INJURY PREVENTION

females. The greater applied moment accounts for sex differences in active knee flexor stiffness, while greater thigh segment mass in males accounts for sex differences in passive knee flexor stiffness and active knee flexor extensibility. Hence, differences in hamstring muscle properties are functions of anthropometric discrepancies, and are not necessarily due to inherent differences in muscle prop- erties between sexes. While these results do not implicate lack of inherent muscle stiffness as a contributing factor to the increased rate of ACL injury in females, male subjects were able to resist a greater load with a similar amount of muscle activity as evidenced by the similarity of knee flexor EMG in conjunction with a greater applied moment in males. This finding may indicate that the male knee flexor musculature is capable of protecting the ACL at a reduced metabolic cost compared to females.

Conclusions It is clear from the findings of Granata et al (9) and Blackburn et al (6) that differences in the viscoelastic properties of isolated male and female musculature are, primarily, a function of anthropomet- ric discrepancies. However, lower active stiffness measured during functional hopping tasks indicates that the female musculature may be less able to stabilise the knee joint and protect against ACL injury. Nevertheless, while these studies have elucidated several gender-specific differences in the resistance of skeletal muscle to stretch, they cannot explain the disproportionately high ACL injury rates at certain times of the female menstrual cycle.

DOES MENSTRUAL CYCLE EFFECT MUSCULO- TENDINOUS STIFFNESS? During the typical 28-day menstrual cycle, the ovaries release vary- ing amounts of oestrogen. Fluctuations in oestrogen are important for normal female physiology but, like any hormone, oestrogen can only facilitate chemical responses in cells which possess the appro- priate receptors. Interestingly, oestrogen receptors have been iden- tified on skeletal muscle. However, the effects of oestrogen on mus- cle viscoelastic properties have, until very recently, been unknown.

In the last month, a study conducted in our laboratory investigat- ing the effects of menstrual-cycle oestrogen fluctuations on leg stiffness was published (21). For this study, eleven physically active adolescent females with an average age of 16.3 ± 0.65 years were recruited. In order to derive the length and consistency of each female's menstrual cycle, each participant was required to keep a diary documenting data on their menstrual histories for 3 months prior to and 3 months following the test period. Using the men- strual-cycle data derived from the questionnaire, each subject was

tested at the four phases of their menstrual cycle: at the first day of menstruation, mid-follicular phase, ovulation day and mid-luteal phase. Venous blood samples collected at the beginning of each test session were used to determine whether hormone values were within the expected range for each of the four phases.

Using methods similar to that described by Granata et al (17), active stiffness was determined as subjects hopped on their domi- nant limb on a force plate at 2.2 Hz. The results demonstrated that active stiffness was significantly lower during the ovulatory phase when oestrogen was at its highest in contrast to the first day of menses and day seven of the follicular phase (8.7 and 4.5%, respec- tively). The decrease in active stiffness at ovulation was thought to increase the likelihood of ACL injury and in support of this notion; the epidemiological study of Wojtys et al (22) identified a greater percentage of ACL injuries during the ovulatory phase compared to the follicular and luteal phases. As described previously, a more compliant musculotendinous system is less able to counteract dele- terious joint forces and may be less effective at shielding the liga- ments from bearing the full responsibility of joint stability where acute strain is applied to the knee. In this respect, more compliant muscles demonstrate an increased reliance on the reflexive response from the contractile components of the muscle due to the decreased contribution from the series elastic and parallel elastic structures. Given that extreme loads are applied to the knee joint within milliseconds during sporting activity, the contractile com- ponents can simply not respond quickly enough to counteract these sudden and potentially damaging forces.

As oestrogen levels can be manipulated via the use of the monophasic oral contraceptive pill (MOCP), it was suggested that this study be replicated in a group of females who are currently using the MOCP. Such a study will delineate whether oestrogen sup- plementation has any role in the prevention of ACL injuries in females and is the current focus of our laboratory. We anticipate that the results of this study will be available in the latter part of this year.

LESSONS FROM THE LITERATURE While a number of neuromuscular factors have been implicated in the ACL gender-bias, several recent studies have attempted to delineate whether musculotendinous stiffness could play a role. Although the results of these studies demonstrated higher quadri- ceps and hamstring stiffness in males compared to females, con- sideration of the inherent size differences nullified most of the dif- ferences between sexes. Nevertheless, lower active stiffness mea-

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