HRM spikes due to high humidity

Water, gel, and in particular sweat are conductors to improve your epidermis' naturally high impedance (because there is no blood circulation there). Artefacts in heart rate monitoring can have lots of different reasons, and there are countless papers online on that topic (ECG, that is, not consumer grade HRM, but the concept is still the same, mind you) if you're interested. I suggest the electrotechnical ones over medical. From what I gathered, and talking to my wife, who's a doctor working in that field (24-hour-ECG, and the like), displacement of electrodes is more often than not the reason for spikes in activity based HR readings. Seeing that our HR straps are mere belts around our chest they are subject to slipping through breathing, throacic rotation, and accumulated moisture in particular (think about the cavity your sternum creates, it's like a canyon for sweat ;) ). Even minor displacement throws ECGs off, which is usually not a problem for a doctor because they know what to look for, and can interpret artifacts. For us geeks otoh, who have to have an accurate reading throughout our activity erratic readings in our recorded activities are our biggest pet peeves.

Thanks for the response. I was wondering if there was anything I could try that would lessen the issue, like wrapping something around the middle of the HRM in the vicinity of the chest cavity, for example.

This is not an issue in drier weather, just during these hot humid months.

Not sure if the problem is 'wetness'. I have always thoroughly soaked mine in water before use, hence it is wet before I even start. When mine does start playing up, and I noticed a high surge at the beginning of my run this morning, I usually change the battery. As an aside, the initial surge dropped off, once I repositioned the strap - https://connect.garmin.com/modern/activity/2772442060. Usually though, I don't see any problems whatever time of the year.

One of the problems with running with chest straps (especially in humid conditions) is the gradual accumulation of salts from sweating in the fabric of the strap. This can make it easier for the HRM to detect stray electrical currents (eg. static) rather than the signals from your heart - although typically such spikes show up early in the run when the skin is still dry.

As a first step, I would recommend giving the strap a thorough hand wash in warm water with a mild detergent to dissolve some of this salt accumulation - sometimes a couple of washes may be required. However, after prolonged use in sweaty conditions, even washing may no longer be effective in removing salts - especially if you live in an area with a "hard" water supply. In which case, a new strap may be required - you can pick up cheap fabric replacement straps for older HRM's from ebay for just a few dollars (keep the HRM sensor part).

The care instructions on the strap state the strap should be rinsed after every use and washed after every seven uses - https://static.garmincdn.com/pumac/HRMSS_Care_ML12.pdf. If you do that, and I do mine in the shower after every use, then salts shouldn't build up.

I can't understand why/how anyone could wear these things repeatedly without rinsing them at the very least. But it is apparent that some people do.

The soft straps do need replacing over time as they wear anyway. I've stuck with the older style HRM-runs for that reason. I only need to replace the strap instead of the whole assembly.

I rinse my straps with fresh water and hang to air dry after every use. I have two straps and rotate them. Once a week I'll put them in a tupperware container filled with warm water and a drop of dish soap. I'll shake it lightly for 1 minute, let set for another 5 minutes and then rinse well. This works great and extends the life of the strap.

I was using the original Garmin heart rate monitor (HRM), with the hard plastic in the front. So, the strap is just a strap and doesn't have any electrodes on it, so its washing and care doesn't really come into play.

I didn't have the right kind of battery to try a new battery, but I did have another backup HRM of the same kind that had never been used. Instead of taking its battery and putting it into the old HRM I thought I would just try using that backup HRM instead. It worked perfectly. So, I think it must have been the battery after all. I'll get the right kind of battery for my original HRM and continue using it.

I never would have thought that a weakening battery would identify itself through heart rate spikes.

As I mentioned I also tried a second or third generation Garmin HRM (not sure which it is -- it has the electrodes on the strap) that had only been used a couple of times and it had the same issue. So, its battery should not have been low. But I'll change it also and see if it makes any difference.

Thanks much for the help!

Well, the second or third generation HRM didn't work. After changing the battery and washing the strap I only made it about 2.25 miles before it started showing random heart rates. I ended up cutting the run short since I am training by heart rate and no longer had a clue what it was. I may wait for a cold weather run to see if it still acts up then also. If so I'll just trash it.

Yet the first generation Garmin HRM that I used yesterday worked perfectly.

I bought a Garmin HRM-1 a while ago and am very happy with it. It was not very expensive. With a couple of spare elastic straps, I hope it will last forever. It replaces a HRM-Run pod (not the integrated type) that quit after years of use (battery and strap replacement didn't help).

If you don't need running dynamics data, or use a RD pod, it can be a really good alternative. No issues with forgetting to soak the strap or having to follow care instructions to the letter. It just works. Static discharge from rubbing clothes layers may still be an issue though. Haven't used it in colder weather yet.

Perhaps I spoke too soon. On this morning's 14 mile run I was getting spikes, but they were not nearly as bad as before I changed the battery. This is the original HRM from Garmin, perhaps what was referred to as HRM-1. Temp was 73F with 72F dew point, so I was soaking wet. But the largest spikes were at 1.7 miles, so I was not very wet at that point.