What is their faulty assumption? That walking shod is natural. While they don’t explicitely state this assumption in their reports, it appears evident to me based on their conclusions.
The Auburn team observed that wearing flip-flops causes “sore feet, ankles and legs.” Indeed, this is often true when shoe-wearing people switch to flip-flops in the summer (it’s sometimes called ‘flip-flop-itis’). Auburn researcher Justin Shroyer notes, “We found that when people walk in flip-flops, they alter their gait, which can result in problems and pain from the foot up into the hips and lower back.” Mr. Shroyer is absolutely correct in that wearing flip-flops alters the human gait; what I believe he fails to recognize is that the alteration is a change back to the natural. Constant shoe-wearing had previously altered the subject’s gait to an unnatural one. Switching to flip-flops does cause pain in some people because the walker is returning to a more natural gait – one to which s/he is not accustomed.
Mr. Shroyer and his colleagues also observed that walking in flip-flops led to shorter steps (i.e., shorter stride lengths) compared to wearing athletic shoes. And he found that, in flip-flops, walkers’ heels hit the ground with less vertical impact force and they tend to grip more with their toes during push-off. These findings are consistent with those of other researchers in the field of foot biomechanics; however, the changes were considered abnormal and negative by the Auburn research team. Scientists are now realizing that these barefoot (and flip-flop) induced changes in walking lead to lower impact forces and less stress to leg joints than shod walking (due primarily to shock absorption by the arch and gait adjustments from tactile feedback), and this is generally recognized as a good thing. In addition, the Auburn researchers imply that it’s unnatural to use the toes for push-off and they don’t acknowledge that chronic shoe-wearing is responsible for the shortened tendons and weak muscles that cause the pain flip-flop users feel in the first place. Instead, because they apparently regard the shod gait as natural, they label flip-flops as “bad” and caution folks against wearing them. This is a logical conclusion if you start with the assumption that walking in shoes is natural. If instead you assume that barefoot walking is the natural condition, then the conclusion you draw from their study is very different.
The Auburn study was performed in 2008. In 2011 we now realize that comparing the 'flip-flop' gait to the 'shod' gait is the wrong comparison. Instead, we should be comparing the 'flip-flop' and 'shod' gaits to the 'barefoot' gait. When we do that, we see that walking in flip-flops is more like walking barefoot and is therefore the healthier shoe option.
Different Assumptions = Different Conclusions
(assuming shod gait is natural)
(assuming barefoot gait is natural)
1. wearing flip-flops resulted in pain in feet, ankles, hips and legs
walking in flip-flops causes abnormal & painful changes in gait
walking in flip-flops mimics barefoot walking & reveals unnatural shoe-induced changes in gait
2. wearing flip-flops resulted in shorter stride lengths
long strides should occur (predominantly in front of the body’s center of gravity?)
short strides should occur (predominantly below and behind the body’s center of gravity?)
3. wearing flip-flops resulted in less heel impact forces
gait pattern should be heel-to-toe and heel should take most impact (like in a shoe)
gait pattern should be short leading to landing flatter on arch which absorbs impact forces (as when barefoot)
4. wearing flip-flops resulted in more toe-gripping action
the body should roll on the forefoot into the next step. (function of the toe appendages?)
the toes grip the ground as the body is pushed into the next step; flip-flops may induce unnatural toe gripping