Rearfoot strikes more frequently apply combined knee valgus and tibial internal rotation moments than forefoot strikes in females during the early phase of cutting maneuvers

Background: Anterior cruciate ligament (ACL) injury often occurs during deceleration maneuvers in sports. Combined knee valgus and tibial internal rotation (VL + IR) moments have been recognized as a risk leading to ACL injury; however, it is unknown how the foot strike pattern (forefoot or rearfoot strike) affects the occurrence rate of the aforementioned combined knee moments during cutting maneuvers. Research question: To test the hypothesis that rearfoot strikes rather than forefoot strikes show a significantly higher occurrence rate of the combined VL + IR moments during the early stance phase of a cutting maneuver. Methods: Twenty-four females performed 60 degrees cutting maneuvers under rearfoot and forefoot strike conditions. Positional data of lower limb markers and ground reaction force (GRF) were collected. Knee varus/valgus and tibial internal/external rotation moments due to GRF were calculated and time-normalized (0-100 %) to the stance phase. The occurrence rates of combined VL + IR moments were compared between rearfoot and forefoot strike conditions throughout the stance (chi-squared test, p < 0.01). Furthermore, the time patterns of the two knee moments and the position of the GRF acting point were compared using the statistical parametric mapping paired t-test (p < 0.0125). Results: Rearfoot strikes more frequently produced combined VL + IR moments than forefoot strikes (maximum occurrence rates: 73.5 % vs. 27.8 %, p < 0.01) during the first 0-40 % of the stance. Both foot strikes consistently showed an increase in knee valgus moment soon after foot impact; however, rearfoot and forefoot strikes respectively applied opposite internal and external rotation moments during the first 0-7 % of stance (p < 0.0125), indicating that the GRF vector that generated knee valgus moment further applied tibial internal rotation moment when it acted posterior to the tibial rotation axis. Significance: The current results suggest that rearfoot strike in cuttings elevates the probability of ACL injury via combined VL + IR moments.