A total of 168 adolescent athletes were recruited in the context of a preparticipation examination before entrance to a federal elite school of sports (Mayer et al., 2012). In addition, 28 recreationally active control subjects were recruited from a local secondary school. Exclusion criteria were the presence of Achilles or patellar tendinopathy, rupture or surgery, rheumatic disease, dyslipidemia as well as acute injury of the knee or ankle. Due to the clinical presence of AT or PT tendinopathy at inclusion (first measurement day: M1) in 11 athletes and 3 controls, 14 subjects had to be excluded. In consequence, 157 healthy adolescent athletes out of 14 different sports and 25 healthy control subjects were included at M1 (Figure 1). Follow-up-examination (M2) took place after 3.2 ± 0.8 years for athletes and after 3.0 ± 0.1 years for controls. At M2 26 athletes and one control subject had to be excluded due to a newly developed diagnosis of tendinopathy in the AT or PT region (Figure 1). Parents of all adolescent athletes signed written informed consent before data acquisition. The study was approved by the Ethics Committee of the local University.

Athlete's examination was conducted during preparticipation and annual health examinations at the university outpatient clinic, Olympic center, responsible for athletes' health in the federal state. For recreational subjects investigations took place at a secondary school by use of the same portable ultrasound equipment. Each examination composed of history, local tendon examination and a brief status of the adjacent joints. In addition to anthropometric data, age and sex, specific training-related data (sport-type, training duration and frequency) were documented in a standardized case report form.

Ultrasound scans were conducted to measure tendon diameter and to detect intratendinous abnormalities (intratendinous blood flow, hypo- and hyperechoic regions). Investigations of both ATs and PTs were performed using high-resolution ultrasonography (Viamo SSA-640A; Toshiba, Tokyo, Japan) with a multi-frequency linear transducer (PLT-704SY) and standardized transducer settings (11 MHz, gain = 93, DR = 50, penetration depth = 3 cm, focus at 0.5 cm). Ultrasound investigations were performed by three trained examiners. Examinations of ATs were investigated in a prone position with feet hanging over the examination table and ankle passively dorsi-flexed to a neutral position. PT examinations were carried out in a supine position with 30° knee flexion, controlled by use of a soft pad. As previously described, both ATs and PTs were longitudinally examined to determine tendon diameters at a reference point 2 cm proximal to the calcaneal insertion (AT 2 cm) and 2 cm distal the Patella (PT 2 cm), respectively (Cassel et al., 2012, 2016). Tendon thickness was reported to be reliably measureable at these standardized locations (Schmidt et al., 2004; Fredberg et al., 2008). Moreover, the diameter at the thickest part of the AT “mid-portion” was measured (AT max) (Cassel et al., 2012, 2016). Longitudinal scans were performed by a strict orthogonal placement of the transducer on ATs and PTs in order to measure “true tendon thickness”, excluding epitenon and paratenon.

For Doppler ultrasound (DU) detection of intratendinous blood flow (IBF) subjects were requested to relax their muscles leading to a slight plantar-flexion ankle angle, while the knee angle stayed at 30° flexion. IBF was measured using power Doppler ultrasonography (Advanced dynamic flow mode) with standardized DU settings (frequency = 4.5 MHz, pulse repetition frequency = 0.5 kHz, color velocity = 4.39 cm/s, color intensity just below the artifact threshold, size of the color box (region of interest, ROI = 3 cm²) (Cassel et al., 2015; Risch et al., 2016). IBF was graded according to a modified “Ohberg score” from 0 to 5 (0 = no vessels, 1 = 1–2 vessels, 2 = 3–5 vessels, 3 = vessels in up to 30%, 4 = vessels in 30–50% and 5 = vessels in >50% within the ROI) (Ohberg and Alfredson, 2002; Hirschmüller et al., 2010). Three examiners with experience in DU examinations performed the investigations.

Adolescent elite athletes were allocated into 4 categories according to their sports (ball sports (B, n = 40), combat sports (C, n = 39), endurance sports (E, n = 20) and explosive strength sports (S, n = 32); Table 2) (Müller et al., 2017). Data was analyzed descriptively by mean and standard deviation of anthropometrics, training and tendon parameters within all athletes (A), sports categories (B, C, E, S) and controls (Co). All tendon data is shown as means of both sides for each thickness parameter. Depending on data distribution (Kolmogorov-Smirnov-Test), differences were tested either by unpaired t-test, one-way and two-way ANOVA (post-hoc Tukey-Kramer-Test/Fishers least significant differences test) or Kruskal-Wallis-ANOVA (post-hoc Wilcoxon test) for non-normally distributed data. To identify potential influencing factors (age, sex, BMI, training amount and years, intratendinous abnormalities) on tendon thickness development a multiple logistic regression analysis was performed (Jmp 9.0 and SPSS Statistics 20). Results with a p < 0.05 were considered significant.

Athletes had higher AT thickness than controls at M1 and M2 (p < 0.001) as well as PT thickness at M2 (p ≤ 0.04; Table 3). Sports categories showed highest AT thickness of B (AT 2 cm at M1, AT max at M1/2) and highest PT thickness of E at M1 (p < 0.04), while there were no statistically significant differences of PT-thickness at M2 (p > 0.05). Male subjects had higher AT and PT thickness compared to females at both measurement days among athletes and controls (p < 0.05; Table 4). Intratendinous abnormalities were present at both measurement days. In M1 IBF (grade 1 and 2) was visible in 5% of PTs, hypoechogenicities in 1.5% of ATs and 2% of PTs. In M2 IBF (grade 1 and 2) was visible in 3% of ATs and 4% of PTs, hypoechogenicities in 2.5% of ATs and 8% of PTs, hyperechogenicities in 1% of ATs as well as PTs.

