The obvious answer seems like the weight gain that accompanies puberty, but I don't think that's the only cause. I've seen some girls (with early growth spurts) hit their peak in 7th grade. How is this possible, and is their any way to prevent it?
If middle school/9th grade girls run less, can we prevent this? Why don't they get faster after more/smarter training? How can a 60 lb 7th grader w/20 MPW training run faster than that same girl as a 100lb 70MPW college runner?