Since the mid-1990s, trail running has been attracting practitioners and is recognised as an athletics modality by World Athletics.1 The International Trail Running Association (ITRA) defines trail running as “a run in a natural environment (mountain, desert, forest) with a minimum of paved roads (must not exceed 20 % of the total route)”.2 The distances and the unevenness (the difference in altitude between the start and the finish line) vary substantially within the sport.3 In the United States, the number of trail runners grew from approximately 4.8 million in 2009 to 9.1 million in 2017.4 The Brazilian Athletics Confederation (CBAt) officially regulated trail running in 2017.5

Associating physical activity and the natural environment, such as trail running, has been reported as a way to maintain physical and mental health.6 Trail running is practiced in natural environments which involves variations in terrain. Therefore, the trail runner deals all the time with biomechanical adaptations during ascents and descents and irregular surfaces.3,7 Thus, the practice of trail running has an inherent risk of developing injuries.

Some studies have shown the prevalence of running-related injuries (RRI) in trail runners.8–10 For instance, the mean 2-week period prevalence of RRIs in Dutch trail runners was 22.4 % (95 % confidence interval [CI]: 20.9, 24.0).11 The point prevalence of RRIs in trail runners was 1.3 % in a study conducted in South Africa.12 Researchers analysed data from five seasons (between 2015 and 2019) of 20–42 km trail running races and reported 1.6 RRIs per 1000 h of running.13 Previous history of RRIs and/or cramps is associated with an increased risk of developing such health conditions during trail running events, which could lead to withdrawing of the trail runner from the race.2

Knowledge about the injuries affecting sports and society helps in the development of risk control and prevention strategies. For example, a group of researchers measured the burden of RRIs, including the prevalence (i.e., 22.4 % [95 % CI: 20.9, 24.0]), in Dutch trail runners.11 The knowledge gathered from this study served as a basis for the recommendation and rationale for the development of a specific RRI prevention programme tailored to this populations.14,15

The objectives of this study in Brazilian trail runners were: (1) to investigate the point prevalence of RRIs; (2) to describe the most common injured body regions; (3) to investigate the period prevalence of cramps; (4) to describe the most common body regions affected by cramps; (5) to describe the personal and training characteristics; and (6) to investigate the history of previous RRIs.

A total of 2491 people accessed the online questionnaire with 1219 people (48.9 %) submitting responses. Only 2 people (0.2 %) did not consent to participate and, therefore, were not presented with the full questionnaire (i.e., parts 1 to 4). Forty-five people (3.7 %) were not eligible, because of less than six months of trail running experience. A total of 104 responses (8.9 %) were removed because they were duplicates.

In total, 1068 Brazilian trail runners were included in this study. The average time to complete the online questionnaire was 14:41 min (IQR: 8:31, 14:44). Table 1 describes the personal characteristics of the participants. The mean age of the participants was 39.5 (95 %CrI: 39.0, 40.0). A total of 34.6 % (95 %CrI: 31.7, 37.4; n = 369) were women and 65.4 % (95 %CrI: 62.6, 68.3; n = 699) were men. Most of the trail runners presented a normal body mass index (BMI) (i.e., 18.5≤BMI≤24.9: 71.5 %; 95 %CrI: 68.9, 74.3; n = 764). A total of 76.0 % (95 %CrI: 73.4, 78.5; n = 812) had a higher education degree.

Before starting to run on trails, 85.1 % (95 %CrI: 83.0, 87.2; n = 909) of the participants reported to running on roads. On average, Brazilian trail runners reported to run 1 time per week on trails (95 %CrI 0, 3); 54.3 % (95 %CrI: 51.3, 57.3; n = 580) of them reported to run between 2 and 4 h per week on trails; and 49.7 % (95 %CrI: 46.7, 52.7; n = 531) reported to run between 5 and 20 km per week on trails (Table 2).

Most of the participants reported trail running experience between 2 and 5 years (53.2 %; 95 %CrI: 50.3, 56.2; n = 568), and the preferred distance related to participating in running events was the half marathon (45.6 %; 95 %CrI: 42.6, 48.6; n = 487) (Table 3). A total of 58.5 % (95 %CrI: 55.6, 61.5; n = 625) of the participants classified themselves as being recreational runners (Table 3). The main motivations to run in general (on and off trails) reported by the participants were: ‘feeling of well-being’; ‘to have fun’; ‘to enhance health’; and ‘to enhance performance’ (Table 3).

A total of 544 RRIs were reported by 419 injured trail runners. The point prevalence of RRIs was 39.2 % (95 %CrI: 36.3, 42.1; n = 419) (Table 4). About 75.2 % (95 %CrI: 71.1, 79.3; n = 315) of the injured trail runners reported training time loss or a reduction in training volume, while 80.0 % (95 %CrI: 76.0, 83.6; n = 335) required medical attention due to RRIs (Supplementary Online Material Table S1). Regarding the characteristics of the symptoms related to the current RRIs, pain (n = 363) was the main symptom, with a prevalence of 34.0 % (95 %CrI: 31.2, 36.9) and representing 86.6 % (95 %CrI: 83.2, 89.7) of the symptoms reported by injured trail runners (Supplementary Online Material Table S1).

