Teeter-totter effect: a new mechanism to understand shoe-related improvements in long-distance running ====================================================================================================== * Benno M Nigg * Sasa Cigoja * Sandro R Nigg * running shoes * elite performance * biomechanics * physiology The effect of shoe mid-sole construction on running performance was discussed with reference to the Nike Vaporfly 4%.1 Drs Burns and Tam described the mid-sole thickness as the major running shoe characteristic that contributes to changes in performance. Here, we highlight the role of the curved carbon fibre plate embedded in the mid-sole, and introduce a new effect on running mechanics, the ‘*teeter-totter effect*’. During ground contact in running, the point of application of the ground reaction force moves anteriorly during the second half of ground contact towards the front end of the curved carbon fibre plate. We suggest that, in this position, the ground reaction force produces a ‘reaction’ force at the heel in upward direction (perpendicular to the direction of the plate; figure 1). ![Figure 1](http://bjsm.bmj.com/https://bjsm.bmj.com/content/bjsports/55/9/462/F1.medium.gif) [Figure 1](http://bjsm.bmj.com/content/55/9/462/F1) Figure 1 Schematic illustration of the teeter-totter effect including the application point of the ground reaction force (red full circle) and its translation (red broken circle and line), the applied force of the runner (black arrow) at the front part of the shoe and the reaction force at the heel of the foot (red arrow) during early/mid-stance (left orientation) and push-off (right orientation). Image modified from nike.com. If the curvature of the plate is designed correctly, the teeter-totter effect will result in a force during push-off that acts at the right location (heel of the foot), at the right time (during take-off) and with the right frequency (depending on the running velocity and the ground contact time somewhere between 2 and 4 Hz).2 This heel force likely contributes substantially to improve running economy. To maximally improve performance with the curved carbon fibre plate requires three main characteristics: 1. The stiffness of the curved plate must be so that the resultant ground reaction force moves far enough anteriorly during the stance phase of running. 2. The pivot point (around which the teeter-totter effect takes place) should not be located too far anteriorly, allowing the shoe sole to act as a fulcrum. 3. The curvature of the shoe sole in the forefoot must be substantial3 but also not too extreme to allow for the desired teeter-totter effect. Model estimations of the magnitude of this teeter-totter effect have been made for a prototype marathon shoe (Salomon Group, Annecy, France). The effect of different curvatures of carbon plates was assessed using a previously described dynamic finite element (FE) model of the leg.4 These model estimations showed an increase in performance, and a forward shift of the ground reaction forces’ point of application for the (curved) carbon fibre plate. This simulation showed an increase of the force acting on the heel in the propulsion phase of the gait. This same leg model was then used on the Nike Vaporfly NEXT% in a preliminary study and a 6% gain in performance was predicted compared with a ‘regular’ running shoe. Even though the ground reaction forces’ point of application shifted slightly more forward with the Nike Vaporfly Next%, most of the gain was due to an increase in the force acting on the heel part of the foot during the propulsion phase of the gait. This 6% gain agreed with results from a field study led by Salomon Footwear teams. This study showed that, when considering regular runners (as opposed to elite marathon runners), the Nike Vaporfly NEXT% shoe helped these runners to shorten their running time by 6% on their habitual running training paths. Therefore, the results of the leg model (representative of average-level runners) correlated with the results of the field tests. These unpublished data and FE model results highlight what we speculate to be the key mechanism that contributed to increased running performance—the curved plate. These curved plates may be modified to yield even faster marathon times. In-shoe pressure measurements in the shoe are a simple way to quantify the teeter-totter effect during running. Compared with a ‘normal’ shoe, using a curved plate should yield higher take-off pressure at the heel during the push-off phase. Due to the high-performance improvement predicted by the FE model compared with previously reported studies,5 independent studies should investigate how systematic changes in plate curvature affect running performance, in the future. Let us consider the high heel in the Nike Vaporfly 4% in relation to the teeter-totter effect. The high heel likely moves the resultant ground reaction force forward during take-off. This corresponds to an increase of the teeter-totter effect and an increase of the force acting on the heel—hence more propulsion. The high heel also allows the use of a more curved plate, which increases the teeter-totter effect. Based on these model calculations one should conclude that the curved stiff plate and the resulting teeter-totter effect contribute greatly to the faster running times the world has seen in athletes who use the Nike Vaporfly 4% and any other shoe constructed similarly. These shoes could propel the heel of the foot during take-off and therefore explain the 4%–6% running economy improvements compared with regular shoes. ## Conclusion We dispute the current dogma that the recent world records in 100 km, marathon, half marathon and 15 km were solely broken due to increased mid-sole thickness. We contend that the effect of the mid-sole thickness is approximately 1%.6 We propose that an appropriate use of a bent carbon fibre plate is the main contributor to running performance improvements, and that these improvements can easily be 6%. Therefore, it appears inappropriate to regulate one specific footwear feature before understanding where these performance advantages originated from.7 However, our current knowledge suggests that, compared with the teeter-totter effect, all other shoe characteristic contributions to performance are small and negligible. ## Footnotes * Contributors All authors drafted, revised and approved the final version of the manuscript. * Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors. * Competing interests BMN is the founder and Chief Science Officer of Biomechanigg Sport & Health Research (BSHR) and SRN is the owner and Chief Executive Officer of BSHR. BSHR is a biomechanics consulting company, where past and present research projects have been funded by Adidas, Salomon Group, On AG, Li-Ning, Mizuno and Brooks Sports. * Provenance and peer review Not commissioned; externally peer reviewed. ## References 1. Burns GT , Tam N . Is it the shoes? a simple proposal for regulating footwear in road running. Br J Sports Med 2020;54:439–40.[doi:10.1136/bjsports-2018-100480](http://dx.doi.org/10.1136/bjsports-2018-100480) pmid:http://www.ncbi.nlm.nih.gov/pubmed/31630088 [FREE Full Text](http://bjsm.bmj.com/lookup/ijlink/YTozOntzOjQ6InBhdGgiO3M6MTQ6Ii9sb29rdXAvaWpsaW5rIjtzOjU6InF1ZXJ5IjthOjQ6e3M6ODoibGlua1R5cGUiO3M6NDoiRlVMTCI7czoxMToiam91cm5hbENvZGUiO3M6ODoiYmpzcG9ydHMiO3M6NToicmVzaWQiO3M6ODoiNTQvOC80MzkiO3M6NDoiYXRvbSI7czoyMzoiL2Jqc3BvcnRzLzU1LzkvNDYyLmF0b20iO31zOjg6ImZyYWdtZW50IjtzOjA6IiI7fQ==) 2. 1. 2. Nigg BM , 3. Mester J Nigg BM , Stefanyshyn DJ , Denoth J . Mechanical Considerations of Work and Energy. In: Nigg BM , Mester J , eds. 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