To the Editor-in-Chief:

The report by Zech et al. (Zech, Argubi-Wollesen, & Rahlf, 2015), examined stability when a highly stable cohort was barefoot and used selected athletic footwear, as so many others have done commencing more than two decades ago (Robbins, Waked, & McClaren, 1992; Robbins, Waked, Gouw, & McClaren, 1994; Franklin, Grey, Heneghan, Bowen, & Li, 2015). But their results differ from most reports in that they found stability was better when wearing shoes - the opposite of what has been reported in all age cohorts except for the extremely old (Robbins, Waked, Gouw, & McClaren, 1992). This can be accounted for by the method they used to infer stability which is invalid. Also, Zech et al. seem to exist in a solitude whereby their report is not placed in context of other similar reports, nor into our current understanding of mechanisms humans use to maintain stable equilibrium.

High stability in humans is the state of weight-bearing with low risk of falling, hence the most valid means of measuring it is quantifying falls under conditions of daily life. This is impractical particularly in stable cohorts, as Zech et al. use, because they rarely fall. Alternatively, stability can be inferred by measures that have been shown to correlate well with falls risk, such as falls from a narrow beam when walking when considering a stable cohort, or falls with one footed standing with less stable cohorts (Robbins, Waked & McClaren, 1992). Stability testing using validated methods have shown that stability in humans progressively declines commencing at 40 years of age – the age when falls risk increases in healthy humans, and footwear impair stability in all but the oldest cohort (Robbins, Waked, Allard & Krouglicoff, 1997).

Zech et al. use a six degree of freedom (x, y, z, pitch, yaw, roll) force platform to infer stability. Force platform output indicates displacement of center of mass over time, which has been expressed in a multitude of similar measures, which in totality are called “sway measures” or simply “sway”. Sway is continuous when standing, but this behavior is not only postural adjustments to maintain stable equilibrium as Zech et al. presume (Duarte, & Sternad, 2008; Loram, Maganaris, & Lakie, 2009). For example, sway declines with each advancing trimester of gestation as fall risk increases (Dunning, Lemasters, & Bhattacharya, 2010; McCrory, Chambers, Daftery, & Redfern, 2010; Olivera, Viera, Macedo, Simpson, & Nadal, 2009; Evenson, & Wen, 2011). Furthermore, when the most stable age cohort stand unstressed (on two feet; eyes open; a firm support base), sway is high and increases further with prolonged standing - often exceeding levels seen in the unstable elderly who are at high risk of falling. Surely any method that is incapable of distinguishing stable from unstable cohorts must be considered invalid. Sway with low stress in stable cohorts is best explained as mainly a measure of plantar load redistribution behavior that protects the plantar skin from ischemic necrosis. As balance stress rises, humans first choose to lower falls risk through reduction of plantar protective behavior to a point where prolonged standing plantar discomfort encourages them to become sedentary. Sway becomes positively related to falls risk once plantar skin protective behavior is extinguished which is nearly impossible to determine with precision, or if specific protocols are validated against actual falls risk in the cohort tested, which Zech et al. did not do. The best available explanation of their results which is consistent with previous reports is that, considering that one-footed standing is not particularly stressful in the cohort they chose to test, the barefoot condition was associated with the greatest sway but also the greatest stability because it was a measure of plantar load redistribution behavior. This is the opposite of Zech et al.'s conclusions.

Zech et al. fail to report developments in the last 25 years regarding how humans use plantar tactile influences in maintaining stable equilibrium. We now know that humans parsimoniously use plantar tactile information mainly from SA II mechanoreceptors to maintain superior stable equilibrium when barefoot but not with footwear), but also to control impact (Robbins, Hanna, & Jones; Robbins & Waked, 1998), and to provide precise foot position awareness (Bart, Van Wezel, Ottenhoff, & Duysens, 1997; Robbins, & Waked, 1998; Robbins, Waked, & Rappel, 1998). Evidence from this comes from the known effect of footwear on plantar shear-stress which is an adequate stimulus for SA II mechanoreceptors, particularly when combined with normal loading (Iglis, Kennedy, Wells, & Chua , 2002; Kennedy, & Inglis, 2002). Direct recording from primary afferents of plantar surface SA II mechanoreceptors support the above analyses (Iglis, Kennedy, Wells, & Chua, 2002; Kennedy, & Inglis, 2002). Also, stability with athletic footwear is positively and negatively related to sole hardness and sole thickness, respectively (Robbins, Waked, & McClaren, 1992). It has been shown that stability is positively related to foot position awareness, which correlates well with frontal plane foot movement when bearing weight on one foot (Robbins, Waked, & McClaren, 1995; Robbins, Waked, Allard, & Krouglicoff, 1998). This led to the hypothesis that athletic shoes destabilize through creating an under-damped state of frontal plane oscillation (Robbins, Waked, & Krouglicoff, 1998). This hypothesis was tested in a report in a geriatrics journal with sole materials differing in resiliency. It was found that stability improves with extremely low resiliency sole materials which produce an over-damped condition (Robbins, Waked, & Krouglicoff, 1998). There is evidence that decline in stability when barefoot with age is likely caused by SA II receptor decline, worsened by footwear created forefoot osteoarthritis in the extremely old which is the only cohort in which barefoot stability is worse than when wearing shoes (Robbins, Waked, & McClaren,1995; Machado, Bombach, Duysens, &Carpes, 2016).

It seems unlikely that Zech et al. are totally unaware force platform limitations of inferring stability, and must be knowledgeable of the above relevant published reports, but chose to use them highly selectively. For example, they mention reports that support their claims (Federolf, Roos, & Nigg, 2012; Koepell, et al., 2004), but not a host of others that refute it (Franklin, Grey, Heneghan, Bowen, & Li, 2015). This suggests that both choice of method and literature review may have been a strategy.

I find it revealing that they conclude:

“….the findings do not support the hypothesis that barefoot standing is beneficial for static postural control. Moreover, the fact that un-cushioned minimalist shoes with direct skin contact at the foot and ankle joint resulted in the lowest postural sway during standing …....”

It may simply be a coincidence that their source of funding was Leguano GmbH, a footwear manufacturer, that makes un-cushioned minimalist shoes, and their conclusions seem to promote the use of these products as offering superior stability. Are Zech et al. involved in clientelism through, on a quid pro quo basis, pandering to their source of funding?

There is a pressing need to further our understanding of human stability particularly in relation to footwear, a relatively recent development in human history that is mainly based on an aesthetic tradition. They substantially affect stability and gait. But scientific advancement requires that authors remain objective and avoid clientelism with their funding sources. The resulting pseudo-scientific advertising presented as original research diminishes the scientific journals which publish reports in good faith, and the authors that honestly report their results in them.

References

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