Fitbit Zip Ionic User Manual page 13

For optimized heart-rate tracking while exercising, keep these tips in mind:
Try wearing Ionic higher on your wrist during exercise for an improved fit and
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more accurate heart-rate reading. Many exercises, such as bike riding or
weight lifting, cause you to bend your wrist frequently, which can interfere
with the heart-rate signal if the watch is lower on your wrist.
Make sure the watch is in contact with your skin.
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Don't wear your watch too tight; a tight band restricts blood flow, potentially
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affecting the heart-rate signal. However, the watch should be slightly tighter
(snug but not constricting) during exercise than during all-day wear.
With high-intensity interval training or other activities where your wrist is
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moving vigorously and non-rhythmically, the movement may limit the
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