Abstract
ke road running, mountain and trail running typically cover longer distances and include uphill and downhill segments that impose unique physio- logical and mechanical demands on athletes.Objectives: This study aimed to identify morphological differences in the patellar and Achilles tendons between trail and road runners. Moreover, the potential influence of weekly mileage and accumulated positive elevation gain on the morphology of both tendons was obtained.Design: Cross-sectional comparative study. Methods: Thirty-three road runners (11 women, 22 men) and thirty- three trail runners (13 women, 20 men) were recruited and their weekly mileage and elevation gain collected. All participants had a weekly training volume exceeding 20 km. The thickness and cross-sectional area (CSA) of their patellar and Achilles tendons were evaluated using ultrasound.Results: Independent samplest-tests revealed significant differences between groups for the Achilles tendon (p< 0.003) but not for the patellar tendon (p> 0.330). Further, Spearman’s correlation coefficients indicated moderate positive correlations for the thickness and CSA of the Achilles tendon with weekly running vol- ume (0.256 and 0.291, respectively) and with elevation gain (0.332 and 0.334, respectively), suggesting a tendency for the tendon to adapt to greater training loads, enhancing its structural integrity and resilience.Conclusions: Trail runners exhibit larger and thicker Achilles tendons, likely due to
coefficients indicated moderate positive correlations for the thickness and CSA of the Achilles tendon with weekly running vol- ume (0.256 and 0.291, respectively) and with elevation gain (0.332 and 0.334, respectively), suggesting a tendency for the tendon to adapt to greater training loads, enhancing its structural integrity and resilience.Conclusions: Trail runners exhibit larger and thicker Achilles tendons, likely due to increased weekly mileage and elevation gain, highlighting the adaptive response to mechanical overload from uphill running. Keywords:trail running; Achilles tendon; ultrasonography; accumulated elevation; cross- sectional area 1. Introduction Running is one of the most widely practiced sports globally [1]. In particular, trail running has seen a surge in popularity, with the number of participants doubling over the past decade [2]. Trail running events include not more than 20% of running on even asphalted roads, with the remainder being unpaved, uneven terrain [2], and thus, it possesses unique physiological and biomechanical characteristics [3]. However, despite its growing popularity, these characteristics have not yet been extensively studied. The biomechanics of trail running cannot be fully understood without analyzing the behavior of the musculoskeletal system, particularly tendons. During running, lower-limb tendons play a critical role in the stretch–shortening cycle [4], which allows the tendons to store and release the energy necessary for efficient running mechanics [5]. Trail running, characterized by elevation changes in the terrain, affects running and thus alters the stress placed on the lower-limb tendons compared to road running. Research has shown that J. Funct. Morphol. Kinesiol.2025,10, 1
J. Funct. Morphol. Kinesiol.2025,10, 1 2 of 10 runners tend to adopt a mid-foot or forefoot strike pattern when running uphill [6,7], which places increased demands on the Achilles tendon (AT) and ankle [8]. During uphill running, the greater involvement of the Achilles tendon is due to a pre-stretch condition during the run cycle, which necessarily results in increased stress on posterior chain tendons and muscles. Conversely, when running downhill, most runners shift to a rearfoot strike pattern [9], which places more strain on the patellar tendon (PT) and quadriceps muscles due to a greater range of knee flexion and increased eccentric braking during landings, thereby contributing to overloading these structures [10]. In this regard, changes in running technique, such as those mentioned in the initial contact pattern, have been identified as key contributing factors to running-related injuries [11]. The main lower-limb tendons of road runners (PT and AT) have been extensively stud- ied [12–15]. A key finding in the literature is that runners tend to have larger tendons, both in terms of thickness and cross-sectional area (CSA), compared to untrained or sedentary individuals [12–15]. These morphological differences are thought to result from the greater mechanical load experienced by runners’ tendons [16], prompting an adaptive response in the form of increased collagen production and extracellular matrix proliferation by teno- cytes [17]. As a result, hypertrophy and increased stiffness have been observed in runners’ tendons [18], which, in turn, may provide a protective effect for muscle–tendon units. In this context, the so-called repeated bout effect evokes the ability of the musculotendinous system to trigger an inherent protective mechanism in response to damage caused by eccen- tric exercise, initiating an adaptive process that enhances resistance to future damage [19]. Furthermore, adaptations in runners’ tendons have been shown to reduce the metabolic cost of running [20] and enhance performance in distance running [21]. The morphological characteristics of trail runners have been less studied compared to those of road runners. One of the few studies investigating this found no significant differences in AT morphology between trail and road runners [22]. Of note, weekly running
runners’ tendons have been shown to reduce the metabolic cost of running [20] and enhance performance in distance running [21]. The morphological characteristics of trail runners have been less studied compared to those of road runners. One of the few studies investigating this found no significant differences in AT morphology between trail and road runners [22]. Of note, weekly running distance was nearly identical between both groups [22], and accumulated elevation was not considered. Given the recent exponential growth in trail running, a more detailed investigation of the main tendons of the lower limbs in trail runners is warranted. Tendon morphology has been shown to influence various biomechanical variables, such as lower limb stiffness or duty factor [23–25]. Therefore, examining the tendon mor- phology of trail runners could provide valuable insights into their running biomechanics and potentially enhance their performance. Additionally, tendinopathies are a common in- jury among trail runners [26], highlighting the need to investigate how sport-specific intrin- sic factors, such as weekly training volume and elevation gain, impacttendon morphology. Given the established link between tendon morphology and running biomechan- ics [16], further research is needed to assess the morphology of the main lower limb tendons in trail runners, taking into account intrinsic factors specific to trail running, such as accumulated elevation or the weekly running volume. The present study has two main objectives: (i) to analyze the morphological differences in AT and PT between road and trail runners, focusing on thickness and CSA; and (ii) to investigate how weekly running vol- ume and accumulated elevation gain influence potential tendon morphology differences. We hypothesize that trail runners, due to higher running volume and greater accumu- lated elevation gain, will exhibit tendons with increased thickness and CSA compared to road runners. 2. Materials and Methods 2.1. Subjects Thirty-three experienced road (11 women, 22 men) and thirty-three trail runners (13 women, 20 men) participated in this study. The inclusion criteria were (i) being 18 years
runners. 2. Materials and Methods 2.1. Subjects Thirty-three experienced road (11 women, 22 men) and thirty-three trail runners (13 women, 20 men) participated in this study. The inclusion criteria were (i) being 18 years
J. Funct. Morphol. Kinesiol.2025,10, 1 3 of 10 of age or older; (ii) running at least three times per week and covering a minimum of 20 km weekly; (iii) having competed in at least two running events per year over the past two years; and (iv) no lower limb tendon-related injuries (e.g., tendinopathies) in the six months prior to data collection. An a priori power analysis [17] was performed to determine the required sample size based on an assumed large effect size (d = 0.8), an alpha level of 0.05, and a desired statistical power of 0.80. The analysis indicated that a minimum of26 participantsper group would be needed to reliably detect significant differences in tendon morphology. To safeguard against potential dropouts due to injuries or scheduling conflicts with competitions, the study recruited a total of 33 runners for each group, ensuring the robustness and validity of the results. Recruitment was conducted using convenience and snowball non-probability sampling. Each participant received a thorough verbal and written explanation of the study’s objectives, methods, and poten- tial risks. Subsequently, all provided written informed consent, in line with the ethical standards set by the World Medical Association’s Declaration of Helsinki. The research protocol was approved by the local Ethics Committee (No. 36/2023). 