Citation: Kolodziej, M.; Willwacher, S.; Nolte, K.; Schmidt, M.; Jaitner, T. Biomechanical Risk Factors of Injury-Related Single-Leg Movements in Male Elite Youth Soccer Players. Biomechanics 2022, 2, 281–300. https://doi.org/10.3390/ biomechanics2020022 Academic Editors: Kerstin Witte and Arnold Baca Received: 30 April 2022 Accepted: 23 May 2022 Published: 26 May 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Article Biomechanical Risk Factors of Injury-Related Single-Leg Movements in Male Elite Youth Soccer Players Mathias Kolodziej 1,2, *, Steffen Willwacher 3 , Kevin Nolte 2 , Marcus Schmidt 2 and Thomas Jaitner 2 1 Department of Strength and Conditioning and Performance, Borussia Dortmund, 44309 Dortmund, Germany 2 Institute for Sport and Sport Science, TU Dortmund University, 44227 Dortmund, Germany; kevin.nolte@tu-dortmund.de (K.N.); marcus2.schmidt@tu-dortmund.de (M.S.); thomas.jaitner@tu-dortmund.de (T.J.) 3 Department of Mechanical and Process Engineering, Offenburg University of Applied Sciences, 77652 Offenburg, Germany; steffen.willwacher@hs-offenburg.de * Correspondence: mathias.kolodziej@bvb.de; Tel.: +49-176-2069-8863 Abstract: Altered movement patterns during single-leg movements in soccer increase the risk of lower-extremity non-contact injuries. The identification of biomechanical parameters associated with lower-extremity injuries can enrich knowledge of injury risks and facilitate injury prevention. Fifty-six elite youth soccer players performed a single-leg drop landing task and an unanticipated side-step cutting task. Three-dimensional ankle, knee and hip kinematic and kinetic data were obtained, and non-contact lower-extremity injuries were documented throughout the season. Risk profiling was assessed using a multivariate approach utilising a decision tree model (classification and regression tree method). The decision tree model indicated peak knee frontal plane angle, peak vertical ground reaction force, ankle frontal plane moment and knee transverse plane angle at initial contact (in this hierarchical order) for the single-leg landing task as important biomechanical parameters to discriminate between injured and non-injured players. Hip sagittal plane angle at initial contact, peak ankle transverse plane angle and hip sagittal plane moment (in this hierarchical order) were indicated as risk factors for the unanticipated cutting task. Ankle, knee and hip kinematics, as well as ankle and hip kinetics, during single-leg high-risk movements can provide a good indication of injury risk in elite youth soccer players. Keywords: injury prevention; risk factor; biomechanical screening; youth soccer; decision tree 1. Introduction The majority of all injuries (70–88%) in youth soccer players occur in the lower extrem- ities, affecting the knee and ankle joints and the thigh and hip muscles [1,2]. Up to 72% of lower-extremity injuries in elite youth soccer players are reportedly non-contact injuries [3]. Altered neuromuscular control during high-risk movements in soccer is assumed to be the underlying mechanism of non-contact lower-extremity injuries [4–6]. In particular, sudden decelerations combined with a rapid change of direction while cutting or landing from a jump are frequent injury situations [7,8]. An injury ultimately occurs in these high-risk situ- ations when tissue stress reaches a tissue’s maximal capacity. Excessive loading can result in mechanical failure and induce acute injuries, such as ligament sprains or muscle-tendon strains [4,9–11]. The assessment of kinematics and kinetics during jump-landing tasks can help identify non-contact injury risk (primarily of the knee joint) in different populations [12–15] defined to separate players at high risk from the rest [16,17]. An altered biomechanical motion of the lower extremities while completing this movement allows the ground reaction forces (GRF) to affect the lower-extremity alignment and is therefore a modifiable injury risk factor [18,19]. More specifically, an increased knee valgus angle, high knee abduction moments, reduced hip and knee flexion angles, internal rotation of the femur on the Biomechanics 2022, 2, 281–300. https://doi.org/10.3390/biomechanics2020022 https://www.mdpi.com/journal/biomechanics