Cyclic variations in multiplanar knee laxity influence landing biomechanics.

PURPOSE Females vary substantially in their multiplanar cyclic knee laxity changes across the menstrual cycle. The biomechanical implications of these cyclic changes are relatively unknown. Our purpose was to first cluster females based on their cyclic changes in anterior knee laxity (AKL), genu recurvatum (GR), varus-valgus (VV), and internal-external (IER) rotation knee laxity across the menstrual cycle. We then compared changes in landing biomechanics from days of minimum to maximum laxity between female clusters and a group of males. METHODS A total of 49 males and 71 females were measured for AKL, GR, VV, and IER and underwent biomechanical analysis of a double-leg drop jump (0.45 m) at two time points: day of minimum (T₁) and maximum (T₂) AKL in each female across her menstrual cycle (males matched in time). Cluster analysis identified four distinct patterns of multiplanar cyclic knee laxity changes from T₁ to T₂ (C₁-C₄). Males were classified as a separate group. RESULTS When landing from a jump, female clusters who increased both sagittal and frontal plane laxity from T₁ to T₂ (C₃, C₄) had 3.7° to 5.2° greater net movement toward knee valgus from T₁ to T₂ compared with females who did not increase both sagittal and frontal plane laxity (C₁) (P = 0.041). Females who increased IER without increasing AKL from T₁ to T₂ (C₂) had -3.7° to -8.1° greater net movement toward knee internal rotation when compared with females who increased both AKL and IER (C₃, C₄) or males who maintained similar laxity from T1 to T2. CONCLUSIONS Changes in knee joint kinematics across the menstrual cycle were dependent on both the absolute and the relative magnitude of multiplanar knee laxity changes. The combination of relatively greater knee valgus coupled with relatively greater external rotation in those with large multiplanar knee laxity changes (C₄) suggests an increased susceptibility to high-risk knee joint positions on ground contact and early in the landing phase.