2.2. Material and Testing For descriptive purposes, participants’ height (cm) and body mass (kg) were deter- mined using a precision stadiometer and a weighing scale (SECA 222 and 634, respectively, SECA Corp., Hamburg, Germany). Additionally, data on the weekly running volume (in km) and the accumulated elevation gain (in m) were collected from each participant (Table). Of note, elevation gain refers to the total accumulated positive elevation, and it is important to stress that the negative elevation change will be equivalent in magnitude, as downhill running is inherently included in this parameter. Therefore, by mentioning elevation gain, we implicitly account for the work done during the descent as well. Table 1.Demographic description of the participants.Road Runners (n = 33) Trail Runners (n = 33) t-Test (df)p-Value Age (years) ˆ 27.9 ±7.13 30.2 ±4.52 343.00 (64) 0.010 Height (cm) 172.9 ±8.44 176.0±8.71−1.48
in magnitude, as downhill running is inherently included in this parameter. Therefore, by mentioning elevation gain, we implicitly account for the work done during the descent as well. Table 1.Demographic description of the participants.Road Runners (n = 33) Trail Runners (n = 33) t-Test (df)p-Value Age (years) ˆ 27.9 ±7.13 30.2 ±4.52 343.00 (64) 0.010 Height (cm) 172.9 ±8.44 176.0±8.71−1.48 (64) 0.144 Weight (kg) 67.7 ±10.47 66.8±8.77 0.41 (64) 0.683 BMI (kg·m −2 ) 22.5±2.24 21.6 ±2.50 1.69 (64) 0.096 Weekly volume (km) ˆ 30 (10) 80 (40) 61.50 (64) <0.001 Weekly elevation gain (m) ˆ50 (30) 2500 (1600) 0.00 (64) <0.001 BMI: Body Mass Index; ˆ Reported as median (IQR). Mann–Whitney U test as Levene’s test is significant (p< 0.05), suggesting a violation of the assumption of equal variances. The assessment of tendon morphological characteristics was carried out using high- definition ultrasound images (longitudinal and transversal views) acquired in B-mode using a wireless 12 MHz linear array probe (GE VScan Air CL, Freiburg, Baden-Wurttemberg, Germany) and with a gain of 100 dB. Each measurement was performed twice by a highly experienced researcher with over a decade of expertise in diagnostic ultrasound imaging. The clearest image, as determined by the examiner, was selected for subsequent calculation of morphological variables. To assess thickness and CSA, the ImageJ software (version 1.54k, NIH, Baltimore, MD, USA) was used [27]. The polygon tool within the software was applied to compute CSA. Given the known correlation between body mass and tendon morphology [28], CSA and thickness values were also adjusted to one-third of the participant’s body mass for the statistical analysis [14]. The PT characterization was evaluated with runners in the supine position, with both knees 30 ◦ bent [24,27]. This
J. Funct. Morphol. Kinesiol.2025,10, 1 4 of 10 position was controlled and guaranteed throughout the examination. A reference of 1 cm distal to the lower pole of the patella, identified by the ultrasound device, was used to assess the tendon thickness and CSA [24,27] (Figure).J. Funct. Morphol. Kinesiol. 2025, 10, x FOR PEER REVIEW 4 of 10 software was applied to compute CSA. Given the known correlation between body mass and tendon morphology [28], CSA and thickness values were also adjusted to one-third of the participant’s body mass for the statistical analysis [14]. The PT characterization was evaluated with runners in the supine position, with both knees 30° bent [24,27]. This po- sition was controlled and guaranteed throughout the examination. A reference of 1 cm distal to the lower pole of the patella, identified by the ultrasound device, was used to assess the tendon thickness and CSA [24,27] (Figure 1). - Figure 1. Ultrasound images of PT (i.e., thickness (A) and CSA (B)). Thickness and CSA are high- lighted in pink. During the evaluation of the AT, runners were in a prone position