INJURY PREVENTION

THE EFFECT OF GENDER AND THE MENSTRUAL CYCLE ON LOWER LIMB MUSCULOTENDINOUS STIFFNESS:

IMPLICATIONS FOR ANTERIOR CRUCIATE LIGAMENT INJURY

By Dr Adam Bryant, BHMSc (Hon), PhD and Assoc Prof Erik Hohmann, FRCS, MD

THE IMPORTANCE OF MUSCULOTENDINOUS STIFFNESS Muscle stiffness is defined as the ratio of change in force in a mus- cle to its change in length (5).

In contrast to their male counterparts, female athletes are at increased risk of incurring a serious muscu- loskeletal injury. In particular the risk of anterior cruci- ate ligament (ACL) injury is estimated to be 2-8 times higher in the female population (1). These alarming statistics have gained the attention of sports medicine researchers the world over and, in an effort to improve the gender equity, a large number of scientific investi- gations have been conducted to delineate the physio- logical mechanisms contributing to the higher ACL injury rates in females. From the outcomes of the published papers to date, it is clear that there is no single causative factor and numerous anatomical and neuromuscular variables have been implicated in the gender bias. For example, females have been found to have a more compliant ACL and a decreased femoral notch width; two anatomical factors that may increase the likelihood of ACL rupture when the femur medially rotates on the tibia during pivoting movements (2,3). Of the neuromuscular factors, decreased synchrony between peak hamstring activation and tibiofemoral shear forces during single-limb landing in females lessens the synchrony between the ACL and the hamstring musculature and increases the risk of ACL strain and/or rupture (4). While a number of additional neuromuscular factors have been suggested, several relatively recent studies have implicated musculotendi- nous stiffness as a piece of the female ACL injury puzzle. This article looks at the evidence.

musculotendinous stiffness of lower limb musculature with the aim of explaining the higher knee injury rates in females.

A stiff muscle will undergo less

stretch in response to an applied force compared to a less stiff mus- cle (6). Given that the musculature surrounding a joint plays a sig- nificant role in determination of joint stiffness (7), stiff muscles may increase the ability to resist an acute perturbation force.

In

contrast, a more compliant (less stiff) muscle would allow for increased muscle lengthening in response to an applied load, which may result in a greater amount of joint translation, increasing the likelihood of joint and ligamentous injury (6).

The attachments of the hamstrings on the posterior fibula and tibia provide resistance to anterior tibial translation and also act to con- trol tibial internal and external rotation (5). As the ACL is stressed with anterior tibial translation coupled with either tibial internal or external rotation (8), the hamstrings and their associated stiffness are likely play an important role in maintaining knee joint stability and the integrity of the ACL (6).

GENDER AND MUSCULOTENDINOUS STIFFNESS Three published studies have investigated the effect of gender on

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Study 1 The first of these studies was performed by Granata et al (9) who investigated the effect of gender on the active stiffness of the quadriceps and hamstring musculature. Twelve male and eleven female volunteers aged 21 to 33 years with no known knee abnor- malities or recent musculoskeletal injuries participated in this study. In assessing both quadriceps and hamstring stiffness, the hip was supported at a flexion angle of 450

and a fixed ankle-foot ortho-

sis was used to secure the ankle in a neutral posture. An accelerom- eter was attached to the heel of the orthosis and weights rigidly attached to the brace using a deLorme boot. Subjects supported weights of 0 kg, 6 kg and 20% maximal voluntary effort thereby generating knee flexion or extension moments. A small perturbation was induced when the investigator applied a brief downward push on the ankle orthosis.

Findings The results of this study demonstrated that gender significantly influenced the active stiffness of the quadriceps and hamstrings

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