Trail running is the most popular off-road (e.g. unsealed surface and natural environment) running event.1 By definition trail running is a foot race in a natural environment including mountains, deserts, forests, coastal areas, jungles/rainforests, grassy or arid plains over a variety of different terrains (e.g. dirt road, forest trail, single track, beach sand, etc.) with minimal paved or asphalt roads.1 Distances range from a few kilometres to more than 80 km and the average inclination should include a minimum of 5 % and not exceed 25 %.1,2

The International Trail Running Association (ITRA) defines trail running as a pedestrian race with a clearly marked route (usually in natural environments and terrains such as mountains, deserts, forests or plains) with a maximum of 20 % of the total route on paved road.3 Participants are semi to fully self-sufficient and are sometimes required to use running packs in races to carry limited nutritional supplies and safety equipment (e.g. clothing or communication).4

Trail running has become increasingly popular and participation has grown substantially in recent years, with an estimated 15 % annual growth rate.5 Only in the US, the number of participants has grown from 4,8 million in 2009 to 9,1 million in 2017.1

At the same time the number of participants has grown, trail running has undergone progressive professionalization as a result of the promotion of national and international federations and the support of important sports sponsors. In 2015, trail running was recognised by the World Athletics (WA) as a discipline of athletics5 and in November 2022 ITRA and WA organised jointly the first World Mountain and Trail Running Championships (WMTRC) in Thailand6; and in June 2023 the second WMTRC was held in Austria7 with more than 70 national teams and 1200 athletes.

Furthermore, trail running has a number of particular risks for the health of athletes that differentiate it from on road races or track and field competitions: uneven surfaces, distance covered, altitude, environmental conditions and endurance effort affected by large elevation gains/losses.8 Therefore, trail running presents a high risk of injury.9 The last systematic review10 reports an injury incidence range 0,7–61,2 injuries/1000 h of running and prevalence range 1,3 % to 90 %.

Due to its recent popularity,11 the number of epidemiological publications about injuries and illnesses in mountain runners is low,1 especially if we refer to professional runners. The current evidence comes mainly from cross-sectional studies obtained in single-day competitions which are focused on injuries and illnesses during competition and whose population is mainly male runners of middle age and amateur level.9,10

The aim of this paper is to describe the epidemiology of injuries and illnesses among professional trail runners during World Mountain and Trail Running Championships 2023.

All the injuries and illnesses suffered during the World Championships by the 77 athletes who made up the Spanish and French national teams were recorded by the corresponding medical team. However the pre-participation questionnaire was answered by 49 athletes (38 Spanish athletes and 11 French athletes). Their demographic and sport characteristics are shown in Table 1.

Table 1.

Demographic and sport characteristics of the athletes (n = 49).

a: years, b: kilograms, c: centimetres, d: body mass index, e: kilograms divide height in metres square f: hours per week g: metres per week.

An injury or illness was suffered by 38 % of athletes during the World Championships. 22 % suffered a musculoskeletal injury and 16 % suffered an illness. By sex, 65 % of injuries and 67 % of illnesses were suffered by male athletes.

Injuries

Among the injuries, 82 % of them occurred during the competition and 18 % during peri‑competition activities (i.e. warm-up and cool-down); and 82 % of them were newly incurred during the competition and 18 % were exacerbation of a not recovered condition.

The distribution by event was: 35 % took place in short trail, 29 % in long trail, 24 % in mountain classic senior and 12 % in vertical. By anatomical location, the most affected was the foot in 47 % of the cases, followed by the ankle and the knee in 12 %. For injury types and diagnoses, the most frequent were lacerations (35 %) and tendinopathy (24 %). (Table 2)

Table 2.

Injury details (n = 17).

F: female, M: male.