with both knees extended and their feet positioned outside of the bed, keeping the ankle in neutral position [25,27]. This position was controlled and guaranteed throughout the examination. Thick- ness and CSA were measured 3 cm proximally to the insertion into the calcaneus, meas- ured using the ultrasound device [25,27] (Figure 2). Figure 2. Ultrasound images of AT (i.e., thickness (A) and CSA (B)). Thickness and CSA are high- lighted in pink. 2.3. Statistical Analysis Statistical analysis was performed using the Jamovi software package (version 2.3.26, The Jamovi Project). The normality of the variables was evaluated using the Shapiro–Wilk test. For variables that met the assumption of normality, a Student’s t-test for independent samples was applied to assess differences in AT and PT morphology between road and trail runners. When the assumption of normality was violated, the Mann–Whitney U test was applied, with effect sizes expressed as rank-biserial correlations [28]. Effect sizes were Figure 1.Ultrasound images of PT (i.e., thickness (A) and CSA (B)). Thickness and CSA
normality, a Student’s t-test for independent samples was applied to assess differences in AT and PT morphology between road and trail runners. When the assumption of normality was violated, the Mann–Whitney U test was applied, with effect sizes expressed as rank-biserial correlations [28]. Effect sizes were Figure 1.Ultrasound images of PT (i.e., thickness (A) and CSA (B)). Thickness and CSA are high- lighted in pink. During the evaluation of the AT, runners were in a prone position with both knees extended and their feet positioned outside of the bed, keeping the ankle in neutral po- sition [25,27]. This position was controlled and guaranteed throughout the examination. Thickness and CSA were measured 3 cm proximally to the insertion into the calcaneus, measured using the ultrasound device [25,27] (Figure).J. Funct. Morphol. Kinesiol. 2025, 10, x FOR PEER REVIEW 4 of 10 software was applied to compute CSA. Given the known correlation between body mass and tendon morphology [28], CSA and thickness values were also adjusted to one-third of the participant’s body mass for the statistical analysis [14]. The PT characterization was evaluated with runners in the supine position, with both knees 30° bent [24,27]. This po- sition was controlled and guaranteed throughout the examination. A reference of 1 cm distal to the lower pole of the patella, identified by the ultrasound device, was used to assess the tendon thickness and CSA [24,27] (Figure 1). - Figure 1. Ultrasound images of PT (i.e., thickness (A) and CSA (B)). Thickness and CSA are high- lighted in pink. During the evaluation of the AT, runners were in a prone position with both knees extended and their feet positioned outside of the bed, keeping the ankle in neutral position [25,27]. This position was controlled and guaranteed throughout the examination. Thick- ness and CSA were measured 3 cm proximally to the insertion into the calcaneus, meas- ured using the ultrasound device [25,27] (Figure 2). Figure 2. Ultrasound images of AT (i.e., thickness (A) and CSA (B)). Thickness and CSA are high- lighted in pink. 2.3. Statistical Analysis Statistical analysis was performed using the Jamovi software package (version
throughout the examination. Thick- ness and CSA were measured 3 cm proximally to the insertion into the calcaneus, meas- ured using the ultrasound device [25,27] (Figure 2). Figure 2. Ultrasound images of AT (i.e., thickness (A) and CSA (B)). Thickness and CSA are high- lighted in pink. 2.3. Statistical Analysis Statistical analysis was performed using the Jamovi software package (version 2.3.26, The Jamovi Project). The normality of the variables was evaluated using the Shapiro–Wilk test. For variables that met the assumption of normality, a Student’s t-test for independent samples was applied to assess differences in AT and PT morphology between road and trail runners. When the assumption of normality was violated, the Mann–Whitney U test was applied, with effect sizes expressed as rank-biserial correlations [28]. Effect sizes were Figure 2.Ultrasound images of AT (i.e., thickness (A) and CSA (B)). Thickness and CSA are high- lighted in pink. 2.3. Statistical Analysis Statistical analysis was performed using the Jamovi software package (version 2.3.26, The Jamovi Project). The normality