Does the Running Economy Really Increase after Ultra-Marathons?

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 andMillet, 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 betweenmaximal oxygen uptake (V̇O2max), the ability to sustain a high percentage of V̇O2max (fractional utilization of V̇O2max), 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, 2012b; Millet, 2012; Perrey et al., 2012). Indeed, although strategies to improve RE aremandatory 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 andmuscle fatigue)may cause the runners to choose strategies that lead to a deteriorated RE in ultra-marathons (Millet et al., 2012a,b). However, although V̇O2max 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, 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.