There are three possible headlines for this blog post:
Running in shoes takes less energy than running barefoot.
The heavier your shoes are, the less efficiently you run.
Running barefoot "offers no metabolic advantage over running in lightweight, cushioned shoes."
The topic is a very cool and thought-provoking new study, from Rodger Kram's lab at the University of Colorado, just published online in Medicine & Science in Sports & Exercise (whose official conclusion is Headline Number 3). While there are plenty of subtleties in the study, the basic idea is very simple. They took a group of runners and measured their efficiency (by measuring how much oxygen they consumed at a given pace) while adding progressively larger weights to their feet. They did this both with and without shoes on. Here's what they found:
One of the findings isn't surprising at all: the more mass the researchers added to each foot (using small strips of lead on the top of each foot), the less efficient the runners became. As previous studies have found, every extra 100 grams per foot made the runners consume about 1% more oxygen. That's Headline Number 2 above, and it's a fact that is often cited by advocates of barefoot running: since shoes typically weigh a few hundreds grams, you should immediately become more efficient by taking off your shoes.
But if you look carefully at the graph, you notice something interesting. All the solid dots (shoes on) are lower than the open dots (shoes off), which means the runners were consuming less oxygen while wearing shoes. In fact, the first solid dot (just shoes, no weight added) is lower than the first open dot (no shoes, no weight added). How can this be, if extra weight makes you less efficient?
The inevitable conclusion -- Headline Number 1 -- is that wearing running shoes somehow makes you run more efficiently, independent of the added weight. If you look at equal-weight conditions, the shoes appear to provide a benefit of about 3-4%. The researchers propose two possible explanations for this effect. One is that the runners spontaneously took slightly longer strides (by 3.3% on average) when they were wearing shoes. These longer strides may have allowed them to be more efficient, thought it's unlikely to account for the whole effect. The other is the cushioning in the shoes: without cushioning, the researchers suggest, the runners' leg muscles have to expend metabolic energy to absorb the impact of each stride. They call this the "cost of cushioning" hypothesis.
A few relevant details about the study. The shoes they used were Nike Mayflys, which are very light shoes, with a mass of 135.6 grams for a men's size 9. The 12 runners included in the study were all experienced barefoot runners, running at least 8K (5 miles) a week barefoot or in minimalist footwear. That's not much, but avoids problems with a barefoot learning curve that may have confused previous studies. Finally, all the runners were midfoot or forefoot strikers, both barefoot and in shoes. This condition was imposed to prevent confounding effects from comparing rearfoot to forefoot striking efficiency.
These conditions raise an important caveat. One of the proposed advantages of barefoot-minimalist running is that it automatically helps to correct overstriding -- an extremely common problem among inexperienced runners. The fact that all these runners were already forefoot strikes suggests that none of them were likely overstriding, which would make them less likely to benefit from barefoot running. It's possible that a truly "random" group of runners might have been less efficient in the shod condition, because more of them would have been dramatically overstriding.
Anyway, the bottom line is: this is a surprising result, at least to me. In the long-running debate over running form and running shoes, many of us had started to accept the claim that cushioning in running shoes serves no purpose. But maybe it does, after all. This debate is far from over!