Thursday, July 1, 2010

Are Porter's More Efficient?

Link to Abstract, Embedded article below

I came across an interesting paper that was looking how and why are Himalayan porters able to move much faster than their Caucasian counterparts...outside of their superior chronic acclimation to high altitudes. Basically, they are more efficient, but why?

Short Answer:

Porters are not only better acclimated and lighter, they also move more efficiently, seemingly due to a more horizontal trunk posture which they are able to control well.

Long Discussion:

Although its the largest factor, it's not all about conditioning.

Evaluation of "performance" in hiking / climbing is often defined at least partially by how fast someone ascends a mountain (i.e. ft / hr, like ascending 4000 ft in 2 hrs). While this may be a decent way to illustrate "performance", it does not necessarily defined cardiovascular fitness, or economy of movement. Factors that may affect performance include

1. Cardiovascular conditioning
2. Altitude and level of acclimatization
3. Bodyweight and backpack weight
4. Efficiency of movement

If you do not consider how all of these parameters may vary between people, you may incorrectly attribute performance differences between people to only one. For example, if one person ascends 4000 ft in 2 hrs, and another guy in 2.5 hrs, is the former in "better shape" than the latter?

Or, was the latter hiking at a higher altitude? Was he carrying an extra 50 lbs of bodyweight or backpack weight? Or was he simply that much less efficient in movement? Someone can be in better cardiovascular shape but slower because they weight more.

In the paper below, the authors were trying to control for #1, #2, and #3 to see if there was a difference in economy (#4). Economy of movement is best defined as:

Efficiency = Power Out / Power In

In climbing, power out is defined as the energy needed to overcome gravity:

Power out = (Bodymass + Backmass) * gravity * distance / time (Watts output)

Power in is defined by the amount of energy consumed, which is related to oxygen consumption.

Power in = Oxygen consumption rate (Liters O2/time) * Constant (Energy / Liter) --> # Watts input

Power out will always be less than power in, which ratios generally around 0.2 - 0.25. That is, your body will consume 4-5 times the minimal energy needed to lift your mass a certain height. Why is effiency so low? Probably a lot of reasons, like a car there are energy losses along the way. Energy loss in heat during muscle contraction, damping of of movement, and conversion from linear (muscle) --> rotational (joints) --> linear (vertical mass movement).

Everyone is different, and the researchers found that porters' efficiencies were higher than caucasians. So aside from already moving faster because of lower bodyweight and better aclimitization, porters also are more efficient in movement! But why?

The study found that porters had better control and less variability in their trunk movement. Less variability means less energy co-contracting muscles and correcting for errors in trunk motion. In addition, it was noted that porters generally had a more forward-leaning trunk than caucasian climbers. Not mentioned in the article, but my own reasoning indicates that this trunk posture should actually be harder to control, making the results even more impressive. There may be an efficiency benefit to leaning the trunk forward, but only if one is able to control it well!


  1. Hi Ze. Saw your comment on Lyle's site and thought you might be interested in a couple of excellent posts by an exercise physiologist/coach:

    Also you might be interested in the series of posts on on a couple of Nepalese porters who are running an ultramarathon in Switzerland.


  2. Thanks Cynthia.

    Also I note that you are in Palo Alto... I'm back and forth between Redwood City and LA. Quite nice place to live up here :)