Physiology of lived experience: Cruising among the Swiss peaks

On the starting line! The challenge ahead is intimidating: completing the SwissPeaks 700 (recently renamed the ‘Odyssey’), promoted as the ‘longest trail in the world’, at a distance of ∼700 km and with nearly 49 000 m of elevation gain. This mountain ultramarathon (MUM) forms a loop around the canton of Valais, Switzerland, beginning on the shores of Lake Geneva and extending to the Aletsch Glacier before returning to the start (Figures 1 and 2). I am the oldest participant on the start list (63 years old) among 100 ‘old horses’ selected for their extensive MUM experience (i.e., finishers of races >300 km with >20 000 m of elevation gain). I am well aware that practical experience and a bit of physiology knowledge derived from my work (as an exercise physiologist focusing on hypoxia and endurance) can offset the ‘biological clock’ and the progressive decline in maximal oxygen uptake with age only in part. This decay begins at ∼30–35 years of age and proceeds at ∼10% per decade (Astrand et al., 1973). Exercise intensity during such long MUMs is very low (typically 50%–60% of maximal oxygen uptake during ascents and probably even lower during descents; Savoldelli et al., 2017). Running economy remains a key determinant of performance in MUMs, as in any endurance event (Millet, 2012), and depends strongly on neuromuscular factors (Barnes & Kilding, 2015). A mismatch between oxygen delivery and consumption in working muscles (i.e., increased muscle deoxygenation) has been reported after MUMs (Vernillo et al., 2017). Finally, in my case, age-related increases in peripheral resistance further limit oxygen utilization (Capelli et al., 2025). The specific demands of mountain ultramarathons also require careful preparation for the extensive downhill mileage and the associated eccentric loads that cause muscle damage and increase injury risk (Carmona et al., 2015), while negatively affecting microcirculation and leading to metabolic disturbances (Vernillo et al., 2017). However, such muscle-damage concerns are somewhat mitigated in extremely long MUMs owing to the very low downhill velocities typically adopted. Indeed, relative muscle ‘preservation’ has been reported as race distance increases; for example, decrements in maximal voluntary contraction of the knee extensors and plantar flexors were smaller in a 330 km (D+: positive elevation gain 24 000 m) race than in a 170 km event (D+: positive elevation gain 10 000 m) (Saugy et al., 2013). More concerning are the potential indirect health consequences of muscle damage (Scheer et al., 2022), including the risks of rhabdomyolysis (Rojas-Valverde et al., 2020) and severe acute kidney injury (Belli et al., 2018; Irving et al., 1990), along with the possibility of persistent long-term cardiac effects, such as myocardial fibrosis, arrhythmias or atrial fibrillation (Burtscher et al., 2022) So, why do it? Why do we register and even pay for standing on the starting line, with no guarantee that we will be among the finishers a few days later? Is it the search for extreme physical and emotional experiences (Bill & Philippe, 2023) or the pursuit of personal goals and the satisfaction of achievement (Roebuck et al., 2018) or because this passion makes us happy when engaged in an open-minded and non-defensive manner (Bill et al., 2024)? Is it an innate psychological drive to explore our physical and mental limits (Roebuck et al., 2018)? External prestige? Or perhaps the fact that extremely long MUMs tend to be less competitive and more intrinsically goal oriented? My own motivation is likely to draw from all these elements, combined with the desire to experience directly the physiological mechanisms that I study in my research. Overall, the small ‘tribe’ gathered at the starting line appeared relaxed, although the magnitude of the challenge undoubtedly elicits fear and anxiety, particularly in the days preceding such an event (Urwin et al., 2021). A final kiss, and we begin! We begin with a long ascent (∼1000 m of positive elevation) that takes us from 375 to 1415 m. During this first climb, my priority is to pace myself wisely, resisting the collective momentum of the group by adopting the most appropriate velocity and step length. I am aware that my vertical velocity will inevitably decline climb after climb, day after day, with minimal change in step frequency but a progressive reduction in step length (Jeker et al., 2020). I try to use my poles efficiently to reduce the mechanical load on my quadriceps and