## 1 Introduction  
Ultra-endurance events are athletic competitions that significantly exceed the duration of traditional endurance events, typically lasting six hours or more. These events include activities such as ultramarathons, Ironman triathlons, and long-distance cycling races. They are defined as those lasting more than six hours, often requiring sustained physical exertion and exceptional stamina [1, 2]. Participation in these events has increased in the last 25 years. The most popular are ultramarathon races, ultarathlons, ultradistance swimming, ultracycling, and cross-country skiing [3]. There is also an increasing number of ultra-endurance mountain races, which are demanding events that produce a high impact on the psychophysiological response of participants [4]. Specifically, ultra-endurance athletes report a large rate of perceived exertion (RPE) [4], a decrease in blood glucose values [5], and an increase in sympathetic modulation analyzed by heart-rate variability (HRV), heart rate [6], muscular pain, and a decrease in cortical arousal [7]. Also, these long-duration events produce dehydration and a decrease in leg muscle strength [4]. They are performed at intensities lower than the anaerobic threshold [8] but produce a large muscular breakdown, as shown by increased creatinine kinase values [9].  
Previous authors have found that different parameters are related to performance in ultra-endurance races [2, 4]. Regarding training programs, traditionally, high-volume training has been the principal paradigm; however, the advantages of implementing high-intensity programs for ultra-endurance athlete performance have been shown [2, 4]. Inaddition, complementary strength training is also related to increased performance and injury prevention [4]. Specifically, in ultra-endurance mountain races, previous authors have shown how stress levels and general mental health are important performance predictors [10]. Regarding body composition, lower levels of body fat are a key factor for finishing ultra-endurance mountain races [10]. In line with this, hydration and nutritional patterns have also been shown to be crucial in these extreme events. For example, athletes that are more hydrated at the start line presented higher performance in ultra-endurance mountain races [2]. Ultra-endurance events often result in substantial energy deficits, producing a catabolic state [11]. Therefore, it is necessary to adapt the body to use fats with principal substrate energy [2].  
Ultra-endurance athletes often face challenges such as hyperthermia that can hinder performance and even jeopardize their health during ultra-endurance events [4]. Heat acclimatization, maintaining proper hydration, and athlete morphology play pivotal roles in preventing hyperthermia [12]. In this context, infrared thermography has emerged as an effective tool for analyzing athletes' thermal responses during movement and for injury-prevention interventions [13,14]. Specifically, in soccer players, an increase of 1.0-1.5 \({}^{\circ}\)C is associated with a high injury risk, and when the values exceed 1.5 \({}^{\circ}\)C, the risk becomes even more pronounced [15]. Moreover, in endurance sports, a rise in temperature is expected at the end of prolonged exercise due to the heat produced by muscular contraction [16,17]. Additionally, ultra-endurance sports induce an inflammatory response that correlates with the duration of physical exertion [18]. Therefore, the application of thermography is crucial for anticipating and preventing potential injury [19]. However, this has not yet been studied in the context of ultra-endurance events.  
Therefore, to better understand the psychophysiological variables related to performance in these events carried out on several consecutive days, as well as the effect of these extreme races on the body, we conducted this research with the primary aim of assessing the physiological, biomechanical, and thermographic responses of male athletes participating in an ultra-endurance race of 4 days.