Does the Running Economy Really Increase after Ultra-Marathons?

This is the authors' abstract. We don't add key points for this paper.

Over the years, ultra-endurance events have increasingly piqued the interest of the scientific community as they are considered an outstanding model to study the adaptive responses to both extreme loads and stresses on the human body (Millet and Millet, 2012).Notably, ultra-marathons, i.e., any event longer than the traditional marathon length of 42.195 km (Millet and Millet, 2012), have seen rising trends in participation (Hoffman et al., 2010;Cejka et al., 2014).It is well-established that endurance running speed depends on the interaction between maximal oxygen uptake ( VO 2max ), the ability to sustain a high percentage of VO 2max (fractional utilization of VO 2max ), and a low economy of running (di Prampero et al., 1986).However, though the running economy (RE) is recognized to be a key determinant of running performance for "classic distances" (up to the marathon) (Saunders et al., 2004), whether or not it is also a primary determinant of ultra-marathon performances remains debated (Millet et al., 2011a(Millet et al., , 2012b;;Millet, 2012;Perrey et al., 2012).Indeed, although strategies to improve RE are mandatory in events shorter than or equal to the marathon distance, optimizing other factors associated with low-intensity endurance (e.g., minimizing damage to lower limb tissue and muscle fatigue) may cause the runners to choose strategies that lead to a deteriorated RE in ultra-marathons (Millet et al., 2012a,b).However, although VO 2max and its fractional utilization have been described as determinants of ultra-marathon performances (Davies and Thompson, 1986; Millet et al., 2011a), it has been argued that the greatest variance in performance (∼85%) was explained when the mean RE throughout was also added (Lazzer et al., 2012(Lazzer et al., , 2014)).Consequently, investigating the effects of fatigue on RE is still a crucial scientific question in ultra-marathon, both for performance optimization and a better understanding of the limits of the adaptive responses of the human body. THE ROLE OF RE IN THE ULTRA-MARATHONTwo different forms of RE have been identified by the scientific literature; as oxygen cost or alternatively, as energy cost.By using one of the two approaches, many studies examined the effect of an ultra-marathon on RE, with equivocal findings (Figure 1).Manuscripts were acquired by searching the electronic databases of MEDLINE, PubMed, ScienceDirect, SPORTDiscus, and Web of Science using the following keywords in various combinations: "energy cost, " "oxygen cost, " "running economy, " "ultra-marathon, " "ultra-endurance."We excluded articles written in languages other than English, as well as articles that have not yet been accepted or published.