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Walk is an activity not an exercise , cause you have yo get to the 60% of your max hr to get fit . Maybe that is what apple watch do.

Advice if you start working out ...
Maybe this could help somebody with this new fitness tracker :D

You just could run or walk at an avg speed and try to speak at the same time. If you could talk normally, go faster, if you cant make a sentence go slower.

OR

To know at what hr you should run to get benefits you could :

1) Get your max HR with 220- age. Get 60% and 70% of that number, and get in that zone while you run

2) The more accurate Karvonen formula:
Target Heart Rate = ((max HR − resting HR) × %Intensity) + resting HR example

For example: I have 55 of resting hr, 31 years old
So:

Minimum Training Heart Rate(60% intensity):
220 - 31(Age) = 189
189- 55 (Rest. HR) = 134
134 x .60 (Min. Intensity) + 55 (Rest. HR) = 135 Beats/Minute

Maximum Training Heart Rate(70% intensity):
220 - 31(Age) = 189
189- 55 (Rest. HR) = 134
134 x .70 (Max. Intensity) + 55 (Rest. HR) = 149 Beats/Minute

Training heart rate zone will therefore be 135-149 beats per minute.
 
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