Thanks for your comments. She has seen a cardiologist and had an ECG. They could not find a problem. Everyone says that it is natural for the heart rate to rise quickly at onset of exercise, which is correct, but the heart rate should drop (or normalize) to a sustainable level appropriate to the level of exertion. i.e., at a light jog it should drop to about 100bpm - 120bpm, but it doesn't, it stays at 170bpm until she completely stops. She is a very fit 17 year old, but when competing she has nothing left, or no 'kick' for the final straight of a race, she is exhausted!. No one seams to know why her heart rate stays at 170bpm regardless of her exertion level.
cheers
Hello!
It is important to calibrate the devices you are using. You should go for a treadmill test near a cardiologist and check it manually before coming to a conclusion.
Try eating a fruit platter or taking a cereal before your workouts, keep yourself hydrated properly and also acclimatize and slowly increase the intensity of your workouts and see if your heart rate stabilizes.
Follow up with a cardiologist if your symptoms continue.
Take care!
I get a similar thing. My resting HR is 60bpm, after a light jog or fast walk it climbs to around 150 immediately.