# Playing Football as a Risk Factor for Lower Leg
## 2 Materials and Methods  
_Design_--The presented survey is a comparative cross-sectional study. The subjects are composed of two groups, FP and REF. The design and methods of this study were previously approved by the local Ethics Committee (code: 19-1571-101) and the study was registered in the German Clinical Trials Register DRKS (Clinical trial registration number: DRKS00020446). Written consent concerning participation, measurement, data acquisition, and publication of the data was collected for each participant. In the case of minors, the participant was represented by their responsible legal guardians. A cross-sectional study design was chosen to analyze the anthropometric (e.g., sex, age) and sport-specific data (e.g., position on field, history of injury) by a questionnaire and to assess the lower leg align ment using an established intercondylar/intermalleolar distance (ICD/IMD) measurement technique [21; 22] as well as a sonographic-assisted digital photograph for determining the Hip-Knee-Ankle angle (HKA). The written consent forms, study guidelines, and questionnaires were distributed to the subjects and in the case of underaged subjects to their legal guardians and were collected before the leg axis measurement. The questionnaire included information regarding participant characteristics (age, height, weight) as well as playing position, dominant leg, training and competition history, level of performance, and injury history. _Study population_--Selection of participants was performed by quota sampling. The investigators were instructed to select the participants within the age range of 15 to 19 years as well as being equally distributed throughout every level of performance (amateur to professional level). Due to the study design, an a priori sample size calculation was not possible because there are no assumptions about the effect size and there was no primary end point with a comparison of two groups. The aim of the study was a initial estimation of the comparability of the two groups. Therefore, the authors aimed for n = 30 for each group, which provided good feasibility and still makes it possible for valid results. Since there are gender differences regarding the timing of the closure of the growth plates and leg axis alignment, only male subjects were selected to minimize confounding factors. The study population therefore consists of a series of male adolescent FP and male adolescent REF aged between 15 to 19 years, matched regarding age and BMI. The minimum age of 15 years was chosen because then at least an already-started growth plate closure of the lower extremities can be assumed and thus the length growth is almost completed [20]. The inclusion and exclusion criteria were defined as shown in Table 1.  
Only outfield players were included in the FP study group, as goalkeepers present a different load and movement profile and therefore might limit the interpretation of the results. Most of the surveyed REF played football during childhood and early adolescence, but they must not have exceeded training exposure of more than 180 min per week at any point during that time; none of them had played since the beginning of their refereeing career. During recruitment, equal attention was paid to ensure that the average BMI of the two groups was similar and within the normal range, since, for example, overweight per se is already associated with a valgus leg axis and could therefore distort the results [6]. The REF all served as both head referee and linesman. _Analysis of the leg axis_--The analysis of the leg axis was performed by measuring the ICD and IMD in a standing position. The athletes were asked to stand shoulder-width apart and then to move the legs toward each other with very small steps until either the malleoli or the condyles touched. This maneuver was rehearsed three times before each measurement to ensure a standardized and accurate procedure. The ICD and IMD