Associations between lower extremity muscle mass and multiplanar knee laxity and stiffness: A potential explanation for sex differences in frontal and transverse plane knee laxity

Background: Compared with men, women have disproportionally greater frontal (varus-valgus) and transverse (internal-external) plane laxity and lower stiffness, despite having similar sagittal (anterior-posterior) plane laxity and stiffness. While the underlying cause is unclear, the amount of lower extremity lean mass (LELM) may be a contributing factor. Hypothesis: Lower extremity lean mass would be a stronger predictor of frontal and transverse plane laxity and incremental stiffness than the sagittal plane. Associations between LELM and stiffness would be stronger at lower force increments. Study Design: Descriptive laboratory study. Methods: Sixty-three women and 30 men with no history of ligament injury were measured for knee laxity and incremental stiffness in the sagittal (90- to 130-N posterior-to-anterior directed loads), frontal (±10-N·m varus-valgus torques), and transverse (±5-N·m internal-external rotation torques) planes and underwent dual-energy X-ray absorptiometry scans to measure LELM. Linear regressions examined the extent to which LELM predicted each laxity and stiffness value, while also accounting for a persons sex. Results: Females (vs males) had greater laxity and less stiffness in the frontal and transverse planes but not the sagittal plane. Lower extremity lean mass was a poor predictor of sagittal laxity and stiffness (R2 range = .021-.081; P >06) but was a stronger predictor of frontal (R2 range = .215-.567; P>01) and transverse (R2 range = .224-.356; P>01) plane laxity and stiffness. Associations were stronger for low (R2 = .495-.504) versus high (R2 = .215-.435) frontal plane stiffness but were similar for low (R2 = .233-.293) versus high (R2 = .224-.356) transverse plane stiffness. Once we accounted for a persons LELM, sex had little effect on laxity and stiffness (change in R2 after removal = .01-.08; P = .027-.797). Conclusion: Less LELM was associated with greater laxity and less stiffness in frontal and transverse planes, which may contribute to the disproportionally higher laxities and reduced stiffnesses observed in females in these planes. Clinical Relevance: Frontal and transverse plane laxity and stiffness may be modifiable through strength training interventions that promote changes in muscle characteristics (eg, muscle cross-sectional area, stiffness) that may contribute to static knee joint stability, thus dynamic joint stability during sport activity. © 2012 The Author(s).