HOAX OR HUNTING FOR HOPE?

– Written by Víctor Rodrigo-Carranza, José María González-Ravé, and Fernando González-Mohíno, Spain

INTRODUCTION

Advanced footwear technology (AFT) like the “Vaporfly” series of Nike or similar shoe models of other companies are characterized by a (1) very light shoe mass, (2) a curved carbon-fiber plate embedded in the midsole to (3) increase the longitudinal bending stiffness (LBS), (4) along with more resilient and compliant midsole foams to provide cushioning and energy return and (5) higher midsole stack height (Figure 1)¹. It is known that the combination of this characteristics can influence running economy (RE)^2,3 and performance^4,5 accompanied by slight biomechanical changes^2,3 such as increased stride length and increased contact time².

AFT usually use different methodologies to increase the LBS (full or partial and flat or curved carbon fiber plate) and different midsole cushioning technologies⁶. However, it has recently been shown that midsole material may be of greater role than increased LBS in improving RE in AFT ⁷.The most used midsole foams in running shoes are ethylene-vinyl acetate (EVA), thermoplastic polyurethane (TPU), and polyether block amide (PEBA). Hoogkamer et al.,⁸ analyzed the mechanical energy return of these three midsole materials, which was 66% for an EVA midsole with air bag, 76% for a TPU midsole and 87% for a PEBA midsole. Thus, PEBA which is less dense, and lighter than traditional foams^8,9 preserves more mechanical energy from each footfall. Low-density foam (i.e. PEBA) allows for adding more cushioning material in the midsole without adding mass and thus allowing for a taller stack height [now limited to 25 mm for spike shoes and 40 mm for competition non spike shoes (https://worldathletics.org/about-iaaf/documents/book-of-rules)] compared to other, traditional, midsole foams (TPU or EVA). As a result, when comparing RE between different models of AFT, the best results have been observed for models using PEBA midsole foam as opposed to traditional materials (EVA or TPU)^6,10 however, PEBA could suffer greater wear of the material with use during the run due to the low density⁶. 

ADVANCED FOOTWEAR TECHNOLOGY AND RUNNING EFFICIENCY IMPROVEMENT

The metabolic cost of running (i.e. RE) which is a determining factor in long-distance events depends on a complex interaction of factors^11,12. The metabolic cost of running is dominated by the cost of generating force to support body weight¹³. When mechanical energy is considered to reduce the metabolic cost of movement, two major strategies have been the focus of attention: optimizing the musculoskeletal system and maximizing the energy returned. During running, maximizing the energy returned, minimizing the energy loss or absorption and reducing the arm of forces on the joints have been the most widely used, mainly recently with the modification of footwear characteristics⁸.

The AFT has been shown to improve the RE based on the mechanisms presented above. While for commercial shoes that combine a carbon fiber plate with compliant and resilient midsole foam, group average improvements in RE of up to 4% have been reported (compared to traditional racing shoes), reported RE improvements from increased LBS by itself are less pronounced [for review see Ortega et al² and Rodrigo-Carranza et al.,³]. This might be because many previous studies increased the LBS of shoes with external insoles or flat carbon fiber plates and not with a curved plate embedded in the midsole foam^14–19.

The most widely analyzed feature of this type of footwear has been related to the modification of the mass of the shoe. Currently, there is a fair consensus that adding/decreasing 100g of mass affects RE by 1%^20,21 at submaximal intensities typical of RE measurements, although at higher speeds this relationship may not be correct²². However, the other features of AFT have been shown not to benefit all runners equally²³, possibly affected by anthropometric, running speed and stride type reasons^2,8,14. For this reason, improvements in RE have been shown to range from 2.8 - 4.2 % with a wide variability of -8.6 to 13.3 % improvement in trained runners^8,23–25 while running at absolute speeds from 14 to 18 km·h^-1 and slightly lower results (less than 2 %) in recreational/trained runners while running from 10 to 14 km·h^{-1 25–27}.

