The sex hormone estrogen may be involved in the anterior cruciate ligament (ACL) injury disparity that exists between women and men. The number of ACL injuries in women occurs at a rate between two and seven times greater than for men in the same activity. Increased anterior knee laxity, defined as the anterior displacement of the tibia relative to the femur, has been associated with increased risk for ACL injury. It has been shown that there is a relationship between anterior knee laxity and the menstrual cycle. However, the repeatability of this relationship between months has not been quantified, but is relevant to ACL injury prevention interventions.

The goal of this study was to quantify the absolute magnitude and fluctuations in anterior knee laxity, rate of force development (RFD), electromechanical delay (EMD), estradiol concentrations, and their relationships over a menstrual cycle and between cycles within subjects. The recruited subject population consisted of 20 females who ranged in age from 18 to 26 years old (22 ± 2 years, mean ± SD) and had a regular menstrual cycle. Absolute magnitude of anterior knee laxity, rate of force development, electromechanical delay, and estradiol concentration were quantified at menses and every day starting 4 days prior to their predicted day of ovulation until 3 days after ovulation for each subject for four months. The absolute values and percent differences in changes in anterior knee laxity, rate of force development, electromechanical delay, and changes in estradiol concentrations were compared between months within subjects. Linear regression analysis was used to determine relationships between anterior knee laxity and estradiol concentrations, rate of force development and estradiol concentrations, and electromechanical delay and estradiol concentrations for each month and compared for each subject. These correlations were compared using Fisher Z-transformation and an equality test of correlations.

Monthly average knee laxity values at menses ranged from 1.52-6.22 mm (3.35±1.29 mm) at 133 N and from 2.13-7.14 mm (4.07±1.38 mm) at 200 N. Monthly average knee laxity values at averaged ovulation ranged from 1.38-5.85 mm (3.53±1.18 mm) at 133 N and from 1.92-6.60 mm (4.28±1.27 mm) at 200 N. Monthly average change in knee laxity from menses to averaged ovulation ranged from -0.55-1.21 mm (0.13±0.51 mm) at 133 N and -0.76-1.35 mm (0.17±0.61 mm) at 200 N.

Monthly average salivary estradiol concentrations at menses ranged from 1.40-9.1 pg/mL (4.0±2.6 pg/mL). Monthly average salivary estradiol concentrations at averaged ovulation ranged from 2.0-10.6 pg/mL (5.2±2.6 pg/mL). Monthly average change in salivary estradiol concentration from menses to averaged ovulation ranged from -1.5-3.0 pg/mL (1.1±1.3 pg/mL).

Menses RFD values ranged from 209.9–2953.8 N/s (1155.3±699.6 N/s) for knee extension and from 242.4–4978.7 N/s (871.2±723.9 N/s) for knee flexion. Averaged ovulation RFD values ranged from 333.3–2497.2 N/s (1147.9±525.0 N/s) for knee extension and from 214.4–3028.5 N/s (836.2±523.6 N/s) for knee flexion. Change in RFD values within a menstrual cycle from menses to averaged ovulation ranged from -1140.2–643.2 N/s (32.6±357.8 N/s) for knee extension and from -3913.6–1521.1 N/s (-52.0±696.8 N/s) for knee flexion.

Menses EMD values ranged from 0.015–0.081 s (0.042±0.014 s) for knee extension and from 0.016–0.161 s (0.053±0.024 s) for knee flexion. Averaged ovulation EMD values ranged from 0.023–0.081 s (0.043±0.011 s) for knee extension and from 0.016–0.094 s (0.051±0.016 s) for knee flexion. Change in EMD values within a menstrual cycle from menses to averaged ovulation ranged from -.053–0.032 s (0.001±0.017 s) for knee extension and from -0.122–0.043 s (-0.002±0.029 s) for knee flexion.

The average coefficient of determination for the relationship between knee laxity and estradiol was 0.14±0.16 at 133 N and 0.13±0.17 at 200 N. The p-values ranged from 0.0086– 0.4909 (0.2208±0.1472) at 133 N and from 0.0006–0.4900 (0.2489±0.1375) at 200 N. Out of 37 conditions, there were 5 p-values less than 0.05 for knee laxity at 133 N and 4 p-values less than 0.05 for knee laxity at 200 N.

The average coefficient of determination for the relationship between RFD and estradiol was 0.15±0.19 in extension and 0.17±0.19 in flexion. The p-values ranged from 0.0003–0.4879 (0.2249±0.1546) in extension and from 0.0062–0.4963 (0.2617±0.1558) in flexion. Out of 37 conditions, there were 7 p-values less than 0.05 for RFD in extension and 3 p-values less than 0.05 for RFD in flexion.

The average coefficient of determination for the relationship between EMD and estradiol was 0.12±0.16 in extension and 0.16±0.17 in flexion. The p-values ranged from 0.0371-0.4987 (0.2932±0.1352) in extension and from 0.0096-0.4964 (0.2569±0.1394) in flexion. Out of 37 conditions, there was 1 p-value less than 0.05 for EMD in extension and 3 p-values less than 0.05 for EMD in flexion.

It was determined if the studied factors would be highly variable between months. The averaged ovulation estradiol concentration will not be highly variable between cycles but the estradiol concentration at menses and the change in estradiol concentration between menses and average ovulation can be highly variable. Knee laxity at menses, averaged ovulation, and the change in knee laxity between menses and averaged ovulation in a given cycle, can be highly variable between cycles. The RFD is highly variable between months about half of the time in both extension and flexion for all compared months for all subjects. Even though EMD is less than highly variable 70% for averaged ovulation in extension, EMD is highly variable between months more than half of the time for menses in both extension and flexion and for the change in EMD between menses and averaged ovulation for all compared months for all subjects. Estradiol concentrations do not correlate well with knee laxity, RFD, or EMD and there can be a significant difference between months when using linear relationship.

In conclusion, estradiol concentrations do not correlate well with specific menstrual stages, menses and ovulation, in 18-26 year old healthy women with a regular menstrual cycle. Knee laxity varies on average between 1 and 3 mm throughout a menstrual cycle in 18-26 year old healthy women with a regular menstrual cycle. Salivary estrogen or estradiol concentration changes alone are not sufficient to predict knee laxity or tendon stiffness changes.