Despite falling levels of societal physical activity, and increased obesity in the general population, a small, but growing number of individuals compete around the world, in the physically and mentally demanding sport of ultra-marathons (Holt, Lee, Kim, & Klein, 2014;Hurdiel et al., 2018; Wardenaar et al., 2018). Such long-distance running events frequently take place in challenging environments, far in excess of the standard marathon distance of 42.20 km, over fixed distances, including 50 km, and 100 km, or time-limited, over multiple days (Knechtle, 2015).Whilst modern athletes, including the very best ultra-marathoners, are potentially homogeneous physically and technically, it is possible that the maintenance of successful performance under pressure is driven by psychological attributes (Cowden, 2016, 2017). Much of the current sports psychology literature pays particular attention to mental toughness,motivation and personality. Indeed, research linked to mental toughness suggests that the mentally tough individual is marked by perseverance in the face of adversity, and an improved understanding of the construct may benefit the endurance athlete by facilitating: (a) effective management of challenges encountered; (b) improved problem-coping skills; and (c) an increased focus on individual goals (Gucciardi, Peeling, Ducker & Dawson, 2016; Perry,Clough, Crust, Earle & Nicholls, 2013). Despite concerns regarding a lack of agreement in defining mental toughness as a psychological construct it continues to be seen as crucial to the success of athletes, and receives considerable research focus (Andersen, 2013; Gucciardi, 2017;Marshall et al., 2017; Vaughan, Carter, Cockroft, & Maggiorini, 2018). Running ultramarathons necessitates significant motivation to dedicate extended periods of time to complete long periods of running, in training and competition, whilst balancing commitments to work,family and friends (Krouse, Ransdell, Lucas, & Pritchard, 2011; Zach et al., 2018). In addition,certain personality types may also impact the ability to handle stress in the absence of a decline in performance and are largely attributable to genetic and non-shared environmental factors(Horsburgh, Schermer, Veselka, & Vernon, 2009; Kaiseler, Polman, & Nicholls, 2012). A growing body of literature has therefore explored the impact of the human genome on phenotypes that underpin endurance performance, including musculature, maximum oxygen uptake, and running economy (Barnes & Kilding, 2014; Sarzynski, Ghosh, & Bouchard, 2017;Valdivieso et al., 2017). Consequently, selective genomic analysis of ultra-marathoners may provide an insight into the factors that define the endurance athlete.Relatively little quantitative, interdisciplinary research has been directed at both psychological and physiological factors, and their interaction, that predict performance in ultra-marathoners,or what drives ultra-marathoners to push their individual limits (Wortley & Islas, 2011). Indeed,despite research yielding an improved understanding of the factors involved in endurance performance, the bulk of studies have been specialised and fragmented, focusing on individual elements of psychology, physiology, biomechanics, and/or genetics (Balagué, Torrents,Hristovski, & Kelso, 2017; Hristovski, 2013). Recently it has been suggested that the introduction of a more dynamic approach to research, modelling, and integrating seemingly disparate disciplines, including psychology and physiology, may benefit sport and exercise science, as it has in other research fields (Balagué et al., 2017). Indeed, cognitive science has had considerable success over the last two decades, utilising knowledge and expertise from diverse and discrete fields of study, including neuroscience, neurobiology, psychology,linguistics and computing, to tackle diverse challenges including modelling memory, language, visual perception, problem solving, and attention (Baddeley, 2012; Eysenck & Keane, 2015;Wan, Chen, Shi, & Zhou, 2018). A more comprehensive exploration of measures including mental toughness, personality, motivation, physiological stress, perception of effort, genetic predispositions, and aerobic fitness may enable sports professionals to provide improved support to ultra-marathoners.At present, an understanding of endurance, synthesising both psychological and physiological factors, remains elusive, possibly as a result of associated research challenges, including cost,resources and available expertise. However, some limited, interdisciplinary research success has already led to an integrated view of fatigue, and the speculation that the termination of aerobic sessions, and endurance performance overall, is likely to be a product of the interaction of multiple psychological and physiological variables (Hristovski & Balagué, 2010; Inzlicht &Marcora, 2016). The Central Governor Model (Noakes, 2007), and the Psychobiological model(Marcora, 2008), have both challenged the pre-existing view that the length of aerobic exercise is determined by muscle fatigue, and suggested a close relationship with perception of effort(Crewe, Tucker, & Noakes, 2008; Inzlicht & Marcora, 2016).The aim of the present research is to provide an integrated understanding of the factors involved in endurance through the quantification of the psychological, and physiological factors that: (a)identify, and affect success in, the ultra-marathoner, and (b) affect the limits of aerobic fitness.Other supplementary aims include, challenging existing psychological models of mental toughness, motivation and personality to successfully identify ultra-marathoners, and provide support for integrating measures from both psychology and physiology, to produce a novel,interdisciplinary Optimum Balanced Performance Model of Endurance Success in ultramarathons.