The prevalence of cramps in the last month was 19.5 % (95 %CrI: 17.1, 21.9; n = 208) and the prevalence of previous cramps (last 12 months) was 36.0 % (95 %CrI: 33.0, 38.8; n = 384) (Table 5). The locations of cramps in the last month and in the last 12 months can be found in Table 5 (prevalences) and Supplementary Online Material Table S2 (distribution in injured trail runners).

Most Brazilian trail runners were men (65.4 %; 95 %CrI: 62.6, 68.3; n = 699) and had trail running experience between 2 and 5 years (53.2 %, 95 %CrI: 50.3, 56.2; n = 568). Regarding exposure to trail running, participants reported running on average 1 time per week on trails (95 %CrI: 0, 3), 2 to 4 h/week (54.3 %; 95 %CrI: 51.3, 57.3; n = 580), and 5 to 20 km/week (49.7 %; 95 %CrI: 46.7, 52.7; n = 531). The overall point prevalence of injuries related to trail running was 39.2 % (95 %CrI: 36.3, 42.2; n = 419), with the knee being the most affected site with a point prevalence of 13.4 % (95 %CrI: 11.4, 15.4; n = 143).

The results of our study showed a point prevalence of trail RRIs of 39.2 % (95 %CrI: 36.3, 42.2). The estimate found in our study was higher than the findings of a prospective study in Dutch trail runners (22.4 %, 95 %CI: 20.9, 24.0).11 A possible explanation for this discrepancy is that the studies had a different design; while our study had a cross-sectional design, the Dutch study had a prospective longitudinal repeated-measurements design. Also, the point prevalence of trail RRIs in South Africa was 1.3 %.12 In this South African study, runners answered the questionnaire two weeks before competitions, and according to the authors, this may explain the low point prevalence, because runners could be prevented from running the race if they reported health problems.12 The prevalence of trail RRIs ranged from 1.3 % to 90 % in a systematic review.9 This large variation may be a consequence of studies using different designs and definitions of injury, according to the authors.9

Regarding the anatomical region affected by RRIs, similar findings were reported in road running, with the knee being the most frequently affected site.25 Trail running factors related to knee overload may partly explain these findings, such as uphill, downhill, uneven terrain, and change directions. In addition, during ultra-trail races (common type of competition in trail running), runners experience high levels of fatigue, as demonstrated in previous studies.26 Fatigue can result in kinematics changes of the knee, contributing to joint overload.27 In a systematic review on the epidemiology of injuries and diseases among trail runners, the foot was the most affected site of injuries.2 This systematic review also presented data on dermatological lesions and gastrointestinal symptoms, common in trail running events.2 The authors speculated that, perhaps, the foot was the most affected site of injuries because there were many blisters and skin lesions on the feet and, then, this finding may have been overestimated.2 Our study focused only on the point prevalence of musculoskeletal injuries of the lower limb during training exposure, which might explain the discrepancy compared to the aforementioned review.

Brazilian trail runners reported cramps in the last month (having the time of data collection as reference) mainly in the triceps surae and quadriceps. Cramps are common medical conditions during ultramarathon trail races.28 Athletes who experience excessive fatigue or cramps during races should seek medical attention for evaluation, avoiding the risk of musculoskeletal injuries.28 Therefore, monitoring cramps in trail runners seems to be a fundamental aspect to compose the evaluation and the surveillance of the health, training, and performance of trail runners.

A nationwide study in Brazil with runners in general (i.e., not specifically trail or road runners) showed a higher proportion of male runners (71.4 %) compared to female runners (28.6 %), corroborating our results.29 The authors of the nationwide study hypothesised that perhaps Brazilian women have a preference for other exercises than running, because the proportion of female exercisers, excluding runners, was actually reverse (i.e., higher proportion of female non-runner-exercisers [56.2 %] compared to male non-runner-exercisers [43.8 %]).29 The proportion of women found in our study (34.5 %) was higher compared to a systematic review on trail running injury risk factors (15.8 %).2 In our study, women reported that trail running promotes a greater sense of well-being and has a more pleasant environment when compared to road running.

Brazilian trail runners trained, on average, 3 times/week on road tracks (paved surfaces). This corroborates our a priori hypothesis that trail runners do not train only on trails, as reported by Hespanhol Junior et al.11 However, a cross-sectional study investigating injuries in the 2019 Skyrun race reported that trail tracks was the most often training surface (76.5 % on trails followed by 55.8 % on tarred/paved surfaces).12 The difference in these findings can be explained by the difficulty of access to trails by Brazilian runners who, usually, live in metropolitan areas. In addition, culturally in Brazil training on flat and paved terrains is adopted by most running coaches, and it is part of the preparation to run on trails.

The point prevalence of RRIs in Brazilian trail runners was about 40 %. Almost 70 % of runners reported previous RRIs in the last 12 months. The last month period prevalence of cramps was about 20 %, while the 12-month period prevalence was 36 %. The triceps surae was the muscle most affected by cramps, representing more than 50 % and almost 60 % in the last month and in the last 12 months, respectively.

The authors affirm that the manuscript is an honest, accurate, and transparent account of the study being reported. No important aspects of the study have been omitted. Any discrepancies from the study as planned have been explained.

Data are available upon reasonable request to LH (corresponding author). De-identified participant data might be available after the consent of all authors and the privacy policy of the Research Ethics Committee of the Universidade Cidade de São Paulo (UNICID).