I took a quick look at the paper. In order to understand it, you need to also look at this paper: "The Relationship Between Lower-Extremity Stress Fractures and the Ground Reaction" (Zadpoor, et al), which contains the definition of "instantaneous vertical loading rate (VILR)".
In a nutshell, VILR is the slope of the impact when the foot initially contacts the ground. Prof. Lieberman found that the VILR is less steep in barefoot runners, since the heel does not make the initial contact with the ground. His finding is illustrated in these two images from his website:
As you can see, that's quite a dramatic difference. Lieberman et al found that nearly all runners change their stride when you take their shoes off: they stop heel striking, and the impact transient goes away. He's not the only one. Hamill, et al in "Impact Characteristics in Shod and Barefoot Running" (Covered by Mr. Burfoot here) had a similar result. Pete the Runblogger summarized the study thus:
"As Steve said, this was a simple study with a simple hypothesis. Basically they showed that if you take a sample of runners accustomed to heel striking and have them run down a runway either in 4mm drop shoes of varying thickness or barefoot, barefoot differs than the 4mm drop shoes. I think most here would agree with this result. You take a heel striking runner accustomed to marshmallow shoes and put them in the Minimus and have them run down the road ten times, I'm guessing that they will still be heel strikers. You have them run barefoot and things change, which is why we all view barefoot as the best way to get a feel for our default stride. So what this shows is that changing to a 4mm drop shoe is not necessarily enough to change your gait in any fundamental way, but going barefoot is. I actually think these results are pretty cool and support what everyone here is saying all of the time. Barefoot is different than a 4mm drop shoe, and unless you work to alter your ingrained motor patterns, going to a low drop shoe in and of itself may not change very much from an impact standpoint, at least over the short term...."So Lieberman and Hamill both found that barefoot runners run differently than shod do. The effect is so reliable that Pete uses it as a parlor trick in his classes. Becker's paper found the same thing: the "center of pressure" for barefoot runners was dramatically different from shod:
|Dramatic change, no?|
So how does Becker, et al make the claim that the VILR does not differ between shod and barefoot? Zadpoor defines VILR in this easy-to-understand graph:
|Clear, right? T is the period of the VILR.|
I don't know. Unfortunately Becker does not explain what they are using for VILR. They also mention that they're using the "Strike Index" from Cavanagh, et al. That paper doesn't include the word "index", so I again don't know what they're talking about...
What I suspect was happening is that their barefoot runners may have been running with a bit of a heel strike, which beginning barefoot-style runners can do, especially if they're running on a softer surface, and are used to heel-striking. They quickly get out of the habit, as it hurts. Again, Pete the Runblogger experienced this first-hand.
"Subjects ran continuous laps around a 25 meter track in the laboratory under both shod and barefoot conditions."
That's the only explanation I can find for why all the running studies but this one find that going barefoot reduces the impact transient in near-100% of runners. Either that, or they weren't measuring the VILR correctly (which I suspect, as they show it as being higher, not lower, for the barefoot runners). I've seen some evidence that barefoot-style running increases the Ground Reactive Force (GRF), while eliminating the VILR. Becker may well be reporting GRF as VILR, but it's tough to tell from that summary.
Why is this whole issue important? Neither Amby nor Becker goes into this issue, but if you read Zadpoor ,which this new study is attempting to refute, you'll see why it's important:
"The results of the fixed-effect meta-analysis of all included studies (Table 4) show that there is no significant difference between the vertical GRF peaks of the stress fracture group and those of the control group (PN0.05, Table 4). However, the AVLR and [VILR] are significantly higher for the stress fracture group (Pb0.05, Table 4). The same conclusion holds when the meta-analysis is performed only within the tibial or metatarsal stress fracture groups, except in the case of the [VILR] of the metatarsal fracture group for which only one study is included (Table 4)."
So there's a non-random relationship between the impact transient, and stress fractures. Eliminating the impact transient should reduce stress-fracture injuries, and the best way to do this reliably is to eliminate the shoes or wear barefoot-style (no heel-cushion) shoes.
In other words, sneakers cause stress fractures.
"Lower-limb stress fractures are among the most common injuries to athletes and military recruits (Fredericson et al., 2006; James et al., 1978; Kowal, 1980; McBryde, 1985; Milner et al., 2006a). According to one report, up to 20% of all sports medicine clinical injuries are lowerlimb stress fractures (Fredericson et al., 2006). Stress fractures are also a major problem for military recruits. According to Ross and Allsopp (Ross and Allsopp, 2002), stress fracture is the most common reason for the loss of training days for Royal Marine Recruits. These injuries cause so many lost training days due to the fact that stress fractures are among the more severe injuries of the lower-extremity musculoskeletal system, and need extended periods (4 to 8 weeks (Brukner et al., 1998)) of refraining from physically-demanding activities for
recovery (Friedl et al., 1992; Jones et al., 1993; Macera et al., 1989; Rauh et al., 2006)."
The smoking gun, in other words.
No wonder Amby likes Becker's study, if it's correct there is no smoking gun, and he's not been peddling snake-oil for the last few decades.
But I think it's pretty clear that it's not correct.
P.S. Related post: Barefoot Running and Shin Splints:
"However, gait retraining using real time feedback has been shown to reduce [tibial shock] during running up to 25%."