Timing, not magnitude, of force may explain sex-dependent risk of ACL injury

Purpose The anterior cruciate ligament is loaded through valgus moment, vertical ground reaction force, and internal rotation moment. The aim of this study was to compare the timing of force peaks during early stance between youth girls and boys. Methods One-hundred and twenty-nine team sport athletes aged 9–12 completed a total of 2540 cutting maneuvers captured with an 8-camera motion capture system. Timing of early force peaks was analyzed within 100 ms after ground contact. Results Genders showed different mean (95% CI) time to peak valgus—(32 ms (30–33 ms) vs 37 ms (36–38 ms), P < 0.001) and time to peak internal rotation moments (36 ms (35–37 ms) vs 38 ms (37–39 ms), P = 0.029) but not time to peak vertical ground reaction force [38 ms (37–40 ms) vs 37 ms (36–38 ms, n.s.)]. Girls showed a smaller time between vertical ground reaction force and valgus moment peaks (mean (95% CI) of 1 ms (1–2 ms) vs 7 ms (5–9 ms), P < 0.001), and valgus- and internal rotation moment peaks (0 ms (− 2 to 1.0 ms) vs − 5 ms (− 6 to − 3 ms), P = 0.0003) but not between internal rotation moment and vertical ground reaction force. Conclusions Concurrent force peaks are more common for girls compared with boys, leading to more frequent multi-planar loading of the knee. Timing may explain sex-dependent risk of ACL injuries. Exposure to repeated cutting movements may result in greater ACL injury risk due to timing of knee forces as well as magnitude. Such exposure should be minimized for at-risk athletes. Level of evidence III.Funding for salaries (funding code 120410021) and equipment to conduct measurements [Qualisys motion capture cameras and software (funding code 903271305), and AMTI force plates (funding code 1203250031)] was provided by The Icelandic Centre for Research (Rannís—Rannis.is). The Football Association of Iceland has provided travel funds to the lead author to present findings at a conference. No funding source was involved in the study design, the execution of the study, the data analysis, writing the report, the decision to publish the results, or writing the article.Peer Reviewe