Long-Length Eccentric Hamstring Capacity as a Modifiable Target for ACL Injury Prevention in Adolescent Female Athletes: A Hypothesis-Driven Narrative Evidence Synthesis and Translational Framework

Anterior cruciate ligament (ACL) injury remains a high-burden event for adolescent female athletes, with substantial consequences for sport participation, knee health, and long-term quality of life. Exercise-based neuromuscular training (NMT) programs reduce ACL injury rates, but real-world effectiveness is limited by implementation and by the fact that prevention packages are necessarily generic. A persistent open question is whether specific, modifiable physical qualities can be targeted to enhance the mechanical robustness of the knee in the joint configurations and loading rates typical of noncontact injury.This narrative evidence synthesis integrates (i) biomechanical and in vitro evidence on ACL loading mechanisms, (ii) prospective evidence on strength balance and ACL injury risk, and (iii) evidence on hamstring adaptations to eccentric training, including training performed at longer muscle lengths. Based on converging lines of evidence, we propose the working hypothesis that insufficient hamstring eccentric capacity at long muscle lengths (joint angles near knee extension and with hip flexion) is a modifiable contributor to hazardous anterior tibial shear and multiplanar knee loading during rapid deceleration, landing, and cutting. We argue that this capacity is frequently under-assessed and under-trained in youth female athletes.We present a translational prevention framework that specifies (1) candidate field-based assessments, (2) exercise progressions emphasizing long-length hamstring eccentrics integrated with established NMT components, and (3) periodization and implementation considerations for adolescent female athletes. We conclude with a research agenda to test whether adding long-length hamstring eccentric loading to standard NMT improves surrogate biomechanical outcomes and reduces ACL injury incidence.