For ATs, thickness did not differ statistically significant between M1 and M2, neither for athletes or controls, nor between different sport categories (p > 0.05; Table 3). For PTs, athletes presented higher thickness at M2 compared to M1 (p < 0.001). In contrast, controls did not show statistically significant differences between measurement days (p > 0.05; Table 3). Differences (M2-1) of AT- and PT-thicknesses between sports categories and controls ranged from –0.3 mm to 0.4 mm (Figure 2). Male athletes had higher PT thickness at M2 (p < 0.001). Sex-specific analysis of sport categories identified male subjects from B, C and S having higher PT-thickness at M2 (p ≤ 0.01; Figure 3). Regression analysis showed an influence of sex on all tendon thickness parameters at both measurement days in athletes and controls, with males having higher thicknesses than females (p < 0.05). Increased PT thickness was associated with higher BMI in athletes. In contrast, factors age, training amount (h/week and years) and presence of intratendinous abnormalities did not lead to a higher AT or PT thickness (p > 0.05).

Existence of intratendinous abnormalities (i.e., hypoechogenicities) usually goes along with enlarged tendon diameter (Docking et al., 2015) and has been reported to predict patellar tendinopathy in athletes (Gisslén et al., 2007; Comin et al., 2013; Visnes et al., 2015). Malliaras and Cook stated that mild patellar tendon thickening (>4.2 mm among men, >4.0 mm among women) may already indicate pathology among active athletes (Malliaras and Cook, 2011). The present study is the first differentiating between tendons with and without structural irregularities when looking at tendon thickness development. However, regression analysis did not show a statistically significant influence of intratendinous abnormalities on tendon thickness, neither in athletes nor in controls. Furthermore, within subgroups of sport categories mean PT thickness values were reported to be below the thickening threshold, postulated by Malliaras and Cook (2011).

In a recently published longitudinal study Visnes et al. followed 141 initially asymptomatic young elite volleyball players (in mean 17 years of age at inclusion) for an average of 1.7 years (Visnes et al., 2015). A total of 22 players developed tendinopathy. The remaining 119 athletes did not show a statistically significant change in PT diameter (differences 0.0–0.1 mm) at follow-up. However, mid-tendon PT-thickness was already on a high level (males: 4.3 mm, females: 3.7 mm). Furthermore, 50% of asymptomatic volleyball players already had intratendinous abnormalities at baseline (Visnes et al., 2015). These findings support data of the present study showing that PT thickness is already determined in earlier stages of adolescence. Moreover, changes in thickness or physiological adaptation by thickness are small and structural changes have to be considered when interpreting physiological adaptations.

Present longitudinal data show higher PT-thickness in follow-up exclusively in males from ball, combat and explosive strength sports suggesting a sex-specific adaptation of PTs due to loading. Sex-influence on absolute thickness values was also detected for AT tendon thickness parameters at both measurement days, irrespective of activity level in athletes as well as in controls. This is in line with data from Visnes et al. who found PT and quadriceps tendon thickness to be statistically significantly higher in 16–20 year old male compared to female volleyball players (Visnes et al., 2015). This fact might be explained by hormonal levels or genetic prerequisites. Increased estrogen levels were seen to be responsible for inhibition of the acute exercise-related collagen synthesis (Magnusson et al., 2007; Kjaer et al., 2009; Hansen and Kjaer, 2016). However, in the present study, lower AT and PT thickness of females were already detected at baseline (mean age of 12 years), where hormonal influences are assumed to be less relevant. Furthermore, differences were visible irrespective of activity level in athletes as well as controls.

When interpreting the results, some limitations have to be considered. Sample size of the longitudinal study was relatively low, especially for the differentiation of several sports disciplines. Furthermore, measurement location of thickness parameters is assumed to be of high relevance when interpreting physiological tendon adaptation by thickness, especially during maturation. According to prior sonographic reliability studies thickness parameters were used at presented sites instead of width or CSA-measurements (Fredberg et al., 2008; Cassel et al., 2012). For ATs, a second measurement location (thickest part in midportion) was chosen to consider possible length changes due to maturation. For PTs, it was decided to take only one standardized parameter in the “mid-tendon” since tendinopathic tissue abnormalities in adolescent athletes usually are visible in the proximal or distal portion of the tendon (Cassel et al., 2015). However, since some studies reported on an increased thickness or CSA in proximal and/or distal PT region following training (Kongsgaard et al., 2007; Couppé et al., 2008; Kulig et al., 2013) future investigations should also consider the insertional regions. Additionally, in order to take length changes due to maturation into account, future investigations might also include measurement of thickest mid-tendon part. Overall, three trained examiners were responsible for the ultrasound measurements, which might have influenced the results. However, investigations on intra- and inter-observer reliability of AT and PT thickness measurements as well as IBF assessment using the ‘modified Ohberg Score’ showed good to high reliability (Fredberg et al., 2008; Risch et al., 2016).

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.