A resistance training program, while beneficial for all children and teens, is particularly important for young adolescent girls, says a 2014 international consensus position statement. (Loyd RS, et al. 2014)
Musculoskelatal growth during puberty, in the absence of corresponding neuromuscular adaptation, says the statement, may facilitate the development of abnormal joint mechanics such as increased knee valgus (e.g. knock knees) movement when landing, and injury risk factors. If not addressed, such risk factors may continue to develop throughout adolescence, the statement says, predisposing female athletes to increased risk of injuries. 
Early participation by female athletes in a multifaceted resistence training program, says the statement, can:
- result in safer movement mechanics and increased posterior chain strength (e,g, glutes, hamstrings, and lower back muscles considered the most influential muscle group for athletes, as they are prime movers of forward propulsion);
- reduce the incidence of ACL injuries;
- help those with low-motor competence catch up with their peers in terms of neuromuscular control;
- induce natural increases in muscle power, strength, and coordination (the so-called "neuromuscular spurt"), which occurs with increasing age in adolescent boys but is not typically seen in females, whose muscular strength, unlike boys, develops in a relatively linear fashion; and
- if timed with maturational growth and development (not just chronological age), induce the neuromuscular spurt, which may improve sports performance and improve biomechanics related to injury risk in young females.
Earlier the better?
The statement suggests that resistance training may be most effective for young adolescent female athletes, citing a recent literature review by a one of the statement's co-authors, Dr. Gregory Myer of Cincinnatti Children's Hospital, which found, based on a review of 11 studies, that the earlier female youth can engage in a well-rounded training program including resistance training the lower the likelihood of ACL injury.
Loyd RS, Faigenbaum AD, Stone MH, et al. Position statement on youth resistance training: the 2014 International Consensus. Br J Sports Med 2014;48:498-505. doi:10.1136/bjsports-2013-092952
Myer GD, Sugimoto D, Thomas S, Hewett T. The Influence of Age on the Effectiveness of Neuromuscular Training to Reduce Anterior Cruciate Ligament Injury in Female Athletes. Am. J. Sports Med. 2012;20(10). DOI:10.1177/0363546512460637 (published online ahead of print October 8, 2012)(accessed October 15, 2012).
