My daughter, also 15 and very athletic, has had the same problems with exercise. Her concussion occurred in early August 2009. She was 7 days with no symptoms at one point and was given the go-ahead to run one lap, see how she felt, up it to a lap and a half the next day, and so forth. She decided to run sprints with her basketball class instead and ended up with major backslide into symptoms.

We waited until her resting symptoms had subsided somewhat (like for the majority of a day) and then started with 2 minutes on eliptical. After five days or so she went to 3 minutes and so forth. After several months of this she is now up to 13 1/2 minutes. Our neuro stressed emotional and physical benefits of exercise and said to increase her exercise as she can tolerate it. She has strict orders to stop if she has the slightest sign of discomfort. A gentle yoga class once a week has been great for her as well.

13 minutes on an eliptical still seems like a horrible slap in the face to an athletic kid, but it's been good for her emotionally to see some progress. I don't know...lots of people on here talk about blood pressue problems with tbi. I'm not sure if we are encountering that or not.

best of luck as you work though this issue.