Humans are well suited to running long distances, having evolved as persistence hunters, capable of covering great distances in pursuit of prey (Scheer, 2019). Ultra-endurance running (UER) tests the limits of the human body and has become increasingly popular over the last few decades, with an exponential increase that has slowed slightly since ∼2016 (Scheer, 2019). This increase is in large part due to an increase in female and master athlete (athletes ages > 35 years) participation (Eichenberger et al., 2012; Knechtle et al., 2012; Zingg et al., 2013; Scheer, 2019). In addition, there has been a similar increase in participation of youth athletes (<19 years of age), although numbers are much smaller compared to adults (Scheer and Hoffman, 2019; Scheer et al., 2020b, c). In 2019 alone, over 669,000 runners finished an UER event and there were over 7000 UER events hosted around the world (DUV, 2019). The exception of course, was 2020, with a significant reduction in UER participation and events, due to the global COVID-19 pandemic (Scheer et al., 2021).

Ultra-endurance running is not a uniform sport but can be defined as a broad category with different types of running activities, such as running events by distance (e.g., any distance in excess of the standard marathon distance of 42.195 km), time (timed- UER, e.g., any events over 6 h) and multi day/multi-stage events (distance or timed events over several days or stages) (Scheer et al., 2020a). UER events can be held on various surfaces (e.g., track, on-road, off-road) (Scheer et al., 2020a) and in extreme challenging environments, putting additional strain on the human body (e.g., extreme cold, altitude, mountain, desert, heat and jungle) (Knoth et al., 2012; da Fonseca-Engelhardt et al., 2013; Gill et al., 2015; Scheer and Murray, 2015; Dawadi et al., 2020; Suter et al., 2020). The most popular race distances are those of 50 km, 100 km, and 100 miles (Cejka et al., 2014; Scheer, 2019; Knechtle et al., 2020), but can also include distances in excess of 1000 km (Fallon, 1996; Schütz et al., 2012; Scheer et al., 2020a), whereas time-limited events often include 6, 12, or 24 h events, with some lasting several days (Hutson, 1984; Bishop and Fallon, 1999; Scheer et al., 2020a). Multi day/multi-stage events are often held in extreme environments, and athletes often need to carry their provisions, resulting in additional weight while running as a further challenge (Krabak et al., 2011; Knoth et al., 2012; Dawadi et al., 2020; Scheer et al., 2020a).

Long hours of UER can lead to excessive stress on the musculoskeletal system, and potentially musculoskeletal injuries (MSKI) (Scheer and Murray, 2015). Average training loads in UER are between 66–83 km/week in adults and around 57 km/week in youth athletes (Kłapciñska et al., 2013; Scheer et al., 2018, 2020d; O’Loughlin et al., 2019). These training demands can lead to overuse injuries, especially when the load exceeds the adaptive mechanisms, affecting predominantly the lower limbs (e.g., patellofemoral pain syndrome, medial tibial stress syndrome, Achilles tendinopathy) (Khodaee and Ansari, 2012; Krabak et al., 2013, 2014; Scheer and Murray, 2015). Acute injuries are less common but may impact race performance (Khodaee and Ansari, 2012; Krabak et al., 2014; Scheer and Murray, 2015).

Musculoskeletal injuries in running have been defined as “running-related (training or competition) musculoskeletal pain in the lower limbs that causes a restriction on or stoppage of running (distance, speed, duration, or training) for at least 7 days or 3 consecutive scheduled training sessions, or that requires the runner to consult a physician or other health professional” (Yamato et al., 2015b), however, definitions of MSKI in UER vary across studies, which make comparisons difficult. Some defined MSKI as a disability resulting in a medical encounter (Krabak et al., 2011; Vernillo et al., 2016), or affecting performance (Bishop and Fallon, 1999). The severity of an injury had been described as severe, if it did not improve with rest (Hutson, 1984), major, resulting in race withdrawal, or minor, when the runner was able to continue the race (Krabak et al., 2011; Vernillo et al., 2016).

