Running to the heart and (carbon fibre) sole of the matter

By Natalie Gallant @GallantPhysio

The world watched in awe as Kenyan Eliud Kipchoge smashed the marathon world record in the 2018 Berlin Marathon, crossing the line with a time of 2:01:39. That’s 78 seconds faster than the previous 42.2 km world record. In my opinion Kipchoge’s phenomenal display of athleticism can be attributed in part to his special genetic makeup, his unwavering commitment to training, supreme ability to remain focused and impeccable time keeping. Nike however might have you think it were his shoes.

Figure 1. The Nike Vaporfly 4%(1)

Kipchoge’s Nike Zoom Vaporfly 4% (Figure 1) boasts ultra-lightweight ‘magically energetic’(2) foam called ZoomX; a foam similar to that used in aerospace insulation and suggested to give an 85% energy return to the runner.(2) A full-length curved carbon fibre plate to stiffen the midsole;(3) allegedly reducing energy expenditure by 1%.(4) Nike promotes the 9 mm heel-toe drop as reducing Achilles strain, (5) overall making a somewhat controversial 4% improvement in running economy when lab tested against previous marathon world record breaking shoes.(3) Watch it here.(6)

So how far can shoe technology advance before the International Association of Athletics Federations (IAAF) deem the shoe an illegal performance enhancer? The IAAF 2018-2019 Competition Rules seem open to debate in regard to what ‘unfair assistance or advantage’(7) may look like. IAAF footwear rulings state:

‘The Athletes may compete barefoot or with footwear on one or both feet. The purpose of shoes for competition is to give protection and stability to the feet and a firm grip on the ground. Such shoes, however, must not be constructed so as to give athletes any unfair assistance or advantage. Any type of shoe used must be reasonably available to all in the spirit of the universality of athletics (…) The sole and/or heel may have grooves, ridges, indentations or protuberances, provided these features are constructed of the same or similar material to the basic sole itself’. (7)

A running shoe is after all an artificial aid, one that we shall continue to scientifically debate and regulate, a point Dr John Orchard makes in his ethical discussion around carbon fibre prosthetics. Carbon fibre or not, it’s important to consider the reverse argument, at what point might our shoes actually hinder performance?

During Nike’s lab test a 50km running limit was placed upon each shoe in order ‘to prevent excessive wear accumulation’.(3) To me this acknowledges that in a relatively short period, there is potential for shoe degradation to occur which may ultimately impact performance. Pick up your trainers and match them sole to sole, it’s likely you’ll notice they have asymmetrical wear. Degradation of our shoes soles changes the materials mechanical properties and as a result runners modify their running pattern.(8) No matter how much shoe companies push the IAAF boundaries with technologically advanced footwear we still have high rates of running related lower limb injuries.(9) Rather than blaming our feet, ‘a masterpiece of engineering and a work of art’ as Leonardo da Vinci described them, consider the shoes material or faulty design features for causing this asymmetrical wear pattern. For example, a raised heel places the ankle in a greater degree of plantarflexion and inversion which increases the chance of lateral shoe wear.(10) Researchers have shown that altering the medial rear foot by just one millimetre, to simulate this lateral foot wear pattern, reduced performance of a single leg heel raise task.(10) Unlike Kipchoge we aren’t all elite sponsored athletes i.e., able replace our shoes for every event, so imagine the impact of lateral shoe wear during 30,000 plus steps of a marathon!

I recently learnt of a tool called the total footwear asymmetry score (TAS),(11) which has revolutionised my thoughts on footwear. A tool with which we can objectively and reliably observe the impact of asymmetrical wear in our patients running shoes. The TAS is calculated by measuring the thickness and hardness of the inner and mid/outer sole using a digital caliper and an Asker C durometer, noting differences between the medial and lateral sides in the same shoe.(11) It’s exciting to think that being able to identify our patients shoe wear patterns early enough could potentially prevent future injury.

This leaves me with two thoughts. Firstly, I wonder how many Kilometres Kipchonge accumulates in his Vaporfly’s before switching to a new pair? I imagine before any asymmetry is detected by his TAS score. My second thought involves Ethiopian Abebe Bakila back in 1960 when he set a world record marathon time of 2:15:16 in Rome, barefoot.(12) What if we could replicate Bakila’s race and have Kipchonge run barefoot? A true test of present day athleticism without artificial aids or performance enhancing technology. I’m just not sure if Nike will fund that project.


Natalie Gallant (@GallantPhysio) is a UK born physiotherapist calling New Zealand home. She lives and works in Auckland at and is Commonwealth Games team physio for the Falkland Islands. When not studying for her Sports and Exercise Medicine PGDip at the University of Otago she can be found trail running the native bush…barefoot (mostly). E-mail: and Instagram: Borntorun_physio

Competing interests



1. Nike. The Shoes Athletes Say Will Change the Future of Running 2017. Accessed 28-09-2018

2. Nike. ZoomX. 2018 Accessed 29-09-2018

3. Hoogkamer W, Kipp S, Frank JH, et al. A comparison of the energetic cost of running in marathon racing shoes. Sports Med 2018;48:(4):1009-1019.

4. Roy JP, Stefanyshyn DJ. Shoe midsole longitudinal bending stiffness and running economy, joint energy, and EMG. Med Sci Sports Exerc 2006;38:562–9.

5. Nike. Introducing the Nike Zoom Vaporfly Elite Featuring Nike ZoomX Midsole 2017. Accessed 28-09-2018

6. Dubois B. Nike Zoom Vaporfly 4 Sub-2H marathon #fake-news. The Running Clinic 2018 Accessed 28-09-2018

7. International Association of Athletic Federations. Competition Rules 2018-2019. Page 69. Accessed 28-09-2018

8. Kong PW, Candelaria NG, Smith DR. Running in new and worn shoes: a comparison of three types of cushioning footwear. Br J Sports Med 2009;43:745-749.

9. Van Gent RN, Siem D, Middelkoop M, et al. Incidence and determinants of lower extremity running injuries in long distance runners: a systematic review. Br J Sport Med 2007;41:469-480.

10. Sole CC, Milosavljevic S, Sole G, et al. Exploring a model of asymmetric shoe wear on lower limb performance. Phys Ther Sport 2010;11:60-65.

11. Sole CC, Milosavljevic S, Sole G, et al. Patterns of mediolateral asymmetry in worn footwear. Footwear Sci 2014; 6:177-192. doi:10.1080/19424280.2014.913694

12. Olympic Games. Abebe Bikila 2018. Accessed 29-09-2018

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