RUNING PERFORMANCE IMPROVEMENTS

Improvements in RE with AFT have translated into performance improvements of up to ~ 10% off in the 3,000 m and 3,500 m tests^25–27. Hébert-Losier et al.,²⁶ found a performance improvement ranging from 3.8% to 8.2% when recreational/trained runners used a model of AFT compared to their traditional footwear. In addition, 11 of 18 recreational runners performed best with AFT. However, in elite runners, improvements in half marathon and marathon events have been found to be close to 1%^28,29 and with about 25% of runners not improving their times when using the AFT²⁹. Therefore, it seems that the improvement in RE and performance when using the AFT is not equal in all runners and possibly influenced by the level of the runner.

A recent study published by our research group²⁵ showed that trained runners improved RE more when running at 13 km/h than national runners (higher level) when running at the same speed using AFT. However, they improved similarly in a 3000 m test when using an AFT model compared to a similar shoe with lower LBS. This may indicate that both levels of runners could benefit similarly when the relative speed is equal (3000 m test at maximum maintainable speed at this distance). However, the improvement is less when the running speed is lower at maximal intensities.

On the other hand, when analyzing the results of retrospective longitudinal studies carried out in 10k, half marathon and marathon performance previous to the launch of this type of footwear it was observed that the performance improvement rate in these events was slowing down³⁰. Furthermore, Kruse, Carter, Rosedahl and Joyner³¹ observed that no potentially transformational changes in training and material had occurred during this period in the evolution of the running times of these events. Related to these findings, we observed a year-on-year improvement of around 0.2% in the road events analyzed prior to the launch (between 2015 - 2016) of the first AFT model (Nike Vaporfly). This is consistent with previous research suggesting that just before the launch of the AFT at Monza 2017, the performance of the world’s elite athletes was stabilizing and reaching the physiological upper limit, as already proven for other animals³². Tucker and Santos-Concejero³⁰ even suggested that achieving the 4% improvement needed to be able to achieve running a marathon in less than 2 hours (in 2016) would take longer than previously anticipated because there were no potential changes in training, as well as doping being more controlled than previous years. However, when comparing average performances between 2017 - 2018, and 2018-2019, the improvement increased by more than 0.5% per year (Figure 2).

These findings suggest that the ~2-6% reduction in energy cost (better RE) with this type of footwear observed in laboratory conditions^3,8,23 translates to actual, but smaller (~0.5-1.4%) improvements in real-road running conditions in elite male 10k, half marathon, and marathon runners. Therefore, the performance improvements provided by AFT correspond to the maximal estimated by models predicting performance improvement based on RE savings, i.e. from the metabolic cost of running, which estimate that at 2:04:00 (h:mm:ss) marathon pace, a 3% improvement in RE translates to a 1.9% faster speed or a final time of 2:01:36²². Therefore, AFT has contributed to improved performance in major men’s road events since 2017.  Our results are similar to those previously observed by Bermon et al.,³³, who conducted an analysis on the top 20 men and women over the same distances with this type of footwear, and lower than those reported by Senefeld et al.²⁸, who examined the finishing times and footwear of the top 50 men and women in the World Marathon Majors before and after the introduction of this type of footwear. These authors found 2% to 2.6% faster times for men and women wearing AFT compared to other running shoes. One possible explanation for the variation in results may be because the metabolic savings caused by AFT are due to a combination of several mechanisms. More specifically, the increase of LBS caused by the carbon fiber plate and the type (curved or flat) embedded in the midsole has been shown to affect different runners differently^2,18,34. Curved and bottom-loaded plates^2,3, similar to the Nike Vaporfly 4% or Vaporfly Next% models used by the majority of athletes in our studies (98.70% of athletes who used AFT) being the ones that have shown the most assistance in previous laboratory analyses^8,23,24.

However, when performing an individual analysis with subjects who ran in consecutive years without and with AFT, and as has been previously analyzed in laboratory tests, not all runners benefited from the use of this type of footwear^23,35. The results of our studies^3–5,36,37 show that about 25% of athletes who ran with and without AFT did not improve their performance. Similar to the values previously found by Senefeld et al.²⁸. This reflects the wide individual variability response with this type of footwear^3,23.

