Module No. 3554
Heart Rate Measurement Precautions
Heart Rate Measurement Functions
The back cover of the watch has a built-in sensor that detects your pulse. This is used to calculate and display an approximate heart rate value. The factors below can cause error in the displayed heart rate value.
How the watch is affixed to the wrist
Individual wrist characteristics and condition
Training type and/or intensity
Sweat, dirt, and/or other foreign matter around the sensor
All of this means that heart rate values displayed by the watch are approximate, and no guarantees are made concerning their accuracy.
The conditions below may make accurate pulse detection impossible.
Exercising in a low-temperature environment or under other conditions that reduce blood flow
Arm tattoos
Use of sunblock cream or lotion, insect repellent, etc.
The heart rate measurement function of this watch is intended for recreational purposes, and should not be used in any way for medical purposes.
Heart rate measurement is automatically disabled when the battery level indicator shows 
or lower.
Movements that Affect Heart Rate Measurement
Activities that involve vigorous movement of the arms only or other certain movements* may affect heart rate measurement and cause errors in the heart rate data displayed by the watch.
Activities involving vigorous wrist movements or wrist rolling such as playing a percussion instrument, tennis, etc.
Warming Up Before Training
Weak blood circulation due to low temperatures or cold hands may affect heart rate measurement.
When performing training that involves heart rate measurement, make sure to warm up appropriately.
Measurement while [ON] is selected for the heart rate measurement setting
While [ON] is selected for the heart rate measurement setting, your heart rate is measured intermittently (every two minutes).
If the watch determines that you are performing some activity (based on behavior analysis by the acceleration sensor), heart rate measurement will be performed continuously (every second).