Im not sure what the exact medical opinion would be, but I think I have experienced some ill effects in my 14yo DD from a strenuous season.
At the end of last season (beginning of August) my dd began to complain of pain. It began after we had completed our travel season where she would normally pitch around 4 complete games on the weekend, sometimes more, plus pitch during the week in league play. During the time the pain first came up, she had pitched on 3 consecutive nights. In the final game for the league championship, she was unable to pitch due to the pain and chance of further injury. It was obvious to me the kid was just plain burnt out and tired from the last grueling 4 months.
We took 4 complete weeks off and then renewed training in early September. We then decided to also embark on a weight training program to build strength and flexibility on all the muscle groups. She has no more pain since the layoff and all is well. Weve since decided that we would attempt to rest more during this upcoming season and continue with a strength training program.
Now...Im not making the claim that repeated pitching in a pretty active schedule means you will get hurt. Rather...I think that a girl can get tired enough to the point where its possible mechanics begin to suffer and you increase the likelyhood of an injury. I do know this...during the middle of the season I thought she was throwing harder than at the end. So....as I indicated, we will back off a little this year and try to get a least some more rest during the week between weekend tourneys.