and were carried "for free", while loads that exceeded 20% gave Nepalese porters an energetic advantage that increases with increasingly larger loads. Consequently, they can carry loads with tump lines that are 30% heavier than the maximum loads African women carry on the heads. Both methods by the way are energetically superior to backpacks.
Two years ago I started to haul firewood with a tump line and canvas sling, and found it far more comfortable than schlepping a log carrier with one or both arms. My Sherpa hauling kit consists of a canvas head band (a homemade swing seat with grommets), and quarter inch cords hooked to a log carrier.
I can haul 60 lb loads, a third of my weight, like a very tired Sherpa porter. I feel more stable with a low hanging load, and find that gripping the head band eases the backward force of the load. When my head feels like it is twisting off, it is quite easy to jerk the load and adjust the headband.
The biggest challenge is lifting the load. Old codgers with bird legs just can't lift 60 pounds from a squat. You need a helper or you should load the wood on an elevated position. The amazing thing is that it isn't a big deal hauling such a load up a hill. You can only walk slowly, but amazingly you soon find your slow-motion stride and don't run out of breath.
I guess I'm a bit of a wuss, since I don't intend to try it barefoot.
Bastiene, G.J., B. Schepens, P.A. Willems, N.C. Heglund. 2005. Energetics of load carrying in Nepalese porters. Science, 308:1755.
Heglund, N.C., P. A. Willems, M. Penta and G. A. Cavagna. 1995. Energy saving gait mechanics with head-supported loads. Nature 375, 52 - 54.
Maloiy, G. M. O., N. C. Heglund, L. M. Prager, G. A. Cavagna and C. R. Taylor. 1986. Energetic cost of carrying loads: have African women discovered an economic way? Nature 319, 668 - 669.