Heart rate monitors are temperamental things. In the past few months I’ve gone through two Garmin Forerunner 220’s, and in both cases the bundled soft strap HRM began to fail in the same peculiar way after about a month (roughly 24 uses). It’s driving me bonkers! Both straps worked great for the first month, but after that I began to see weird behavior on runs lasting more than an hour. Somewhere during the second hour, the pulse rate would start decaying steadily down, regardless of how hard I was exercising. Sometimes it would jump up again to a normal-looking value, but more often it would continue decaying downward from around 150 all the way to the 40’s, and stay there until the end of my workout. It rendered the HRM nearly useless for long runs.

An hour-long total decay, with eventual recovery:

Partial decay at the end of a marathon, with some earlier gaps:

Total and permanent decay at the end of a workout:

This pattern of failure seems different from the types of HRM trouble I’m more familiar with, which involves bogus (usually too high rather than too low) pulse readings at the beginning of a workout. That problem is normally caused by poor electrical conductivity between the strap and the skin, and can be solved by wiping on some spit or electrode gel, or just waiting until the skin gets sweaty. DC Rainmaker has a good explanation of this and other common HRM problems. But I don’t think that’s the problem here, and I always use electrode gel anyway.

What on earth might cause a problem like this, that only strikes after an hour or more into a long run, and only after a month of use for the heart rate monitor? When the first HRM started malfunctioning a couple of months ago, I tried cleaning the strap, tightening the strap, replacing the battery, and updating the firmware. Nothing worked, and I eventually exchanged the watch and HRM for a replacement. Now just over a month later, the replacement has started to do the same thing. Ack!

If I had to guess, I’d say it’s somehow related to the accumulation of salt from sweat, which builds up over time and shorts out the conductive thread woven into the strap. That would be consistent with a problem that doesn’t appear for a month (it takes a while for enough salt to accumulate), nor until running more than an hour (it takes a while for the strap to become saturated with sweat). The Garmin soft strap does say right on it that it should be machine washed after every seven uses, which I admit I’ve never done. But washing it once this problem appears don’t seem to fix it.

Some Amazon reviewers reached a similar conclusion, though their symptoms sound different from mine. Some found partial success in washing or soaking the strap, and others by cleaning the snap contacts on the transmitter with a deoxidizing cleaner. If I can’t find a reliable answer soon, I’ll probably have to throw the Garmin HRM in the trash and look for a compatible HRM from Polar or another vendor that doesn’t suffer from this strange decay issue. Sad.

Update July 13:

I’ve concluded the Garmin soft strap HR monitor is flawed by design, and I can’t recommend it to anyone. After lots more experimentation, I confirmed the problem is at least partly due to sweat and salt build-up in the strap fabric. Unfortunately, washing the strap doesn’t really fix the problem. In my tests, hand washing with soap in a sink of warm water didn’t help at all, and machine washing with other laundry (per Garmin’s instructions) only partially helped. After washing, the problem returned again within a matter of days, if not sooner.

The final straw was a 6-hour endurance race I competed in yesterday. The day before the event, I machine washed the strap and air dried it while lying flat, so it would be ready to go for the race. Despite the washing, the heart rate data started flaking out after the first hour. It recovered twice, but eventually it went completely nuts and gave me totally wrong HR data for the last 2.5 hours of the race. I was not happy.

Maybe the sweat and salt become so embedded in the strap’s fabric that even a machine washing can’t completely remove them? Or maybe the salt permanently damages the conductive threads in the fabric? Perhaps it’s some kind of mechanical failure unrelated to sweat and salt.

One thing’s for sure: after going through two of these soft straps, both of which worked great for the first month after purchase, I’m confident the problem has nothing to do with how I’m wearing the strap, or moisture on my skin, or the battery. It’s some kind of cumulative failure that’s inherent in the soft strap HRM design. Washing may provide temporary relief, but eventually the HRM will become a useless piece of junk. Stick with the tried-and-true hard strap design – the Garmin soft strap just doesn’t work.