The watch may not measure your heart rate if it determines that you are not wearing the watch correctly on your wrist.
The watch uses the output of various sensors to determine whether or not it is affixed to your wrist correctly. It measures your heart rate when it determines it is affixed to your wrist.
If you do not want to measure your heart rate while the watch is on your wrist, select "OFF" for the "Detect wear on the wrist" setting. This enables heart rate measurement.
In addition, you need to properly fasten the watch to your wrist to measure your heart rate. For details, refer to "Fastening the Watch to Your Wrist" in the User's Guide.