calf muscles, ‘saving my legs’ for what lies ahead (Giovanelli et al., 2023). This strategy promotes a longer, slower leg extension during push-off and increases contact time, which is likely to reduce the rate of force production and provides metabolic benefits (Vernillo et al., 2014). When compared with uphill walking without poles, the combined effects, including reduced internal work, counterbalance the increased energy cost of involving both upper and lower limbs. Consequently, using poles at moderate intensity has minimal impact on overall metabolic expenditure while still reducing mechanical work (Giovanelli et al., 2019). This helps to explain why the vast majority of runners adopt poles in long mountain ultramarathons. Night falls, but the first night outdoors does not yet bring significant sleep deprivation. Owing to workload and preparation for a scientific experiment, I had been unable to implement an effective ‘sleep banking’ strategy (i.e., increasing sleep duration in the days before the race) (Poussel et al., 2015). I am therefore slightly anxious about how sleep restriction might affect me during the forthcoming nights. After several hours, in the middle of the night, only a scattered few runners remain visible across the mountains. I soon reach the first major challenge: a very steep descent of D−: negative gain 1300 m. Like many ultra-trail runners, who often present numerous low-grade cartilage and meniscal lesions (Della Rosa et al., 2024), I had experienced chronic knee pain in the months leading up to the event (Diermeier et al., 2019; Knechtle & Nikolaidis, 2018). This had substantially reduced my running volume before the race. Having undergone meniscus surgery a few years earlier, I knew my knee would be a major limiting factor and my most likely reason for withdrawal. I therefore adopted an extremely conservative downhill pace, reducing joint load with careful pole use. Although limiting, this had been anticipated in my preparation, during which I deliberately targeted body‑weight reduction to decrease mechanical stress on the knee while also decreasing fat mass to improve my power-to-weight ratio, thereby enhancing uphill velocity (Lemire et al., 2021) and partly offsetting the inevitable reduction in downhill speed. This carefully controlled pacing strategy was essential, not only to remain comfortably ahead of intermediate cut-off times but also to retain a buffer, should unforeseen problems slow me down later. The first difficulties began after 2 days and ∼150 km of ascents and descents. I was feeling reasonably well, apart from some unpleasant nausea, which is a common exercise-associated gastrointestinal symptom in ultra-endurance events (Costa et al., 2025). We (Millet et al., 2025) recently hypothesized that the prevalence of exercise-associated gastrointestinal symptoms is particularly high during the early phases of MUMs owing to a convergence of several factors, including excessive carbohydrate intake before or early in the race, pre-race anxiety, inappropriate pacing, shifts between hypo- and hypervolaemia, altitude exposure and environmental temperature fluctuations. Of these six factors, none appeared especially relevant in my case. I consumed substantial protein rather than carbohydrates only; I felt confident with modest performance expectations; my pacing was deliberately protective; I was well acclimatized; and the weather (unfortunately) was consistently wet and cold. Although exercise-associated gastrointestinal symptoms are a major cause of withdrawal from ultramarathons (Costa et al., 2016), the origin of my nausea over these days, and why it eventually resolved, remain unclear. Certainly, an area for future investigation! Between 350 and 380 km, despite several relatively long sleep periods (averaging 5 h per night), my physical energy (and parallel motivation) declined markedly, owing to persistent knee pain and relentless poor weather (fog, cold and rain). Ultrarunners are known to have a high tolerance to pain, partly attributable to reduced pain‑related anxiety and fewer avoidance behaviours; pain is ‘part of the game’ rather than a threat. Mental toughness scores are higher among ultramarathon runners than in other sports (Brace et al., 2020). I might be an exception regarding this particular psychological faculty. My options were limited: stop and sleep (never withdraw without first attempting a nap, because you might change your mind afterwards); withdraw; or engage in a targeted pain-specific resilience strategy. Fortunately, Laurent (a highly