By Dr. Brian McDonough, Medical Editor

PHILADELPHIA (CBS) - How much exercise is too much exercise? In the past that was not a very important question to ask because the vast majority of people stopped exercising when they were fatigued, but things have changed a bit.

First, for older people who work out there are now a subset of people who train to excess for marathons and triathlons. I am not saying that this training or competition is bad, but sometimes people go too far and run the risk of serious injury – but at least adults have a choice.

I tend to worry more about kids. Many young people are being coached by overly aggressive parents and they are being pushed far too hard. Think about little league baseball or travel soccer. We have all heard stories of children with high pitch counts or playing three games a day.

Back before the days of parental involvement and perhaps over involvement, kids stopped playing when they were tired and the injuries were far reduced.