Regarding the injury mechanism, direct contact with an object (i.e. roots or stones) or with another athlete causing athlete to slip and fall was the most frequent cause of injury in 42 % of the cases; followed by friction and sweating in 33 % of the cases and repetitive transfer of energy or overuse in 25 %. Considering the mode of onset, 59 % of the injuries onset gradual and 41 % sudden after a trauma. (Table 2)

As for the severity of injuries, 35 % of the injuries did not affect sport performance and 65 % of injuries did. Of them, 35 % forced athletes to stop less than fifteen minutes but 30 % had to withdraw from the competition. The diagnoses of the withdraw athletes were: rib fracture, shoulder dislocation, skin avulsion of the heel and 2 exacerbations of previous tendinopathy. (Table 3)

Table 3.

Diagnosis and characteristics of the athletes leading to withdraw (n = 9).

	Age	Sex	Event	Location / System	Injury Mechanismo/aetiology	Diagnosis	New or exacerbation
Injury (n = 5)	32	F	long trail	Foot	Overuse	Plantar tendinopathy	Exacerbation
	36	M	long trail	Knee	Overuse	Patellar tendinopathy	Exacerbation
	39	F	long trail	Foot	Overuse	Skin avulsion	New
	32	M	long trail	Chest	Acute after trauma	Rib fracture	New
	23	M	vertical	Shoulder	Acute after trauma	Shoulder dislocation	New
Illness (n = 4)	22	F	classic	Termoregulatory	Hyperthermia or dehydration	Heatstroke	New
	33	M	long trail	Termoregulatory	Hyperthermia or dehydration	Dehydration	New
	29	M	long trail	Termoregulatory	Hyperthermia or dehydration	Dehydration	New
	30	M	long trail	Neurological	Carbohydrate intake defficiency	Hypoglycemia	New

F: female, M: male.

The main results of the study are: (i) 38 % of the athletes had an injury or illness during their participation in the WMTRC 2023; (ii) 22 % suffered an injury and 16 % an illness; (iii) 24 % of the athletes suffered an injury and illness that forced them to stop sport performance and 12 % had to withdraw due to a major injury or illness; and (iv) foot was the most affected body region and hyperthermia and dehydration were the most frequent aetiology of illness.

Our interpretation of these data is that there is a high risk for professional athletes, more than one in three, to suffer an injury or illness during an important trail running championships. In relation to the above, approximately almost one in four athletes are in risk of suffering an injury and illness that affect their sport performance and more than one in ten of withdrawing due to a major injury or illness.

As far as researchers of this study are aware, our study is the second one exclusively focused on professional runners, that is, athletes who are (i) fully dedicated to training and competing in this sport within a federative structure and (ii) compete in national and international events. The first study14 investigated pre-competition health status of Spanish athletes the month prior to the WMTRC 2022. It concluded that more than a half of trail runners suffered an injury or health problem and one in ten had to modify their participation in training and competitions in a moderate or severe way.

The measurement of the severity of an injury or illness is an important concept in epidemiological studies. The most widely used severity measure in sports medicine is the duration of time-loss from training or competition. Bahr et al.12 defined time-loss as “the number of days that the athlete is unavailable for training and competition, from the date of onset until the athlete is fully available for training and competition”. Authors of the study think this measurement in trail running is accurate for major injuries and illnesses that make the athlete unavailable for training and competition for one day or more; and it underestimates those injuries and illnesses that limit athlete's performance in terms of intensity or time less than one day (minutes or hours). We think this is especially relevant in trail running for two reasons: the long distance of the races and the existence of refreshment stations that allow minor health problems (i.e. blisters or mild dehydration) to be resolved throughout the race. For this reason we propose a classification that identifies those injuries and illnesses that force the athlete to lose a few minutes during the competition, but allow them to finish it.

In relation to incidence of injuries and illnesses, this presents a very wide range in published studies, between 1,3 % to 90 %.9,10 Authors of the study think that the reasons for such a variation could be: (i) heterogeneity of methodologies in previous studies and inconsistent definitions of epidemiological concepts as injury or illness across studies; (ii) heterogeneity of trail running as sport: uneven surfaces, long distances covered, altitude and changing environmental conditions make each race different, even the same race could be different every year; and (iii) the number of epidemiological publications about injuries and illnesses in mountain runners is still low.

