Literature

We have known for some years that the energy cost of walking and running is related primarily to the work done by muscles to lift and move the limbs. Furthermore, energy cost is typically independent of running speed. This being so, it is of interest to look at the relative importance of leg length, and it can be predicted that longer legs reduce energy costs because "longer legs [-] reduce the amount of up-and-down movement in a stride, and so also reduce the force needed to push down with each step." A mathematical model for calculating energy costs for bipeds and quadrupeds has been developed by Herman Pontzer and published in The Journal of Experimental Biology. "'All things being equal, leg length is one of the major determinants of cost', says Pontzer, adding that if two animals are identical except for leg length, longer legs are more efficient."
The Nature news report of this paper has the title "These legs were made for walking", which evokes a design recognition response in the reader. However, intelligent design is not being considered. Rather, the emphasis is on adaptation driven by enhanced energy efficiency. "The fossil record shows that two million years ago, there was a big increase in leg length in early humans," says Pontzer. He suggests that a reason for this increase could have been the energy saved by having longer legs."
It should be noted that the link between energy efficiency and leg lengthening is not a very compelling argument because the Darwinian approach to adaptation has to find a link with passing on ones genes (and this requires some dubious just-so stories). More importantly, it is good practice in science to consider multiple hypotheses and to find ways of evaluating them. One often notes arguments by Darwinians making the claim: "an intelligent designer would not do it this way", always leading to rejection of the intelligent design hypothesis. Here is a case where there are good reasons, supported by a mathematical model, why an intelligent designer would do it that way. So, one asks, should the evidence for an ID explanation of why humans have legs that are relatively long be considered for evaluation within science?

Predicting the energy cost of terrestrial locomotion: a test of the LiMb model in humans and quadrupeds.
Pontzer, H.
Journal of Experimental Biology, 2007, 210, 484 -494.

Summary: The energy cost of terrestrial locomotion has been linked to the muscle forces generated to support body weight and swing the limbs. The LiMb model predicts these forces, and hence locomotor cost, as a function of limb length and basic kinematic variables. Here, I test this model in humans, goats and dogs in order to assess the performance of the LiMb model in predicting locomotor cost for bipeds and quadrupeds. Model predictions were compared to observed locomotor cost, measured via oxygen consumption, during treadmill trials performed over a range of speeds for both walking and running gaits. The LiMb model explained more of the variation in locomotor cost than other predictors, including contact time, Froude number and body mass. The LiMb model also accurately predicted the magnitude of vertical ground forces. Results suggest the LiMb model reliably links locomotor anatomy to force production and locomotor cost. Further, these data support the idea that limb length may underlie the scaling of locomotor cost for terrestrial animals.

See also:

Blackburn, L. LONG LIMBS COST LESS, Journal of Experimental Biology, 210, 0ii (2007)

Odling-Smee, L., These legs were made for walking. Model opens up research into efficiency of motion. news@nature.com, 19 January 2007; | doi:10.1038/news070115-9