RE is strongly influenced by anatomical and biomechanical differences³⁸ and thus to subject-specific responses to different LBS^19,35. It has been shown that increased LBS has greater impact on metatarsophalangeal and ankle joint mechanics². Therefore, gait patterns and the degree of LBS may play an important role in the responder effect of runners. Willwacher et al.,¹⁴ found that runners show two different strategies to increase LBS (with carbon fiber flat plate) of footwear as a function of triceps sural strength capabilities. One possible explanation for these differences may be that the increase in midsole LBS does not follow a linear relationship with RE using flat carbon fiber plates¹⁸. Some authors suggest that the optimal LBS would be one that does not disturb the natural flexion of the metatarsophalangeal joint due to an increase in the moment arm of the ground reaction force and a reduction in the arm of the joint torque at toe-off¹⁶. This could explain the wide variability in the benefit of using AFT²⁴ and even why some runners did not benefit. However, these speculations have been made with flat carbon fiber plates and not curved as the models used in our studies, so there could be other related factors.

MECHANISMS BY WHICH THE AFT COULD IMPROVE RUNNING PERFORMANCE

Although improvements in RE and performance are multifactorial^9,39 and are likely to be the result of the interaction between the characteristics of the shoe itself⁷ and the runner²⁶ there is scientific debate about the relative contribution of the various features of AFT to performance.

However, the mechanisms by which AFT improves RE and performance are currently unclear^2,40. Previous authors have summarized the benefits of this type of shoe by suggesting that performance improvements occur without a biomechanical explanation⁴⁰, others have summarized the possible effect that each of the characteristics described by the AFT could have^41,42 and others authors have tried to explain a possible mechanism as the determinant factor by which AFT improve RE with theoretical assumptions, but without experimental data⁴³.

For example, Nigg et al.^41,44 proposed that the main feature of AFT that causes the performance improvements in long-distance running is the “teeter totter” effect resulting from the stiffness of the plate and the curvature of the forefoot region (rocket). Although there is no experimental data directly supporting this mechanism. On the other hand, Hoogkamer et al.⁸ suggest that the high-energy return properties of the foam are the key contributor to improved running performance in the AFT. Furthermore, there is a lack of consensus on how much the increase of LBS affects RE through the use of different methodologies².

POSSIBLE RISK OF INJURY WHEN USING AFT

As we have explained in this article, the use of AFT has been shown to improve running performance with possible biomechanical modifications. However, these biomechanical alterations could cause some negative effects mainly on the foot and ankle. Recently, the debate is growing as to whether this type of footwear could cause injuries in runners^45,46. At present, there is little scientific knowledge on this subject. Tenforde et al.⁴⁵ show 5 clinical cases in which they justify the possible effect of carbon fiber plate shoes on navicular bone stress injury.  However, these are retrospective analyses that do not allow attributing the injury relationship to this type of footwear. Hoenig et al.,⁴⁶ made an opinion commentary in which no conclusive results are provided. Therefore, there is currently insufficient scientific knowledge to be able to justify that AFT can increase the risk of injury. Nevertheless, as suggested by Dr. Adam Tenforde of Harvard Medical School, a gradual transition to this type of footwear is recommended similar to those previously suggested with minimalist footwear which may consist of performing less than 10-15% of the volume in the first few months of use and progressively increasing over at least 1 year to balance with conventional training footwear.

CONCLUSION

The combination of the characteristics described by the AFT have shown improvements in running economy of around 4% in different levels of runners. Improvements in running economy have translated into ~1% performance improvements in main road events (10k, half marathon and marathon) in elite athletes and ~5% improvements at shorter distances (i.e. 3,000 m) in recreational runners. However, the mechanisms by which these types of shoes improve running performance are currently unclear. In addition, there is also little scientific evidence analyzing the effect of AFT on injury. Therefore, it is currently known that this type of footwear improves performance, but more research is needed to identify the mechanism by which it improves running performance and the possible negative effects of wearing AFT.

Víctor Rodrigo-Carranza Ph.D.^{1, 2}

Post-Doctoral Researcher

José María González-Ravé Ph.D.¹

Full Professor

Fernando González-Mohíno Ph.D.¹

Assistant Professor

1.              Sport Training Lab

University of Castilla-La Mancha

Toledo, Spain

2.             Integrative Locomotion Laboratory

Department of Kinesiology

University of Massachusetts

Amherst, MA, USA

Contact: victor.rodrigo@uclm.es

Header image by Snapwire (Cropped)