Submaximal low-strain cyclic loading induces localized inelastic deformation & diminished energy dissipation in the anterior cruciate ligament

Submaximal loading during routine activities is an understudied contributor to evolving mechanics preceding acute Anterior Cruciate Ligament (ACL) injury. This study characterizes the history-dependent mechanical response of the anteromedial (AM) bundle of the ACL subjected to repeated submaximal low-strain cyclic loading and intermittent recovery periods. This loading regime represents early-stage behavior often referred to as preconditioning, which is important for achieving steady-state mechanics but also for understanding the onset of irreversible changes. Digital image correlation (DIC) reveals the development of localized inelastic deformation in regions corresponding to clinically observed acute ACL tears. Complementary repeated cycle-recovery (RCR) experiments reveal that inelastic deformation and normalized hysteresis follow a dual-regime pattern, with pronounced early-cycle attenuation followed by a linear-log response. These findings indicate that submaximal loading induces irreversible mechanical changes on short time scales and establishes a mechanistic link between physiological relevant load histories and increased site-specific susceptibility of the ACL.