GENDER BIAS IN ACL INJURY

musculature. In this respect, females exhibited 56-73% of the effective stiffness of male subjects and the gender difference was significant at each of the load conditions. At higher joint loads, the gender difference was amplified and the authors argued that this result has implications for diminished joint stability and muscu- loskeletal injury during functional loading tasks. In this respect, the stiffness-versus-moment gradient must exceed a minimum value to maintain stability (10), with increased gradients associated with greater joint stability (11).

The lower active rotational joint stiffness in females compared to males was attributed to several geometric and hormonal factors. Given that rotational stiffness is related to linear stiffness of the muscle by the square of the moment arm, a smaller knee and there- fore a shorter moment arm in women may lead to a lower rotation- al stiffness. Material differences in musculotendinous stiffness properties were also thought to be related to gender differences in hormone concentration. However, while reduced stiffness has been demonstrated during phases of high oestrogen versus progesterone concentrations in smooth muscle of the bovine uterine artery (12), this hypothesis could not be substantiated in skeletal muscle at the time of publication.

Study 2 While the results of Granata (9) provided some interesting insights into the gender differences in active stiffness of the quadriceps and hamstring musculature, the methodology incorporated controlled open kinetic chain measurements of the isolated knee. However, neuromotor control can voluntarily and reflexively modulate muscle stiffness (13,14) during functional performance tasks through mus- cle recruitment strategies (15) or postural adaptation (16). With this mind, Granata and colleagues undertook a second study to investigate whether female subjects demonstrated less leg stiffness in functional tasks (17). It was hypothesised that lower active mus- cle stiffness in female subjects observed from previous analyses (9) would translate to lower leg stiffness in hopping tasks at preferred frequency hopping tasks compared to the male subject group.

Eleven male and 11 female volunteers (aged 21 to 31 years) with no reported knee abnormalities or recent musculoskeletal injuries participated. Measurements of leg stiffness were determined by requiring subjects to perform two-legged hopping on a force plat- form. Subjects were asked to hop in place without shoes and with their hands on their hips at three separate hopping frequencies. Hopping was performed first at their preferred rate, then at 2.5 and finally at 3.0 Hz. Controlled frequency hopping was easily achieved by performing the tasks in time with a digital metronome. Hopping frequencies of 2.5 and 3.0 Hz were selected in addition to the pre- ferred hopping frequency as they have been previously determined to be higher than the average preferred hopping frequency for humans and thus reveal greater leg stiffness values (19).

Findings The results of this study revealed that stiffness values were signif- icantly greater in males (33.9 ± 8.7kN/m) than in females (26.3 ± 6.5kN/m), with significant differences at each of the hopping fre- quencies. Lower stiffness values in females were explained primari- ly by differences in body weight given that female subjects weighed an average of 83% of the male subject population. In a mass-spring model of harmonic motion, the stiffness must change in proportion

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to the system mass in order to maintain a constant frequency. Similarly, when the frequency of hopping increases, stiffness must also increase to oscillate the mass (17). Other factors that may have contributed to the gender difference in leg stiffness included muscle recruitment strategies and leg posture during hopping. With respect to the latter, functional leg stiffness is influenced by knee flexion angle (19) and kinematic data describing landing strategies suggest possible gender differences in knee and ankle flexion angles at contact (20).

Women and men demonstrated similar hopping frequencies when permitted to perform at self-selected rates. When frequency of hop- ping was not constrained, neither was the leg stiffness. The female subjects could have chosen to hop with similar leg stiffness as the male group in these preferred hopping conditions by performing the task at higher self-selected frequencies. Instead, the women choose to hop at a similar preferred frequency as the comparatively heav- ier men by recruiting less leg stiffness. Why did the women hop with identical preferred frequency and duty cycle as the men? One possible explanation is inherent active muscle stiffness differences between genders. Hence, in accordance with the results of the non- weight bearing stiffness measures reported in the original study (9), the results of this study illustrates lower active stiffness in females is also evident in functional tasks. Given that joint forces go well beyond the stabilising capacity of the joint capsule and ligaments during weight-bearing and sporting activities, the active muscles must provide assistance to stabilise the joint. Hence, lower active stiffness in females would effectively increase ACL strain during functional loading conditions and may explain, in part, the disproportionately high rates of ACL injuries in women.

Study 3 In the final study that has compared the viscoelastic properties of male and female musculature, Blackburn et al (6) revisited the iso- lated stiffness of the hamstrings. The active and passive stiffness together with the active extensibility of the knee flexors of 15 male and 15 females was determined to identify the contribution of the contractile and non-contractile components towards overall active muscle stiffness. Passive knee flexor stiffness was assessed in a seated position by moving the knee from 900

to 00 of flexion, and

calculating the slope of the moment-angle curve. Passive motion at 5 degrees per second was achieved using an isokinetic dynamome- ter. Active knee flexor stiffness was assessed using a method simi- lar to that described by Granata et al (9). However in this instance, a mass equaling 10% of the subject's total body mass was placed on the shank at the malleoli. Active knee flexor extensibility was assessed with the subject laying supine with the hip fixed in 900

of

flexion. From this position, subjects actively extended the knee as far as possible. The absolute angle defined by the longitudinal axes of the tibia and femur was measured at maximal knee extension using a manual goniometer, and was subtracted from 1800

.

Findings Males displayed greater active and passive knee flexor stiffness, and lesser extensibility compared to their female counterparts. However, no sex differences were present when active knee flexor extensibility and passive knee flexor stiffness were normalised to thigh segment mass, nor when active knee flexor stiffness was nor- malised to the applied moment. Thigh segment mass and the applied moment were both significantly greater in males than in

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