Detecting sickness

I had a fever on Wednesday, the stress meter did go up abnormally that day and tracked my temp. That said many things will cause the stress meter to be high. Doing a new exercise, work issues, having too much to drink, ... it's interesting in retrospect but it's not a clear indication by itself that you are ill. My HR also tracked my temperature pretty closely as well. Below you can see a small snip that does shot the effect on a very short timeframe. I went to bed after my fever receded and it came back while I was sleeping, I took a fever reducer and was fine in the morning. You can see that perfectly reflected in the graph.

In my opinion it can be a lot of coincidence, analyzing technically the watch makes a reading of the pulsations, movements and to be generous quasicorporal temperature.
I do not know exactly what parameters the watch takes to specify stress, in my case it indicates a lot of stress when I am almost without performing movements but with high pulsations such as performing an exam, being nervous, etc.

The "stress" indicator on my VA3 is essentially a random number generator. I wouldn't rely it to tell me anything significant about my health.

The stress score is based on heart rate variability (HRV). I don't know if any other factors are considered.

The idea is that if you are healthy and relaxed, the time between each heartbeat will vary a lot. But if you become stressed or sick, the time between heartbeats becomes more constant. Some athletes use HRV to plan their training. They will not do a hard training pass on a day with low HRV.

I use a phone app called HRV4Training which uses the phone camera to measure the heartbeats, and here I can see how the BPM will increase to above 60 when I inhale and decrease to below 50 when I exhale.

I have only measured my HRV for a little more than a month. During that time I have been hit by both some mild overtraining and a virus. Both have shown up on the graph, and now while I am recovering from both, I see my HRV increasing on the graph.

So HRV is usable to some extent, and I look forward to getting some long term experience. I only use it for fun so far. I am not basing my training on the measurements.

The problem with the "stress" score is that we humans are very good at finding correlations and patterns, even when there aren't any. It's easy to look retrospectively at, say, a horoscope, and find places where the predictions appear to match reality. If I rolled dice every day, and plotted the numbers on a graph, I could probably find something in my life to match those numbers with.

The thing that really killed my interest in the Garmin "stress" measure was that it is completely, totally different between my VS3 and my VA3, which suggests that the HRV data is not being collected correctly, or interpreted correctly, by at least one of these devices. If not both.

I only trust information if I have some way to verify that it is at least approximately correct. From a physiological point of view, it's far from clear what HRV really measures even when it is measured with quantifiable accuracy using lab equipment; and if the data isn't collected or processed correctly anyway, then it's of even less value.

There is another thread about this, suggesting that an update changed the stress measurements on the 935 an maybe vivoactive 3 as well: https://forums.garmin.com/forum/into...e-stress-level
LarsTheBear this might be an explanation for the differences between VS3 and VA3.
AllanOlesen67 I am not sure about this either. I know that it's supposed to be about HRV, but often i have the feeling, that i am getting stress scores, if my heart rate is above my resting HR and rest if it's close to my resting HR. Sure there are more factors that come to play.

I too take the measurements with a grain of salt. It's possible the different readings your seeing between the VS3 and VA3 could be the sensors (placement or design) and improved technology with the newer VA3. I don't have a VS3. Are the sensors the same as the VA3? I've noticed a big difference between the sensor readings on the FR 235 & 935.

Here's a clip from the support page on what the HRV is suppose to read.

Heart-Rate Variability (HRV) or R-R is recorded by all devices that use Firstbeat technology. HRV is only used for the calculation of Physiological Measurements such as the VO2 Max Estimator, Stress Score, Performance Condition, and Lactate Threshold. HRV is not available for analysis by the end consumer using Garmin Products[SUP].[/SUP]

AllanOlesen67 I am not sure about this either. I know that it's supposed to be about HRV, but often i have the feeling, that i am getting stress scores, if my heart rate is above my resting HR and rest if it's close to my resting HR. Sure there are more factors that come to play.

I think they are somewhat correlated. On mornings where my resting heart rate is higher, my heart rate variability is usually lower.

So resting heart rate may be a just as useful indicator of stress, ilness or overtraining. And also easier accessible.

I would expect HRV and heart rate whilst inactive to be very strongly related. In healthy people, both are controlled by parasympathetic nervous tone, and its effect on the vagus nerve. In a healthy person, if you block the vagus nerve (don' t try this at home, kids) the heart rate when inactive will be about 100 bpm and constant. Vagal nervous activity causes both the reduction in rate and the increase in variability.

I have attached a picture showing my HR and stress from Connect, one above the other. This is from my VS3. The horizontal red line I drew by eye, based on nothing in particular. However, if you look at how far above the red line my HR is, it's almost a perfect match for the stress chart. The higher my HR is above the red line, the higher the stress. You can even see the individual blips in the stress chart during the day, matching up with slightly elevated HR when I'm walking about my house. I was pretty active in the evening (shooting, as I recall), and that shows up both in the HR and stress.

I suppose the benefit of using HRV rather than HR alone is that you don't (maybe) have to guess where the "red line" goes.

For all that, HR and HRV are influenced by all manner of factors, none of which it will be possible for a watch to discriminate. I think it is necessary to be very careful about interpreting this data, whether it is collected properly or not. By putting the HR and stress charts together, it's easy to see that mild exercise like walking about is interpreted as "stress".

FWIW the VA3 and VS3 HR sensors look the same -- they are both three little green lights. However, the watch bodies are very different. I suspect the smaller body of the VS3 will more easily lie with the sensors all touching the skin, than the much larger VA3. But this is just guessing.