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Document Title: BarberWestin-AJSM-Mar06.shtml
Article Title: Jump-Land Characteristics and Muscle Strength Development in Young Athletes --
A Gender Comparison of 1140 Athletes 9 to 17 Years of Age
Authors: Sue D. Barber-Westin, BSc, Frank R. Noyes, MD and Marc Galloway, MD
Publication: American Journal of Sports Medicine, Baltimore, Maryland
Date: March 2006.
Volume 34, pages 375-384
Keywords: Female, male athlete, pediatric, child, muscle, alignment, limb symmetry, ACL-injury prevention, knee angling, jump technique.
(Reference-denoting numbers appear in the same font and point size as the document text. As with all Knee Library documents, this article is provided in full-text form, complete with all figures and tables.)
Comments: This penetratingly insightful and very highly thought-provoking study looked the effects of gender and chronological age on muscle strength and neuromuscular characteristics in a group of over a thousand athletes in the 9-17 age bracket. Barber-Westin et al. found that most of athletes in this age group land with knees angled inwards (which can be a predisposing factor to ACL injuries); however, this was seen not only in females, but in males as well. It was noted that maximum hamstring strength (as a function of body weight) in females occurs by age 11, whereas this same milestone is reached 3 years later in males. The similar intergender lower-limb-alignment upon landing (taking into account that females do have a wider pelvis on account of childbirth enablement) means that some other factors must underlie the 4- to 8-fold higher incidence of ACL injuries in female athletes. Possible factors noted by various researchers, and discussed in detail in other articles, include vertibular-system "firmware" issues, too-narrow intercondylar notches, a proclivity towards lax joints, and a monthly hormonal cycle with ligament-weakening estrogen spikes. (Note that female-athlete knee-injury-prevention training programs such as Cincinnati Sportsmetrics focus on developing hamstring strength, improving jump height, and landing jumps with knees well-flexed and with knees angled outwards. Such training has been proven highly effective in preventing ACL injuries in females, and so it is clear that the aforementioned aspects do play a significant role in some way.) The authors note that further study is needed, especially longitudinal studies which follow a given group of athletes for extended periods of time. Ideally, such a study would entail about 1000 athletes, starting at about age 9 and carrying through until about age 30 or so. It would be very intriguing to correlate the data collected in such a study to any subsequent knee injuries in the group. (Another excellent article dealing with female athletes, and focusing especially on the topic of hamstring strength in proportion to quadriceps strength, is Ahmad-AJSM-Mar06.shtml. Other superb articles on these topics are Noyes-AJSM-Feb05.shtml, Chappell-AJSM-Mar02.shtml, Mandelbaum-AJSM-Jul05.shtml, Chappell-AJSM-Feb07.shtml, and BarberWestin-AJSM-Dec05.shtml.)
ABSTRACT
Background: Many authors have speculated that altered neuromuscular control and strength of the lower extremity are responsible for the gender disparity in knee ligament injury rates.
Hypothesis: Significant increases in normalized quadriceps and hamstrings strength and limb symmetry on single-legged hop test occur with age. No gender differences in strength occur until age 14 years, after which boys generate greater peak torques than do girls. Age and gender do not influence lower limb alignment on a drop-jump test.
Study Design: Cross-sectional study; Level of evidence, 3.
Methods: We studied the effects of age and gender in 1140 athletes, 9 to 17 years old, on muscle strength and neuromuscular control during functional activities. Isokinetic quadriceps and hamstrings strength was measured at 300 deg/s. Limb symmetry was assessed with single-legged hop tests. A video drop-jump test determined lower limb alignment in the coronal plane.
Results: Extension peak torques significantly increased with age; maximum strength was noted in girls at age 13 years and in boys at age 14 years (P < .001). Although maximum flexion strength occurred in boys at age 14 years (P < .001), girls had only slight increases from ages 9 to 11 years (P = not significant). Boys aged 14 to 17 years had significantly greater normalized isokinetic strength than did age-matched girls. No age or gender effects existed in limb alignment on the drop-jump test or limb symmetry on single-legged hop testing.
Conclusion: Maximum hamstrings strength was noted in female athletes by age 11 years, compared with age 14 years in male athletes, and a distinct lower limb valgus alignment existed in the majority of all athletes on landing. The absence of a gender difference in lower limb alignment on landing suggests other factors may be responsible for the gender disparity in knee ligament injury rates.
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