- optical hrm work by Photoplethysmography (ppg)
- this is the volumetric measurement of an organ.
- LED in sensor illuminates the skin; and the amount of light transmitted or reflected is measured; which reflects change in blood volume
- change in volume --> change in pulse pressure --> heart beat
- SO! when you run in cold conditions where there is a temperature gradient in between your watch and the ambient running environment
- this causes liquid vapor from your body heat to condensate onto the lens of your optical watch sensor
- as a result, the LED transmission and reflection is altered
- arteries are located deeper in the body than veins, which can be close to the surface of the skin.
- Photoplethysmogram optically generated from your sensor is going to be off; and instead of detecting blood volume changes in your artery; it is detecting volume changes in your veins.
- veins are a low pressure system relative to arteries, an the hemodynamics are more easily influenced by arm motion
- hence, optical sensor now detects vein hemodynamic changes from arm pumping that occurs with every step, causing CADENCE LOCK
- that's why your HRM locks onto cadence-- it isn't a software bug in your garmin but a hardware limitation.
some questions my friends asked me:
1. How does the PPG, when it's working correctly, know to look at the arteries instead of the veins?
PPG actually uses an averaging algorithm between your veins and arteries. The veins are much less responsive to changes in blood volume; and as noted earlier are a lower pressure system so influenced more by up down arm movements so you get the cadence lock. However, when the reflection and transmission of the light to sensor is altered by condensation, the veins are picked up more than the arteries and are therefore represented as a higher percentage of the average-- this is why when you get cadence lock the HR doesnt coincide directly with the cadence; you still get some averaging in from detected arteries
2. Do you think the condensation is forming on the inside or outside of the watch? If it's inside, isn't the watch supposed to be waterproof? If it's outside, wouldn't it be removed from your skin running on it?
The condensation is forming on the outside of the watch. Your watch is much warmer (assume body temp since it is worn close) than the ambient environment; so it is radiating heat. The atmospheric moisture condenses at a rate greater than that at which it can evaporate, resulting in the formation of water droplets. You can see a similar effect on leaves in the early morning forming dew.
This is also why cadence lock is more commonly experienced on humid days/days with high dew point. If you chill your watch before your runs you can stave off HR cadence lock at least until your watch warms up to body temperature. Do this by leaving your watch overnight near a window sill, or sticking it in a fridge for 1/2 hour.
