Last year, Asics released a special version of its up-market, heavily cushioned Kayano training shoe that the company says will tune itself to accommodate changes in a woman's menstrual cycle. If that strikes you as an absurd idea, it should. The shoe is both silly and expensive. But it is also potentially revolutionary.
True to form, shoe-skeptical runners have greeted the Kayano with mockery. Born to Run author Chris McDougall wrote that Asics had found "new depths to the cesspool" of marketing shamlessness, the minimalist-shoe website Zero Drop said it sounded like something from the Onion, and Phil Maffetone, a doctor and barefoot running advocate, called the Kayano a hoax.
[As an aside, I'm not exactly sure why the Kayanos are catching heat now—the original, terribly written article that appears to have sparked their outrage was published in early 2010, shortly after the Kayanos were first released.]
The thrust of their criticism, which is probably correct, is that a heavily cushioned, heavily supported shoe like the Kayano may not prevent injuries, as Asics implicitly says it does. Worse, it has a gimmick, the expensive menstrual-tuning feature, which is loosely (but poorly) grounded in science. Researchers have long known that estrogen levels, which vary over the course of the menstrual cycle, cause changes in muscle and tendon stiffness. Those changes, in turn, may create slight variations in women's arch height. The Kayanos, Asics says, counter those "periodic changes in the shape of the woman's arch" with more or less arch support, depending on the time of month.
But what's gimmicky about about the Kayano 16 is also exactly what's interesting, and it is something that McDougall and many minimalists are inclined to miss.
For years, the key question for minimalist runners has been whether modern running-shoe design has backfired and created more problems than it has solved. This is a long-running debate, and as many shoe companies bring out minimally cushioned and minimally supported shoes, it is increasinlgy being waged retrospectively.
What's lost is a sense of what shoe design of the future might bring. If the engineers of yesterday got things wrong, it stands to reason that an engineer with a proper understanding of biomechanics could create a shoe that actually works: that is, one that prevents injury and enhances performance. The most promising idea here is closely related to Asics's shoe-tuning concept, and it comes, as blogger and biologist Pete Larson notes, from Canadian biomechanist Benno Nigg.
Somewhat controversionally, Nigg believes that running injuries are partly a function of resonance, or the frequency of impact forces as they travel through the body. Each stride produces vibrations, and, according to Nigg, runners unconsciously work to keep those vibrations within a certain, personal range, either by adjusting pace, varying muscle stiffness, or altering body position (bent knee versus straight knee, etc.) Deviation outside that range may cause injury, so Nigg's idea is to create a shoe that self adjusts to keep impact frequencies stable. Much of this is outlined in Nigg's new, self-published book Biomechanics of Sports Shoes.
It is a concept almost completely ignored in the barefoot running debate, but it is worth exploring. There is a fairly robust literature showing that runners alter their stride with fatigue, and one potential reason this happens is to maintain ideal resonance frequency. (A fatigued muscle may require a different frequency than it did while rested.) Theoretically, a self-tuning shoe could anticipate that change in frequency and adjust its hardness or softness accordingly. Again, theoretically, this might translate to performance increase, as the runner could maintain an efficient stride past his or her normal fatigue limit. It could also reduce injuries.
The Kayano probably isn't that shoe. It tunes itself to harden or soften its arch based on an assumption that runners need arch support in the first place. Unfortunately, as far as I know, there is little credible research that arch support, tuned or otherwise, is effective in preventing injuries among distance runners.