Discussion
The relative increased risk of anterior cruciate injury in women compared with men in twisting and cutting sports has become well recognised and well documented in the literature.1 ,2 While numerous factors including anatomic alignment, hormonal influences and anatomic structure (small notch, smaller ligament volume) have been considered and in some cases implicated as causative factors, targeting neuromotor retraining of core and balance deficiencies has been shown to be an effective means of injury prevention in these at-risk athletes.3–5 ,14 Specific studies of adolescent and young adult athletes have revealed similar sex-related variation of core and balance control when comparing trained and untrained athletes.10 While a large majority of existing studies have identified the gender variations contributing to ACL risk, few have targeted skeletally immature cohorts. Stracciolini et al18 performed a cross-sectional review of children presenting to a dedicated children's sports medicine centre over 10 years. They concluded, in contrast to what is seen in late adolescence and early adulthood, boys aged 5–12 years had a higher ACL to total injury rate than girls (p<0.01). However, over time, the proportion of ACL to total injuries increased at a higher rate for girls, and by the age of 16–17 years, the ratio exceeded that seen in boys. In 2011, the National Athletic Trainer's Association did an extensive review of the literature in coming up with their position statement on overuse injuries in paediatric sports and found the highest level of evidence to support the conclusion that balance, coordination, strength and flexibility deficits are directly correlated to overuse injuries in paediatric athletes.14 Admittedly, the type and severity of injuries may be slightly different in the lighter, shorter, paediatric population due to lower energy mechanisms; nonetheless, targeting the core deficits would seem to be a reasonable target.
Little research is available to suggest when balance and core deficits present in the adolescent population and how it progresses over time, and at what specific age do girls begin to lag behind boys.6 ,16 ,7 ,19–21 Barber-Westin et al22 analysed children aged 9–10 years and noted a large majority of both sexes landed with increased valgus position from a drop-jump test and suggested that neuromotor retraining might be indicated. The study did not conclude a specific age or time frame by which such a programme should be initiated and whether that time was different between sexes.
In this study, we elected to use single-leg balance test on a stable and unstable surface as well as a modified drop jump as our measure of lower limb neuromotor control. These measures have been previously described and validated in the literature as valid and reproducible in adult populations.7 ,10–13 ,19 They have also been commonly used as a screening tool for lower extremity balance and control in direct relationship to ACL injury risk. Barber-Westin et al22 concluded that regarding single-leg squats, ‘sexed-based strength differences are not expected to become apparent until the mid-teens and children may not master complex motor skills until ten to twelve years of age’. More specifically, they found ‘most kids aged nine to twelve had abnormal drop-jump tests where they fall into the knock-knee position. More girls than boys fell into a knock-kneed alignment during the drop-jump’. Croce et al6 concluded that knee muscular response strategies differ by developmental level but they did not identify a sex-related difference during jump landing. Hewett et al7 documented deficits in drop-jump testing for female athletes at the middle school and high school levels. In our study, the female athletes did not perform worse than their male counterparts. When compared with collegiate athletes, our findings did confirm the relatively poorer performance of preadolescents compared with mature athletes; nonetheless, most of the preadolescent participants were able to complete the task. We were also able to identify poorer performance of the tasks in the youngest subgroups of our cohort, but were able to demonstrate improvement with maturity.
More specifically, we were able to identify a gradual improvement in single-leg balance and drop-jump performance for girls and boys with the girls making a slightly earlier progress between age groups 6–7 and 8–9 years compared with the boys who appeared to make their significant progress between age groups 8–9 and 10–11 years. In contrast to Barber-Westin et al,22 we were not able to demonstrate a deficit for females in the age groups compared. More specifically, we were not able to identify deficits which would place female participants <14 years of age at increased risk of ACL injury due to neuromotor deficits. Indeed, by visual inspection of the graphics, there appeared to be a slight predilection to improved performance by the girls compared with the boys, but this did not achieve statistical significance.
The concept that preadolescent females would have improved neuromotor performance and coordination is supported in the developmental literature. According to Largo et al,17 ,23 the impression that girls are better coordinated than boys is mainly based on two facts. First, ‘girls carry out movements in complex and adaptive tasks somewhat more rapidly than boys. Also, girls show fewer associated movements during all motor activities, and therefore movements appear more harmonious’. Girls perform complex, sequential movements on a pegboard more rapidly than boys. Girls can also stand on one leg longer, and show fewer associated movements than boys.17 ,23 These findings are likely consistent with the findings in this study which reveal a slight predilection of improved performance in age-matched preadolescent girls compared with boys. Ultimately, a key finding for the purpose of this paper is not so much that the girls may perform slightly better than boys which is interesting, but rather that the deficits seen later in the more mature adolescents and young adult females are not yet present in this younger age group.
This study, as is the case for most research, has some potential weaknesses that should be acknowledged and taken into account by the reader. First, while the neuromotor tests performed have been validated and used in other studies, they have not necessarily been validated in this younger age group. For the case of this study, it was felt that they were the best tests available for the project. Second, in an effort to assess the effect of childhood development, we chose a convenient educational grouping which represented a general, but not exact, age grouping and a general, but not exact, educational maturity grouping. It did not represent an exact assessment of physical maturity; nonetheless, it does paint a picture of the effect of development and maturity over time. Finally, as in the case of many comparative studies, it would be nice to have greater numbers in each group to enhance the power of subsequent conclusions.