I'll give the quick summary to this story as I've covered the increase seen in repetitive strain injuries in other posts.
A study done (1) at the Children's Hospital Boston showed that girls aged 9 to 15 who accumulated 8 hours or more per week of high impact sports and sports training, like, running, jumping, basketball, volleyball etc, had a twice the risk of stress fracture compared to those doing 4 hours or less of high impact activity.
This may seem like another no-brainer, however if were that obvious there would not be the increase in repetitive strain injuries, like stress fracture, that are being seen in North America's youth who participate in sport.
The researchers point out concern for young athletes who are being given exercise loads that are beyond the bodies ability to adapt. It isn't that high impact is bad on it's own, it's that too much of it is bad.
Young athletes are often pushed into competing in and training for multiple sports simultaneously. The researchers say that it isn't unusual to see children training and competing for 20 hours per week.
(1) Arch Pediatr Adolesc Med -- Abstract: Prospective Study of Physical Activity and Risk of Developing a Stress Fracture Among Preadolescent and Adolescent Girls, April 4, 2011, Field et al. 0 (2011): archpediatrics.2011.34v1
Repetitive, high-impact sports linked to stress fractures in girls