Hamstring Stiffness and Knee Laxity Across the Menstrual Cycle in Females with ACL Reconstruction

Abstract

Previous research in healthy, uninjured females has been equivocal as to whether knee flexor stiffness (FLEX) changes across the menstrual cycle (MC). However, once an athlete sustains an ACL injury, they are 6 times more likely to suffer a second ACL rupture to either the contra or ipsliateral limb. Given this information, females with a history of ACL injury are at greater risk of rupture compared to the general population, which creates a strong rationale for utilizing them in research. If reproductive hormones influence ACL injury risk by altering the properties of tissue, then females that have suffered a non-contact ACL injury may be more sensitive to hormonal fluctuations than females from the general, uninjured population. PURPOSE: To determine if FLEX and knee laxity (LAX) change across the menstrual cycle (MC) in ACL reconstructed females. METHODS: 20 females with noncontact, unilateral ACL rupture completed the study. Each subject was tested twice across the MC at periods associated with low (3-5 days post menses) and high (within 3 days post ovulation) estrogen concentrations. MC phase testing order was randomly assigned and the PI was blind to MC phase at the time of testing. The uninjured limb was used for testing. FLEX was assessed by quantifying the damping effect imposed by the knee flexors on oscillatory knee flexion/extension induced by perturbation. LAX was assessed via the KT 1000 knee arthrometer in the anterior direction at 133N of force. Blood hormone levels (estrogen [E], progesterone [P], and free testosterone [FT]) were assessed with radioimmunoassay procedures. Separate paired t-tests were performed with MC phase as the within subject factor (α <.05). RESULTS: [E] and [P] were higher at ovulation than post-menses ([E]: 31.1±13.7 vs 70.3±54.7 pg/mL, p=.009, [P]: 0.51±0.3 vs 3.92±4.27 ng/mL, p=.003). LAX (6.6±1.4 vs 7.3±1.3 mm, p=.03), and FLEX (178.6±35.7 vs 199.0±37.9 Nm/rad·kg-1, p=.03) were also increased at ovulation. [FT] was equal at both time periods (0.8±0.3 vs 0.9±0.2 ng/mL, p=.41). CONCLUSIONS: Increased knee flexor stiffness at ovulation may represent an effort to increase knee joint stability in compensation for increased anterior knee laxity. These factors change across the MC in females with a history of ACL injury and may increase the risk of a second injury. Supported by the NASM.