Scosche Rhythm

The Scosche Rhythm+ is an optical Heart Rate Monitor that is too flawed for me to recommend it. If you really want to have heart rate data, and needed to avoid a heart rate strap, then this may be your best option, but be aware of the issues before you purchase.

1 Pros

• The Rhythm is more convenient than a traditional heart rate strap. It sits comfortably on the forearm just below the elbow, and is unlikely to cause any chafing problems.
• Because it supports both Bluetooth and ANT+, the Rhythm+ will work with the vast majority of devices and phones.

2 Cons

• The biggest problem I found with a Rhythm+ is that it frequently gave wildly inaccurate readings. See below for more details.
• In addition to the wild errors, the Rhythm+ is not accurate enough to provide useful Heart Rate Variability (HRV) data. HRV is used by a number of devices for estimating fitness and recovery, and the Rhythm+ will prevent these features working correctly. See below for more details.
• Unlike a traditional heart rate strap where the battery will last for months, the Rhythm+ only lasts for around seven hours. This means that you have yet another device to keep charged.

3 Accuracy

I compared the Rhythm+ to a traditional heart rate monitor strap so that I could plot the recordings together to see the differences. I have found that the Rhythm+ gave erroneous readings too often for me to recommend it. (I suspect that the optical sensor has a sampling frequency, as you can see the light flashing. If the frequency is too low, it is possible for sampling errors to occur, which might explain why the errors seem to occur at the start of the run, as the heart rate is rising rapidly.)

This graph shows a fairly typical error I've seen with the Rhythm+. For the first a few moments the Rhythm+ gives a correct reading, but then rapidly fails, giving a wildly high reading. After a few minutes things settle down and the Rhythm+ tracks reasonably well. I tried attaching the Rhythm+ a few minutes before the run, but that didn't help.

Is a close up of the comparison between the chest strap and the Rhythm+. As you can see, the Rhythm+ is reasonably accurate, though not quite up to the standards of the chest strap.

In this graph the Rhythm+ is in blue, and you can see the typical startup problems. This graph is of a High Intensity Interval Training session where my heart rate is faring much more rapidly, and you can see that the Rhythm+ is not quite keeping up. The lower red line represents the difference between the two readings.

Sometimes the Rhythm+ will lock on to the heart rate and then lose it again, repeating this a few times before stabilizing to a good reading.

Another close-up, showing that the Rhythm+ at its most accurate is generally pretty close to the chest strap, and is probably good enough for most situations.

Rarely, the Rhythm+ will never achieve an accurate reading, as you can see from this graph.

4 Heart Rate Variability

Because HRV is becoming a critical component of many of the advanced features of the Best Running Watches I investigated the Rhythm+'s HRV support in depth. It turns out that the Rhythm+ does transmit HRV data, but not accurately enough to be viable.

• I used the LightBlue iOS app to grab the raw Bluetooth data stream from the Rhythm+ and from the Polar H7 heart rate strap.
• I converted the raw data to binary and interpreted it using the Bluetooth heart rate specification. The iOS app will log the data to a text file which can be pasted into Excel for analysis.
• For any given heart rate, the Rhythm+ will send a beat interval that is constant, not reflecting the true interval between beats. Interestingly the interval sent is close to but not exactly the same as you would expect if the heart rate was the exact integer. For instance, with a heart rate of 60 the Rhythm+ sends 1024 not 1000. (For HR=61 is sends 1007, not 984, etc.)
• To check the output from the ANT+ feed I simply used the live recording plugin to SportTracks. Here you can visually see that the beat to beat interval is the changing based on the heart rate and not reflecting the variability.

A sample of the debug output from the Rhythm+.

A sample of the debug output from the H7

Here you can see that the beat to beat interval is staying the same for a given heart rate.