Fitbit Zip Ionic User Manual page 12

For optimized heart-rate tracking while exercising:
During a workout, experiment with wearing the device higher on your wrist (2
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finger widths) for an improved fit. Many exercises such as bike riding or
weight lifting cause you to bend your wrist frequently, which could interfere
with the heart-rate signal if the watch is lower on your wrist.
Wear your Fitbit device on top of your wrist, and make sure the back of the
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device is in contact with your skin.
Consider tightening your wristband before a workout and loosening it when
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you're done. The wristband should be snug but not constricting (a tight
wristband restricts blood flow, potentially affecting the heart-rate signal).
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