My daughter is 12 years old, she plays soccer competitively, and she runs four miles every day. She has been known to stress her body by exercising too much. Lately, she has been wanting to run further to improve her endurance. I have been hesitant to let her, because I am afraid she will stress her body. Is there a way to know if she is working too hard? Is there some sort of formula to see how far she should be able to run? Please advise.
A Scandinavian researcher, Bengt Saltin, found in a 1995 study that active Kenyan school children covered on average 8–12K [~5-8 miles] a day. That's a weekly total of 35–50 miles.
William Roberts, M.D., medical advisor for the Twin Cities Marathon, has conducted several studies on young marathoners, a self-selected group of young high-mileage runners, and has come to the same conclusion. "Kids and mileage basically makes us nervous," Roberts says, "but from all I've found, there is no harm being done; they are not getting hurt. If it is their choice, keep doing it."
A study on overuse injuries and burnout in child athletes published in the June 2007 issue of Pediatrics points out, "Endurance athletic events (triathlons, marathons, and half marathons) are becoming more popular in the United States, and legitimate concerns have been raised for the safety of youth participating in these events." The report concludes, "There is, at present, no scientific evidence that supports or refutes the safety of children who participate in marathons. There are no recorded data on injuries sustained by children who run marathons. … Ultimately, there is no reason to disallow participation of a young athlete in a properly run marathon as long as the athlete enjoys the activity and is asymptomatic."
Warming up, sleep, stretching (do it after, not before!), hydration, and proper shoes/surface are more important and would be the main factors in her getting hurt or physically "stressed". At her age though, she shouldn't need to train for endurance. Better to practice actual skills like juggling a ball, passing/receiving with a wall or partner, or skills to beat an opponent.