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Is Toe Spring in Footwear Really Related to Injury?

A recent study by Professor Daniel Lieberman and colleagues is nothing short of misleading and confusing.

Professor Daniel Lieberman has a track record of publishing information that at best could be called misleading, or at worst, scientific misconduct, an accusation levelled by Dr Steve Robbins here.

Nature Magazine seems to be supportive of this behaviour, and has, in my view, lost considerable credibility by publishing articles that continue to support unsustainable conclusions.

Both Lieberman and Nature are at it again with a recent article titled:

Effect of the upward curvature of toe springs on walking biomechanics in humans Sichting, Lieberman et al Scientific Reports volume 10, Article number: 14643 (2020) 

It is open access and may be read in its entirety here.

It tests the general hypothesis that shoes with a toe spring will affect stiffness of the foot-shoe-complex and the total work done at the MTP joints.

The paper also tests two additional specific hypotheses.

Hypothesis 1 is that during midstance, the stiffness of the medial longitudinal arch will increase with greater toe spring angles since the dorsiflexed position of the toes activates the windlass mechanism.

Hypothesis 2 is that during the propulsive phase, increasing toe spring angles will gradually decrease the total angle through which the toes rotate and subsequently decrease the total work at the MTP joint.

A fair bit has already been written and spoken about this article, so I shall limit my input to correcting some of the misleading information in the article, and leave you all to draw your own conclusions.

It does not take long for the inaccuracies to begin.. in fact, they begin right at the very start of the article!

“Although most features of modern footwear have been intensively studied, there has been almost no research on the effects of toe springs. This nearly ubiquitous upward curvature of the sole at the front of the shoe elevates the toe box dorsally above the ground and thereby holds the toes in a constantly dorsiflexed position.”

 

 

Lieberman and co then justify this statement with this graphic.

 

This is an outrageous exaggeration of what we see in a modern running shoe and is drawn in such a manner as to grossly exaggerate the facts and underpin the first of the author’s many erroneous claims. It also sets the tone for the entire experiment, which, on the basis of this single misrepresentation should be immediately brought into question.

In fact what this drawing represents is a shoe with close to a “zero drop”, a style of shoe Lieberman himself champions.

Graphic 1

The black lines show the midsole thickness has no gradient, as is evident in nearly every commercially available running shoe on the market.

There is little or no differential in the gradient or ‘drop’ between the midsole of the heel and the midsole of the forefoot. The graphic represents no commercially available running shoe I am aware of, and is designed only to over exaggerate what ‘might’ happen to the toes if such a shoe did exist.

Below is what a real running shoe toe spring looks like – check the strobel board line for reference, in these Hoka shoes in the top photo, it’s almost flat and the bottom photo of the Nike is what I would refer to as subtle.

Recent model Hoka running shoes

 

Recent model Nike running shoe

 

The toe spring demonstrated in graphic 1 above (in what is supposed to be a scientifically rigorous paper) is at least 40 degrees, whereas the industry standard for running shoes is 15 degrees or less! Therefore the statement in this paper that the normal toes spring of a running shoe “holds the toes in a constantly dorsiflexed position” is incorrect and misleading. At the very worst, the toes might be held in a very small degree of dorsiflexion, but certainly less than 10 degrees and nothing like the gratuitous graphic above!

OK.. so what did they actually do?

Participants walked on the treadmill barefoot and in four pairs of custom-made sandals with varying degrees of toe spring angle. The sandals consisted of a top sole, rubber outsole, foam midsole (thickness 2 mm), and curved fiberglass plate that ran the length of the sandal and curved upwards at the ball of the foot to the tip of the sandal. The upwards curvature under the toes was either 10°, 20°, 30° or 40°.

Now this is what happens when an evolutionary biologist starts to talk about athletic footwear.. first, you must know the processes. And, what is a huge and glaring error here is that toe spring is never.. not ever discussed in terms of degrees at a manufacturing level. It is always discussed in terms of millimetres, and this does NOT translate to degrees.

So, right away we have a fundamental error.