It is well established that physiological demands within this broader spectrum of UER may vary, depending on event type (e.g., distance, surface, and elevation changes) (Davies and Thompson, 1986; Bassett and Howley, 2000; Millet and Millet, 2012; Scheer et al., 2020a). For example, the fractional utilization of VO₂max in a 24 h race is between ∼40–50% (Millet et al., 2011), whereas shorter races of 6 h duration can be run at ∼70% of VO₂max (Giovanelli et al., 2017). If these higher running intensities in UER increase the risk of MSKI is currently unknown. Similarly, UER with large elevation changes, especially prolonged downhill running sections, place particular demand on the musculature with prolonged eccentric muscle action, leading to increased release of muscle enzymes (creatine kinase) and muscle damage (Dewolf et al., 2016; Giandolini et al., 2017; Vernillo et al., 2017). Such variability makes it likely that MSKI and injury rates will also be affected by the different types of UER events, like timed-events, multi-day events and continuous UER events. Similarly, injury rates and diagnosis of MSKI during competition and training may also vary, as during competitions athletes typically push themselves to the limit. Several review articles of illness and injuries exist about ultramarathon running, however, they have not specifically reviewed MSKI in different types of UER events and analyzed MSKI in competition and training (Khodaee and Ansari, 2012; Lopes et al., 2012; Krabak et al., 2013, 2014; Hoffman et al., 2014; Knechtle and Nikolaidis, 2018a, b). Proper treatment of injuries and illnesses in UER is important for avoiding long-term issues (Krabak et al., 2014) and therefore it is important to examine and summarize MSKI within these broad categories of UER during competition and training.

The aims of this scoping review were therefore: (1) to examine the current evidence of MSKI, providing a synthesis of the most common MSKI by anatomical region and specific diagnosis; (2) categorize MSKI by type of UER activity (competition: time-limited; multi-stage; continuous UER events) and training); (3) describe knowledge gaps in the literature and provide advice on potential further research.

This review is based on the recommendations for scoping reviews, with the purpose of identifying and mapping the available evidence and identifying knowledge gaps (Armstrong et al., 2011). As such we used a broad research question, for example: what are the musculoskeletal injuries in UER? What are the different anatomical distributions and incidence/prevalence of MSKI in UER competition and training? Our aim was to review the existing literature, summarize those findings, identify knowledge gaps, as done in previous scoping reviews (Armstrong et al., 2011; Granacher et al., 2016). The review considered scientific papers that investigated MSKI in UER. UER is defined as a broad category with different types of running activities, such as running events by distance in excess of 42.195 km (standard marathon distance), timed-events over 6 h durations, and multi-day or multi stage running events, on all surfaces (road, off-road, track) and terrains (Scheer et al., 2020a). All studies that examined MSKI in UER were included irrespective of participants age and/or sex, however, detailed information of MSKI according to sex or age was not available in the majority of studies, therefore a comprehensive breakdown of MSKI according to these parameters was not possible. However, those studies that did provide specific results on age and/or sex aspects were highlighted in the respective sections in the discussion (Micklesfield et al., 2007; Scheer et al., 2020d).

An electronic literature search was performed using different databases (PubMed, SPORTDiscus, Web of Science) from January 1st, 1984- September 30th, 2020. The following search terms were used: ‘ultra endurance running’ or ‘ultra running’ or ‘ultramarathon’ or ‘trail running’ or ‘ultra trail running’ and ‘injury’ or ‘musculoskeletal injury’ or ‘muscle injury.’ We identified a total of 771 studies meeting our initial search criteria. Figure 1 gives an overview of article selection process in accordance with PRISMA (Moher et al., 2009). After removal of duplicates, the abstracts of the remaining 229 studies were reviewed. Only studies written in the English language were considered. Studies that did not examine MSKI in UER, did not provide any data on general or specific MSKI, provided data on mixed population (e.g., different running populations including UER and non-UER) were excluded. Similarly, studies that only examined muscle damage from biochemical markers or muscle cramps, as well as case reports were not included in this review. Studies on blisters, dermatological or other illnesses and injuries were excluded. A total of 35 full texts were assessed for eligibility and after review, a total of 13 studies met our inclusion criteria and were therefore included in the review. All abstracts and manuscript were reviewed by the two authors.

