Polar s810 can measure your heartbeat intervals. The differences in these periods reflect your autonomic nervous system when it regulates your heartbeat. This measurement is most useful at rest or at low intensities, because we are talking about very small differences (msecs.) Your relaxation rate indicates the state of your physical recovery. If you are overtrained, it might very well be reflected on the Polar s810 monitor.
Polar s810 agrees with clinical ECG
In a clinical perspective you will normally use an electrocardiogram (ECG) to measure the differences in length between each heart beat. This is called the R-R interval, referring to the time between two R waves in the ECG. You don’t have to understand what these waves mean, but if you have a special interest in this topic, there is an explanation here: electrocardiogram.
Heart rate monitors can protect you from overtraining
Polar s810 provides a valid measurement of the R-R intervals and calculation of the heart rate variability compared to ECG. This variability can be used as a sign of health and recovery level. I think it is interesting that a heart rate monitor can provide so precise info because it gives you the possibility to notice when you need more rest before the next training schedule. Thus, it can protect you from overtraining. In theory there should be a correlation between the state of physical recovery and your heart rate variability, and it is also concluded to be true in a couple of scientific studies.
If we take a look back at the last decades there has not been much progress in heart rate monitors: They showed the heart rate and stored some data in files (and not much compared to other digital things these days). Introducing precise details like Polar s810 is interesting and hopefully Polar watches will get competition from other products, so there will come more useful info that will make our bike training more scientific.