TOWARDS A BETTER UNDERSTANDING OF THE ROLE OF FATIGUE ON ACL INJURY RISK

The anterior cruciate ligament (ACL) originates on the posterior-superior portion of the medial facet of the lateral femoral condyle and inserts anteriorly and medially to the tibia's intercondylar eminence, preventing the femur from sliding from the tibia anteriorly. Non- contact ACL lesions are one of the most devastating injuries in sports, and they often occur during abrupt changes of direction, like turns or cutting maneuvers and landing movements. ACL injuries during sports movements usually result from the interaction of several extrinsic and intrinsic risk factors, which may change dynamically due to physical fatigue. The overall scope of this dissertation was to better elucidate the effect of physical fatigue on the biomechanical risk to sustain an ACL injury during sports movements. Since fatigue has been defined in several ways, the presented protocols intended to replicate, still considering the laboratory environment limitations, the real sport-like scenario miming different harmful athletic movements (e.g., change of direction, landing from a jump). Furthermore, the procedures aimed to stimulate both the central and peripheral mechanisms responsible for the fatigue onset in an athletic population composed of team-sports athletes from the amatorial to the elite level. The first study evaluated the effect of a 5-minutes continuous shuttle run protocol involving changes of direction on the kinematics of the lower limbs in recreational athletes, showing kinematic changes previously suggested to increase the risk of ACL injury in cutting tasks, such as the substantial reduction of hip and knee flexion, as well as the increase of hip adduction and hip internal rotation. The second project aimed to replicate and ameliorate the first one with the involvement of elite female soccer players, who performed the shuttle run till exhaustion to evaluate both the kinematics and kinetics of the lower limbs. The repetition of turning actions induced alterations in lower limb kinematics in all the tested players and generated changes in lower limb kinetics in 85% of the players. The nature and extent to which lower limbs mechanics were altered throughout the test were highly variable among individuals. Biomechanical modifications mostly occurred in the sagittal plane, but adaptations in the coronal and transverse planes were also detected. A subset of participants showed a drift of pivoting limb kinematics that tightly matches with the known ACL injury mechanism, while other players displayed less definite, minimal, or even opposed behaviors. Furthermore, a systematic review was carried out to explain the associations between cognitive function and ACL injury-related biomechanics and introduce the context for the third project. Five studies out of six found some form of a cognitive-motor relationship, with worse cognitive performance associated with an increased injury risk from the dual-task scenario. At the same time, the remaining study did not identify interactions between cognitive function, secondary tasks, and landing mechanics. The last original project was performed during the visiting period at the Neuromuscular Biomechanics Laboratory of the Montana State University and aimed to assess the combined effect of cognitive performance and physical fatigue during an unanticipated landing task, in order to define the interaction between these potential risk factors. The peak knee flexion angle decreased throughout fatigue progression, demonstrating a potentially harmful movement pattern when the athletes were fatigued, especially at the end of the protocol. In addition, this study highlighted some interesting fatigue effects during unanticipated landing, especially relationships with the participants' baseline cognitive performance.