THE EFFECT OF THE FEMALE MENSTRUAL CYCLE ON LOWER EXTREMITY NEUROMUSCULAR PERFORMANCE AND ANTERIOR KNEE LAXITY

Participants: E. M. Wojtys, L. J. Huston

Keywords: knee, muscle, menstrual cycle, female

Introduction

Females appear to suffer 4-8 times the ACL injury rate for the same sport as males. Numerous explanations and hypotheses have been put forth without convincing objective evidence. One striking difference between male and female athletes is the female hormonal cycle. During the course of this cycle there is a well-defined cascade of hormonal effects. Both the absolute levels of estrogen and progesterone and the ratio of these hormones to each other change dramatically during this time. In the early phase of the menstrual cycle (days 1-9), concentrations of both hormones are low. Ovulation is preceded by a midcycle surge of estrogen during days 10-14. During the last 15 days of the cycle, progesterone levels rise significantly. Interestingly, estrogen has been demonstrated to affect soft tissue strength, muscle function and the central nervous system. Consequently, such interactions may play a role in the increased female susceptibility for soft tissue injury and the etiology of the increased incidence rates of ACL injury.

The purpose of the present study was to determine the effects of the menstrual cycle on lower extremity muscle strength, endurance and reaction time, as well as A-P knee laxity. The null hypothesis was that no differences exist in these parameters during the course of a monthly cycle in a group of young, recreationally active females.

Materials and Methods

Twelve normally cyclic female volunteers (average age 27.7 + 2.9 years) were examined during the course of one complete menstrual cycle. Twelve age-matched males served as a control group. Individuals were tested four times during their cycle: Day 1, Day 12 (mid-cycle), Day 24 (premenstrual), and again on Day 1. All participants underwent isokinetic dynamometer strength, endurance, and time to peak torque tests at 60° and 240°/second, KT-1000 arthometer measurements for anterior tibial translation (ATT), and an ATT stress test designed to monitor lower extremity muscle function while tracking anterior tibial translation.

A specially designed device was manufactured to measure anterior tibial translation in reference to the femur in response to an anteriorly directed force. A 30 lb. step force was applied to the proximal third of the posterior aspect of the leg, while two linear potentiometers measured relative tibial displacement; one potentiometer placed on the patella with the second placed on the tibial tuberosity. Lower extremity muscle function was recorded using surface EMG's at five muscle locations: lateral quadriceps (LQ), medial quadriceps (MQ), lateral hamstrings (LH), medial hamstrings (MH), and the gastrocnemius (G). Lower extremity muscle response in terms of spinal and cortical reaction time could then be quantified.

All data were analyzed statistically by multi-way analysis of variance (ANOVA) with the Bonferonni post-hoc test for repeated measures. Statistical significance was defined at a level of p<0.05, but was adjusted down for multiple comparisons.

Results

No significant differences were found in lower extremity muscle strength, endurance, or time-to-peak torque between days during a complete cycle. Spinal and cortical reaction time also showed no significant variation. However, statistical differences in knee laxity were found during the course of one cycle: laxity was greatest on day one of the cycle with a notable decrease by mid-cycle and then rising by day one again (Table 1). This trend was consistent for the KT-1000 and potentiometer data (relaxed and contracted muscle states). Interestingly, this trend was not seen with the male control group.

Table 1: Mean (SD) Knee Laxity Through One Cycle

* significantly tighter than Day 1, 24, 1(repeated) of female group.

Discussion

Days 1, 12, and 24 were selected due to their significant differences in terms of estrogen and progesterone activity. During menstruation (day 1), the level of estrogen is low. At mid-cycle (days 10-14), the estrogen level is at its peak while the level of progesterone is at its lowest. During the late lute phase (days 22-26), the progesterone level is at its peak.

Conclusions

This investigation demonstrated little variation in muscle strength, endurance, and reaction time between days 1 and 12 of the menstrual cycle, when estrogen concentrations rise dramatically. In this study group, significant differences in knee muscle performance were not found that could explain the disproportionate rate of ACL injuries reported during the mid-cycle of the menstrual cycle.