According to this study the incidence of injury is 22 % and the incidence of illness 16 %. As regards the body region and in line with majority published series,9,10 the lower extremity was the most affected, with the foot first, followed by the ankle. Regarding illnesses, the majority of the published series9 indicate the digestive system, with symptoms as nausea, vomiting, abdominal cramping or pain, to be the most affected system. In our study, the thermoregulatory system is the most affected. This includes digestive symptoms such as nausea or vomiting, but not only, it also includes headache, dry mouth, hyperthermia or even heat stroke.

On long trail race day (June 9, 2023), the Innsbruck weather station15 recorded a minimum temperature of 9 °C coinciding with the start time of the race, and a maximum temperature of 23 °C throughout the day. Furthermore, athletes ran through mountains where temperatures were close to 0 °C. So trail long athletes withstood a thermal variation of around 20 °C throughout more than 9 h of competition. Few studies reported data about specific diagnosis of dehydration16,17 and they focused only on the symptoms as nausea, vomiting or abdominal pain, not reporting the diagnoses. In order to minimize the risk of dehydration, one of the last reviews18 recommends for endurance athletes a water consumption of 400–800 ml/h and a sodium consumption of 300–600 mg/h.

Our study describes the epidemiology of injuries and illnesses among professional Spanish and French trail runners during WMTRC 2023, as well as their impact on athletes’ performance during the competition. As far as researchers of this study know, athletes’ performance is not reported in previous studies. In our study we can group the main causes of withdrawal into three: dehydration, exacerbating of previous overuse injuries and acute musculoskeletal injuries.

If we compare the results with other sports, the incidence of injuries and illnesses is higher than in most of them. The recently published study by Edouard et al.19 collects injuries and illnesses in 9 different sports from the last European championships in Munich 2022. In athletics the incidence of injuries is 3.5 % and illnesses 2.6 %. In cycling the incidence of injuries is 7.5 % and illnesses 0.5 %. Only the incidence of injuries is higher in triathlon with 24 %.

Regarding the most recent systematic review,10 the majority (80,8 %) of participants subject of the studies included, were men, so women have not been properly investigated. Notwithstanding the above, our study is balanced in terms of sex (i.e. 52 % men and 48 % women) and results by sex are: male athletes suffered 65 % of injuries and 67 % of illnesses, and 66 % of athletes leading to withdraw were male. These results are consistent with the results published on other sports. In athletics,20 the rate of injuries registered was significantly higher in male athletes during 14 top level international championships. The results are also coincident In elite skiers.21

The origin of this sex-related differences on incidence is unclear. Sex differences in the morphological, physiological and biomechanical characteristics could be part of the explanation. Regarding trail running we hypothesize that a possible reason of this higher incidence in male athletes may be due to their greater running speed. On the one hand, this can lead greater metabolic and water consumption, and on the other, it can increase the risk of acute traumas and injuries resulting from them.

Most articles published on trail running epidemiology collect data through surveys self-filled by athletes.9,10,22-25 This could lead to recall bias. In our study, team doctors collected data from injuries and illnesses directly, through an internationally validated form, which is completed by team doctors, so that, the quality of the information collected is more accurate.

For future epidemiological research in elite trail running, we recommend using international consensus statements such as the “International Olympic Committee consensus statement”,12 with accurate definitions and systematic data collection methods. In addition, the authors encourage medical teams from national trail running federations to further research in next top-level championships in order to achieve higher levels of evidence.

Several potential limitations should be acknowledged when interpretating the results of this study. There is a selection bias because only healthy athletes and in good physical condition were able to compete in the Championships We believe that such circumstance may understate overuse injuries and chronic illnesses against acute injuries and new illnesses.

It is a cross-sectional study with a small sample size. Future prospective studies with a larger sample size would be necessary to establish more solid conclusions in the epidemiology of professional trail running injuries and illnesses.

An injury or illness was suffered by 38 % of the sample of professional mountain runners during the World Championship. Foot is the most affected anatomical location and the thermoregulatory system the most affected organ system.

Due to a major injury or illness 12 % of the professional trail runners were unable to finish the competition. The principal cause is dehydration followed by overuse injuries and acute traumas.

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.