The definitions of MSKI varied across the different studies, with some providing no specific definitions (Scheer and Murray, 2011; Graham et al., 2012; McGowan and Hoffman, 2015; Dawadi et al., 2020), whereas other defined MSKI as a disability resulting in a medical encounter (Krabak et al., 2011; Vernillo et al., 2016), or affecting performance (Bishop and Fallon, 1999). All studies reporting MSKI during a race were either documented by the attending medical team (through self-referral/self- reporting system via medical encounters) or by a routine daily medical assessment questionnaire (Hutson, 1984; Fallon, 1996; Bishop and Fallon, 1999; Krabak et al., 2011; Scheer and Murray, 2011; Graham et al., 2012; McGowan and Hoffman, 2015; Vernillo et al., 2016; Dawadi et al., 2020). Studies examining MSKI in training studies defined injuries as either an athlete reporting the diagnosis based on a health care provider encounter or through athlete self-assessment (Micklesfield et al., 2007; Hoffman and Krishnan, 2014; Malliaropoulos et al., 2015; Scheer et al., 2020d).

The main findings of our review were: (i) MSKI in UER are common, mostly affecting the lower limbs and are of overuse in nature; (ii) MSKI differ between competition and training, with multistage events, predominantly affecting the lower leg, foot, and knee, while timed events mainly affect the ankle, Achilles tendon, and knee; (iii) Short continuous UER events off-road have the highest incidence of MSKI, mainly affecting the foot and ankle, while long continuous UER affect the knee and ankle; (iv) During training back, knee and bone stress injuries are common. Detailed description of MSKI incidence (rate of occurrences of new cases) and/or prevalence (number of MSKI at a particular time) are provided. The manuscript is organized accordingly describing the different pertinent research sections.

Thirteen studies examined MSKI in UER and are included in this review. Table 1 and Table 2 provide detailed information about the study, participants (age and sex), and MSKI. Nine studies (Table 1) provide information from UER races/competitions (total n = 723 participants), while four studies (Table 2) provide information from MSKI mostly during training of UER athletes (total n = 1606 participants). Detailed information of MSKI according to sex or age was not available in the majority of studies, therefore a comprehensive breakdown of MSKI according to sex or age was not possible. One study (Micklesfield et al., 2007) specifically examined bone stress injuries in female UER, and another MSKI in youth athletes (Scheer et al., 2020d) and for those studies results by sex and age were discussed in more detail in the appropriate sections. The vast majority of MSKI involve the lower limb (Fallon, 1996; Krabak et al., 2011), and are generally minor across all types of UER (Fallon, 1996; Bishop and Fallon, 1999; Krabak et al., 2014). MSKI are predominantly overuse in nature (e.g., tendinopathies), even during competitions, with incidences in the region of between 98–100% (Fallon, 1996; Bishop and Fallon, 1999), demonstrating the exceptional demand on the musculoskeletal system during prolonged running, in contrast to ‘true’ acute injuries (e.g., muscle strains) that may be expected during competition in other, more explosive sports (Huxley et al., 2014; Krabak et al., 2014; Scheer and Murray, 2015).

The aims of this scoping review were therefore: (1) to examine the current evidence of MSKI, providing a synthesis of the most common MSKI by anatomical region and specific diagnosis; (2) categorize MSKI by type of UER activity (competition: time-limited; multi-stage; continuous UER events and training); (3) describe knowledge gaps in the literature and provide advice on potential further research. The main findings of our review were that MSKI were mostly overuse injuries, predominantly affecting the lower limbs with different injury patterns and diagnosis between different types of UER activities.

Musculoskeletal injuries in UER are common, mostly affecting the lower limb and are of overuse in nature. MSKI differ between competition and training, with multistage events, predominantly affecting the lower leg (medial tibial stress syndrome), foot, knee (patella femoral pain syndrome (PFPS), while timed events mainly affect the ankle (tendinitis of the foot dorsiflexors, Achilles tendinopathy), and knee (patellar tendinopathy). Short continuous UER events off-road have the highest incidence of MSKI, mainly affecting the foot (plantar fasciitis), ankle (ankle sprain), whereas in long continuous UER the main anatomical site of injury is the knee (PFPS), and ankle. During training the back, knee and bone stress injuries are common. Future considerations include examining MSKI in different UER events, environments and surfaces, and on larger study populations. Establishing risk factors, examining sex differences and using a standard reporting system of MSKI in UER are also important.

VS performed conception and design of the study, literature search and analysis, and manuscript writing and editing. BK performed literature search and analysis and manuscript writing and editing. Both authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.