of the variables was evaluated using the Shapiro–Wilk test. For variables that met the assumption of normality, a Student’st-test for independent samples was applied to assess differences in AT and PT morphology between road and trail runners. When the assumption of normality was violated, the Mann–Whitney U test was applied, with effect sizes expressed as rank-biserial correlations [28]. Effect sizes were calculated and interpreted following Cohen’s criteria, where values of 0.2, 0.5, and ≥0.8 represented small, medium, and large effects, respectively [29]. Pearson’s correlation coefficients were calculated to evaluate the relationship between weekly running volume,
J. Funct. Morphol. Kinesiol.2025,10, 1 5 of 10 elevation gain, and tendon morphology. Partial correlation analyses were conducted to control for the potential confounding effects of elevation gain on weekly running volume and vice versa, providing a clearer understanding of the independent relationships between each variable and tendon morphology. Additionally, Spearman’s correlation coefficients were used to assess the associations between weekly volume, elevation gain, and tendon morphology in the absence of normality. The statistical significance was set atp< 0.05. 3. Results 3.1. Patellar and Achilles Tendon Differences Between Road and Trail Runners Both absolute and relative PT thickness and CSA showed no significant differences (p> 0.330) between road and trail runners (Table). Table 2.Tendon morphology comparisons between road and trail runners. Road Runners (n = 33) Trail Runners (n = 33) Difference (Mean [95% CI]) p-Value Patellar Tendon Thickness (mm) 3.35 ±0.49 3.47 ±0.48 0.12 (0.12–0.36) 0.330 CSA (mm 2 ) 82.29 (23.26) ˆ 84.55 ±17.51 4.91 § 0.516 Thickness normalized (mm kg −1 ) 0.07 (0.01) ˆ 0.08 ±0.01 0.01 § 0.203 CSA normalized (mm 2 kg −1 ) 1.87±0.33 1.91 ±0.40 0.05 (0.13–0.23) 0.596 Achilles Tendon Thickness (mm) 4.93 ±0.59 5.33 (0.54) ˆ 0.46 § <0.001 CSA (mm 2 ) 49.61±6.51 66.70 (18.3) ˆ 17.70 § <0.001 Thickness normalized (mm kg −1 ) 0.11±0.02 0.12 (0.03) ˆ 0.01 § 0.002 CSA normalized (mm 2 kg −1 ) 1.11±0.17 1.59 ±0.31 0.47 (0.35–0.60) <0.001 CSA: Cross-sectional area.p-values are reported via Student’st. ˆ Reported as median (IQR). Mann–Whitney U test as Levene’s test is significant (p< 0.05), suggesting a violation of the assumption of equal variances. § Difference in medians due to the non-normal distribution of one of the groups. Here,p-values are reported using the Mann–Whitney U test. For the AT, significant differences were observed in both absolute and relative thick- ness, and for CSA, significant differences were observed between road and trail runners (p< 0.001; Table). 3.2. Influence of Weekly Volume and Elevation Gain on Patellar and Achilles Tendons No significant correlation between weekly training volume and elevation gain on PT thickness and CSA, in absolute or relative values,
AT, significant differences were observed in both absolute and relative thick- ness, and for CSA, significant differences were observed between road and trail runners (p< 0.001; Table). 3.2. Influence of Weekly Volume and Elevation Gain on Patellar and Achilles Tendons No significant correlation between weekly training volume and elevation gain on PT thickness and CSA, in absolute or relative values, was found. Also, when controlled for elevation gain, weekly training volume did not correlate with any PT morphology variable. Similarly, when controlling for the heterogeneous weekly training volume between road and trail runners, no significant associations were found for the PT (Table). There was a significant correlation between weekly training volume and elevation gain on AT thickness and CSA in absolute and relative values (all, r > 0.327,p< 0.004). However, when controlling for elevation gain, a significant association between weekly running volume and both the absolute thickness (Spearman’sρ= 0.256;p= 0.040) and CSA of the AT (Spearman’sρ= 0.291;p= 0.019) was found. There was also a significant association between elevation gain and the CSA of the AT, both absolute (Spearman’s ρ= 0.332;p= 0.007) and relative (Spearman’sρ= 0.334;p= 0.007). In contrast, no significant associations were detected for the AT thickness (Table)
Description
Trail runners exhibit larger and thicker Achilles tendons due to increased weekly mileage and elevation gain.