Hi there and thanks for posting this question
First of all sometimes machine heart readings are innacurate, sometimes they unexpectedle jump for no reason. I remember i would be concerned many times so I bought a Polar hear rate monitor and you can see how heart rate readings differ sometimes. However this doesnt mean that she may have a spike in irregular heart beating so if you feel that her heart rate rises for no reason you should go to a doctor to check her heart and if she is safe for exercising. Normally they conduct a stress test and monitor how the heart rate behaves.
Regarding to your second part of your question here is a description of what we believe is a good way of finding out your daughte's target heart rate. This information wa sin my book Super Body, Super Brain so try to substitute the formula with your daughter's age
Steps to (Karvonen Formula) My Target Heart Rate
*Subtract your age from 220. 220 – 34 = 186
*Check your resting heart rate. 46 (yes, my heart rate is that low!)
*Subtract that from the figure in Step 1. 186 - 46 = 140
*Multiply that number x 65 percent for 140 x .65 = 91 for the low end
your low range and x 85 percent for your 140 x .85 = 119 for the high end
*Add the resulting figures to your resting 91 + 46 = 137
heart rate. That will give you the low end 119 + 46 = 165
and the high end of your Target Heart Rate. My target heart rate = 137-165.
Hope this helps!
Thank you so muh for your answer Michael and for taking the time
to teach me a good system to set my mind at rest. That was so helpful..