I recently compared two optical heart-rate monitors, the inexpensive Coospo HW706 armband and the Amazfit Active 2 smartwatch, against a Polar H10 chest strap. The H10 is useful here as a reference. Chest straps measure the heart’s electrical signal rather than trying to infer heart rate optically through the skin, and in normal conditions the Polar is about as close as we can get to a benchmark outside a laboratory.
The ride was also quite a useful test: a mixture of riding through town, short harder efforts, a long steady climb, forest cover and a long descent.
The Coospo is a relatively inexpensive optical armband, typically available for around €35 to 40 from Amazon or AliExpress. The attraction is obvious. An optical armband is generally more comfortable than a chest strap, and the Coospo can broadcast heart rate to a bike computer.
During the early part of the ride the Coospo followed the Polar H10 closely. On the long climb it was, for significant periods, almost exactly on top of the Polar trace. Unfortunately, that didn't last. Around an hour into the ride, the Coospo suddenly started reporting substantially incorrect values. It later recovered, then lost the heart rate again.
This isn't the first time I've noticed this behaviour. It is actually one of the reasons I eventually bought a Polar Verity Sense.
That makes the Coospo difficult to recommend for serious training use. A monitor which is accurate for 45 minutes and then suddenly isn't is arguably worse than one which is consistently slightly inaccurate, because there is no obvious way to know when the numbers can be trusted.
Interestingly, it handled a few short harder efforts reasonably well. The route included accelerations over a road bridge and railway bridge and the Coospo followed these quite closely, although it tended to miss the very top of the peaks. For shorter sessions or fairly steady training it may therefore be perfectly usable. I just wouldn't want to rely on it for analysing a long ride.
This may be an issue with this particular model or with device or maybe it just doesn't like me? Without other tests I wouldn't write off Coospo as a brand.
What surprised me in this test was just how well the Amazfit Active 2 Watch tracked the Polar H10. I've tended to regard wrist optical heart-rate monitoring with some suspicion, particularly for cycling, where wrist position, vibration and movement can all make optical measurements more difficult. The Amazfit initially confirmed some of those prejudices. For roughly the first ten minutes it took a while to settle down and establish a convincing heart-rate trace. I've seen this behaviour before with the watch.
Once it had settled, however, it was remarkably good. On the long climb the Amazfit followed the Polar H10 extremely closely. Looking at the traces afterwards, there were surprisingly long sections where you would struggle to distinguish one from the other.
Both optical devices appear to lag the Polar slightly — perhaps around eight seconds — which isn't particularly surprising. Optical heart-rate algorithms usually involve some filtering and smoothing. For steady efforts that delay isn't especially important.
The Amazfit was less convincing during very short hard efforts. It can miss the peak of a sprint, so I wouldn't use it to analyse ten- or twenty-second intervals. For efforts lasting a minute or two, though, I think it is quite usable.
The descent was also interesting. The Amazfit became somewhat less stable, although at points the Polar trace itself also looked surprisingly erratic. One possibility is that after a long descent the chest strap had dried slightly or lost ideal skin contact. Towards the end of the ride the Amazfit again missed some smaller changes in heart rate, but overall I came away much more impressed than I expected.
For general endurance training, climbing and longer intervals, the heart-rate measurement from the Active 2 appears genuinely useful.
The Amazfit's weaker point is GPS The Heart rate was the pleasant surprise. GPS was less impressive. I recorded the ride simultaneously using the Amazfit and a dedicated bike computer. Under open sky the two tracks were quite similar. Once the route entered dense woodland, however, the difference became obvious. The bike computer continued to follow the road reasonably accurately while the Amazfit started wandering away from the actual route. More importantly, once the watch lost a good satellite fix it sometimes took a while to recover, even after emerging from the trees.
I've noticed the same problem in built-up areas. This matters if you're using GPS distance or speed for interval training. Poor positioning beneath trees or between tall buildings can produce inaccurate distances, and therefore misleading pace or speed calculations, especially when running. Not great for the Parkrun with all the tree cover.
On this particular ride the Amazfit recorded roughly 2 km less distance than the other devices. So if I'm using the Active 2 for running intervals, I would prefer to do them somewhere with a clear view of the sky rather than in dense woodland or between tall buildings. For cycling I would still use a dedicated bike computer for GPS recording.
Can the Amazfit replace a heart-rate strap? That depends what you're doing. For maximum accuracy, particularly for short intervals or detailed heart-rate analysis, I would still use the Polar H10. A chest strap reacts almost immediately to changes in heart rate and doesn't have the same problems with movement and optical signal quality. But for normal endurance training the Amazfit Active 2 performed far better than I expected.
The watch can also rebroadcast its heart-rate measurement to another device. That means you can wear the Amazfit on your wrist and transmit heart rate to a bike computer. Given how closely it tracked the Polar during most of this ride it looks like quite a useful option.
The comparison changed my opinion slightly about wrist-based optical heart-rate monitoring. The Coospo HW706 can be accurate when it is working properly, but on longer sessions I've now seen it lose the correct heart rate often enough that I don't completely trust it. If you want a dedicated optical monitor to use with a bike computer, I would be inclined to spend more on something such as the Polar Verity Sense or an equivalent device from one of the established manufacturers.
The Amazfit Active 2 was the real surprise. After a slightly hesitant start, its heart-rate trace followed the Polar H10 extremely closely for much of the ride. It isn't perfect, very short efforts remain its weak spot but for endurance riding, climbing and longer intervals I think the heart-rate data is perfectly usable.
Its GPS is considerably less convincing under trees or among buildings, so I wouldn't regard it as a replacement for a good bike computer. But as a relatively inexpensive smartwatch which can also provide and rebroadcast reasonably accurate heart-rate data, the Active 2 did much better than I expected.