LETTER TO THE EDITOR Comment on: ‘‘How Biomechanical Improvements in Running Economy Could Break the 2-Hour Marathon Barrier’’ Robin B. Candau 1 • Anthony M. J. Sanchez 2 • Fabio Borrani 3 • Alberto E. Minetti 4 Ó Springer International Publishing AG 2017 Dear Editor, The recent article by Hoogkamer et al. [1] on the possi- bility of setting a new world marathon record below the mystical 2-h mark appears robust and has the immense merit of detailing several mechanical factors that describe performance in running. This work represents a compre- hensive review on running economy; however, several comments should be considered. By using projections based on statistical analyses, the authors noted faster evolutions of running records than expected in various studies conducted on the topic. As already mentioned in 1925 by Hill, extrapolation of world records must be performed with caution [2]. The mismatch between projections and actual records may be explained by the fact that projections are based on the characteristics of the runner’s sample at a specific time and not when the records occur. The improvement in marathon performance seems to be primarily related to the increase in the number of runners in the last decades, notably East African runners [3]. The record has fallen by 1–5 min/decade since 1960 when Africans entered international competition. Of note, performance improvements since 1980 also likely reflect an increase in competitive opportunities that allowed top athletes to earn a living in running [4]. African runners have relatively long and light lower limbs, which are associated with reduced internal work [5]. Indeed, the optimal lever arm of the ankle muscle extensors minimizes the energy cost of running [6]. However, Hoogkamer et al. focused their analysis on mechanical factors that could optimize running energy cost such as the weight of the shoes, the slope of the terrain, wind, or drafting. Dennis Kimetto, who holds the world record for a marathon, needs to increase his velocity by 2.5% to the required velocity of 5.86 m/s to run a sub-2-h marathon, and the authors pre- dicted he needed to improve his running economy by 2.3%. The authors based this prediction on the equation of Mar- garia et al. [7] and assume implicitly that the energy cost is stable during the entire marathon. This is obviously not the case for a run conducted until exhaustion [8], and espe- cially during a marathon since energy cost increases sig- nificantly by 16 ± 13% over such a distance [9]. Thus, we cannot neglect the amplitude of the increase in energy cost, whereas the gains in performance required to break the current record are that of only 2.5%. The authors propose a 32-s reduction in marathon time through a reduction of 100 g in shoe weight but this appears difficult to conceive since the mass of shoes has already been optimized. It is well known that barefoot running has its concerns with marked mechanical impacts on the locomotor apparatus, thus limiting this option. One adaptation to these mechanical impacts is to decrease leg stiffness and to run with the knee joints on average more & Robin B. Candau robin.candau@umontpellier.fr 1 University of Montpellier, INRA, UMR 866, Dynamique Musculaire et Me ´tabolisme, 2 Place Viala, 34060 Montpellier, France 2 Laboratoire Europe ´en Performance Sante ´ Altitude, EA4604, Department of Sports Science, University of Perpignan Via Domitia, 7 avenue Pierre de Coubertin, 66120 Font-Romeu, France 3 Institute of Sport Sciences of University of Lausanne, Faculty of Biology and Medicine, University of Lausanne, Lausanne, Switzerland 4 Laboratory of Physiomechanics of Locomotion, Department of Pathophysiology and Transplantation, University of Milan, Via Mangiagalli 32, 20133 Milan, Italy 123 Sports Med DOI 10.1007/s40279-017-0761-8