Sex differences in the physiological response to acute anterior cruciate ligament overuse.

Young female athletes are at least two times more likely to suffer a non-contact anterior cruciate ligament (ACL) injury than males, and one and a half times more likely to have a recurrent injury. Primary factors contributing to this disparity are less stiff and weaker ACLs, and greater knee laxity than males. Also, some evidence suggests females may exhibit a muted response to repetitive, high-intensity activity compared to males. Here, we test the hypothesis that female ACLs would accumulate more extracellular matrix (ECM) damage and show a delayed reparative response compared to males under equivalent submaximal fatigue loading. Using an adolescent mouse model (C57BL/6J), ACLs were cyclically loaded to induce an acute submaximal overuse injury (n = 20 per sex). ECM damage was assessed via immunofluorescence, apoptotic activity via immunohistochemistry, and gene expression changes through RNA-sequencing at 24 and 72 h post-injury. Female ACLs showed significantly greater collagen denaturation than males (P = 0.05), with no significant difference in apoptosis. Transcriptomic analyses suggest sex-specific healing strategies. Females followed a slower, more regulated reparative response, whereas males exhibited a more aggressive repair approach emphasizing mitosis, cell proliferation and migration. These findings may explain higher female ACL failure rates as greater matrix damage combined with a slower repair response could lead to injury propagation if reloading occurs prematurely. By contrast, the faster male response might reduce recurrence risk but increase fibrosis potential. If confirmed further, these potential physiological differences may require the implementation of sex-specific strategies for training and recovery regimens to prevent overuse injuries and optimize healing outcomes in young athletes. KEY POINTS: Females are at least twice as likely to suffer an anterior cruciate ligament injury (ACL) relative to males when participating in the same, or comparable sport. Sex differences in ACL structure and function are primary factors for increased female injury risk. Here, we show that the female mouse ACL accrues more collagen matrix damage than males under comparable fatigue loads. Additionally, female mice demonstrated a slower, more regulated ACL reparative response, while male mice exhibited a more aggressive repair approach emphasizing mitosis, cell proliferation and migration. These results, if translatable, may in part explain the sex-disparity in ACL failure rates with greater matrix damage and a slower repair response increasing the risk of reinjury, and a faster male response potentially reducing injury recurrence risk with further physical activity.