Mio Sport SD Manuel D'instructions page 12

If your HR varies, note that fluctuations are normal in a healthy heart;
in fact, the healthier your heart is, the more variation you could see.
However, if MIO's calculations start to vary significantly from what you
have come to expect, replace MIO's battery (see Section 16).
To optimize readings, occasionally clean the back of your MIO case and the
Sensor Ring with a clean, soft cloth.
08. MAnuAl HeArt rAte loCk
To prevent the Smart Touch Heart Rate sensor from accidentally reading
your heart rate (e.g. the sensor comes into contact with clothing or skin),
you can use the Heart Rate Lock feature. This will also help to prolong
your MIO's battery life.
note: If you try to measure your heart rate when this lock is activated, the
'HR Lock' screen will display to remind you to unlock manually.
9 English
» To turn the HR Lock ON, in any mode (except
while changing settings), press and hold the
stArt button. The HR lock icon will display.
» To turn HR Lock OFF, press and hold stArt
button.