experienced friend) ‘dragged’ me slowly all the way to Lengritz (2416 m), where hot drinks restored my morale. Beyond his words, ‘We move slow but we keep moving’, this anecdote confirms the importance of the social interactions (in this case, with fellow runners) on the response of the athlete to MUM-related stressors (Harman et al., 2019). Functional ‘teams’ arise spontaneously, offering reciprocal support. In mountain sports, such social relationships are mainly instrumentally oriented and valuable for coping with both physical and environmental challenges. Such complementary alliances proved equally important at ∼580 km, at the technical pass of ‘fenêtre d'Arpette’. From pass to pass and valley to valley, the cumulative distance increases, with the most demanding sections still ahead. Favourable weather is forecast, and the anticipated psychological benefit of reuniting with my daughters on the trail approaches (Figure 3). Having contact with family members helps to cope with anxiety when the runner experiences difficulties (Harman et al., 2019). The psychological and emotional support that my family provided, particularly given that they had travelled a long distance to be present, was exceptional. Such affective social interactions play a key role in helping athletes to cope with the demands of an activity often perceived as solitary (Harman et al., 2019). Around 500 km, I began to experience substantial fluid retention, with progressively increasing oedemas visible in the lower limbs and facial region (Figure 3). This resulted in a substantial increase in body mass between the start and the finish of the race. Oedemas are commonly reported during ultramarathons (Bracher et al., 2012; Vitiello et al., 2015; Zanchi et al., 2016) and are the observable consequences of the increase in total body water (Knechtle et al., 2008), including plasma volume (i.e., pronounced hypervolaemia) (Krumm et al., 2025). I had not experienced this phenomenon during my previous MUMs, and the underlying physiological mechanisms remain unclear. Given that pro-inflammatory responses can contribute to hypervolaemic processes (Robach et al., 2014), it cannot be ruled out that the chronic knee pain experienced during the race acted as a primary trigger. The consequences for renal function might be substantial and remain understood only in part. In contrast, the short-term effects (Linder et al., 2025; Pasternak et al., 2023; Vauthier et al., 2024) and, potentially, long-term effects (Hodgson et al., 2017; Rojas-Valverde et al., 2020) of hypohydration-induced acute kidney injury are better documented, including its possible association with cardiac dysfunction (Burtscher et al., 2022). The emotional impact at the finish line was profound, and the journey across the Swiss peaks proved highly instructive. As a physiologist, directly experiencing both the advantages and the drawbacks of such an extreme MUM provides valuable insights that might help to refine my understanding of the associated physiological stressors. This experiential perspective could contribute to the development of strategies aimed at mitigating the deleterious consequences of prolonged MUMs (e.g., sleep deprivation, inflammatory responses, pronounced hypervolaemia and oedemas, and low-grade knee cartilage damage), while simultaneously acknowledging and optimizing the beneficial adaptations elicited before and during the race, mainly the increased level of physical activity (Stamatakis et al., 2025; Tarp et al., 2024). To conclude, in my view, few stimuli offer a more powerful health-promoting leverage than sustained moderate-intensity exercise in mountainous environments, because it combines the well-established benefits of green exercise (Laezza et al., 2025; Pretty et al., 2005) and those of moderate-altitude exposure (Burtscher et al., 2024; Mallet et al., 2025). How green it was, how slow it was, and what a long period of exercise at altitude it turned out to be! (Figure 4). We were truly fortunate to have such an adventure right on our doorstep. Sole author. Thanks to Julien Voeffray and Noémie Remacle for imagining and organizing this event. Thanks also to all the volunteers who brought it to life. I wish also to express my sincere gratitude to all those who supported me along the course (Alexa, Antoine, Bastien, Forrest, Floriane, Olivier, Marie-France, Yohan, Jonas and Esther) and to my dedicated heart team (François, Armelle, Perrine, Manon, Solène, Alex, Félix and Lucien), whose encouragement proved invaluable throughout the race. None declared. None.