The authors then state that:

“The sandals were designed to mimic the stiffness and shape of toe springs commonly found in commercially available shoes”

Now this again is very misleading and I am uncertain if the authors have the necessary knowledge of the shoe manufacturing process. Specifically

“toe spring is NOT built into the shoe, it is built into, and solely determined by, the last”.

This is a very important point, because if the spring is built into the shoe, it will of course dorsiflex the toes by the amount of spring present. HOWEVER, if it is build into the last (remembering it will be 15 mm  tops), the toe spring is countermanded by other factors including the drop of the shoe and thickness of the insole (sockliner).

Because of this, the position of the digits in most commercially available running shoes is likely to be very close to neutral!

I also feel a major oversight of this study is that the experimental conditions were barefoot and the four sandals, but traditional running shoes were excluded.

How then is it possible to amortise the results back to toe spring in running shoes?

What did the study find?

  • there was no change in arch stiffness due to toe springs
  • greater toe spring angles decreased the total range of MTP joint rotation
  •  toe springs seem to be able to compensate for the negative effects of stiff shoes on MTP work requirements
  • Although the study designed sandals with toe spring angles of up to 40°, and a bending stiffness that is similar to conventional shoes, the maximum MTP joint dorsiflexion angle caused by toe springs was less than 10°
  • negative work was highest during walking barefoot and with 10° sandals (which most closely approximates the toe spring of modern running shoes)

What did they conclude?

  •  “that toe springs have important heretofore unrecognized biomechanical effects on foot function that merit consideration, especially since they have become increasingly exaggerated in modern athletic shoes”.. and they then cite this reference.. from exactly 40 years ago!
  • The authors also appear to have not read the articles they are citing. A great example is the paper by Scott Wearing and James Smithers. The authors specifically cite this paper saying “Etiologically, plantar fasciitis is recognized as an injury caused by excessive and repetitive loading of the foot’s longitudinal arch“. In fact the Wearing paper concludes

    “scientific support for the role of arch mechanics in the development of plantar fasciitis is equivocal”

    and

    “tensile failure is not a predominant feature in the pathomechanics of plantar fasciitis”!

    Scientific rigor is not negotiable when reviewing papers for publication. Errors of this magnitude should not be missed, especially when citations appear to be manipulated to satisfy the authors own bias.

Then, things really start to unravel, as the authors attempt to connect toe spring with injury, specifically plantar fasciitis (which was NOT any part of the study).

They note that:

the decrease in negative work at the MTP joints suggests that intrinsic foot muscles have to perform less eccentric muscle work to control MTP joint dorsiflexion during the propulsion phase of gait

Well fair enough, but the 10 degree sandal, which closest approximates a running shoe, demonstrated an INCREASE in negative work, the same as the barefoot condition!

Then the kicker..

“Thus, habitually wearing shoes with toe springs could inhibit or de-condition the force generating capacity of intrinsic foot muscles. While the direct consequences of weak foot muscles are not fully known, it is likely that they could increase susceptibility to flat foot and associated problems such as plantar fasciitis”

I can only make the following observations

  • the study did not measure foot strength
  • the study did not look at plantar fasciitis, make clear what this diagnosis entails, nor make any compelling link to the main outcome factor, that the 20, 30 and 40 degree sandals reduced the negative work at the MTP joints (although along with barefoot it INCREASED with the 10 degree sandal).
  • they state that plantar fasciitis is caused by “excessive and repetitive loading of the foot’s longitudinal arch”. No such cause is currently established.
  • they state that while toe springs may increase comfort by reducing the effort of the foot muscles, they may increase susceptibility to plantar fasciitis and other foot-related problems. There is zero justification for this conclusion in this study as presented.

I try to see the positives in every study I read, and I always try to learn. With this one though I just have an overwhelming sense of confirmation bias and an agenda to paint modern running footwear as something it is not. Does it really matter?

Well the final comment should go to the banner headline in the UK Guardian newspaper:

And because lots of people read this stuff, it is important we get it right so we would encourage you all to pass this article onto those who may be interested in such topics.

 

Written by Simon Bartold

www.bartoldclinical.com

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