Over the past several weeks I have been testing both the Movesense Medical module as well as the Heart rate plus device to see if they can track the non linear HRV index, DFA a1 accurately during a 20 minute incremental ramp. The ramp protocol has been discussed before - after a 20 minute warmup, begin with a starting power at 130 watts then a 5 watt per minute rise up to 230 watts ending at 20 minutes (100 watts over 20 minutes). In the past my DFA a1 .75 value has been consistently in the 170-180 watt range with a heart rate about 130. My intent here is to compare the Polar H10 to the Movesense Medical as well as the "Heart rate plus" modules to see if they are in that ballpark. Although it is difficult to wear two chest belts simultaneously, it can be done - but be prepared to throw out some results if they slide into each other (and cause too many artifacts).
First off, it's been an interesting journey. The initial results with the Medical module were not consistent, occasionally having corrupted output files, abnormal ECG wave structure and HRV results that did not make sense. Some of the difficulties involved using too high a bluetooth sample rate. Although I have been able to get 500 Hz to work, it is less reliable than 250 Hz. However, even with a good sample rate and bluetooth transmission, the ECG waveforms were not normal originally.
Here is an example of what the QRS should look like (taken from a medical grade 1000 Hz ECG):
Now for some samples of the Medical module:
This is a bad waveform with a deviation to the right recorded at 500 Hz:
This is a good waveform with a sample rate of 500 Hz:
Does sample rate matter?
There have been several publications examining the importance of sample rates for HRV and even DFA a1. They do not indicate huge disparities in results comparing rates of 128 and 1000 Hz. But, they have been done at rest and no study has looked at what happens during exercise. The problem with lower sample rates is that the waveform loses some smoothness and definition. Remember, with the lower sample rate, less information about the wave structure is present.
Here is a 200 Hz sample from the Medical module:
Here is a Hexoskin sample taken form the .wav file using Audacity:
- This is a great example of a low-ish sample rate (256 Hz). Each data point that makes up the waveform structure is now exposed
Of course, if we zoom out (on the 200 Hz Movesense above), the structure looks fine:
My concern was that the lower sample rates could affect an index of self similarity (DFA a1) if the level of fine detail was lost. In addition, the distortion of the wave (tilt left or right) could also lead to some change in the RR timing.
The software engineer in charge of the Movesense Medical device was kind enough to work with me on this issue. After some adjustments in preprocessing filters (new firmware), both the waveform and bluetooth transmission seem better.
One of the interesting subjects I was introduced to was the effects of pre processing on the final wave or RR. Depending on the device, internal software, hardware filtering, and processing, the final result could be quite different. For starters, the Heart rate plus device, although capable of ECG display, does not have the on board hardware and software that the Medical Module has. This leads to a potential issue in HRV accuracy as we are about to see.
DFA a1 curves generated from each device as a pure RR sequence to a Garmin watch:
Both devices may have similar RR acquisition and processing (when not in ECG mode) with an internal sample rate of 125 Hz. The following was recorded using each module as just a heart rate unit with a Garmin watch receiving the bluetooth signal of RR time (like a Polar H10).
Heart rate plus:
Here is the same ramp done using the Medical Module:
- The results are very close, DFA a1 .75 at: 191 vs 187 watts and 133 vs 131 bpm.
- This is near my historic averages
The nice thing about both ramp recordings was the absence of any lost beat artifact:
For comparison, here is a Polar H10 recording session (on a different day):
The artifacts were about 1.5% overall, but more prevalent toward the middle - end:
- This artifact rate was actually very low for the Polar device (I usually get 2-4%).
- The DFA a1 power and heart rate at a .75 value were also close to both Movesense devices. Some of the discrepancy could be from artifact.
- DFA a1 = .75 at 195 watts/135 bpm for the Polar vs 191/187 watts and 133/131 for the Movesense units.
- Very close indeed despite not being a validation study using multiple ramp trials
ECG recording from both the Medical module and Heart rate plus:
As opposed to RR times series, the following uses the ECG waveform to determine HRV
Heart rate plus, ECG recorded with a sample rate of 500 Hz:
- Wow, this is really off the mark.
- The DFA a1 was below .75 from the start and dropped to very low values right away. I did repeat this and it was the same.
- The Heart rate plus module does not seem to accurately measure DFA a1 during exercise.
Lets look at the raw waveforms in Kubios:
- There are absolutely no artifacts or arrhythmia in the 20 minute ramp.
- The waveform appears fine.
- However, apparently, the heart rate plus device does not have the necessary hardware (and software) to reproduce a precise ECG derived timed signal.
Movesense Medical module as ECG recorded at 500 Hz:
- DFA a1 .75 was reached at 188 watts/131 bpm heart rate. This was about the same as the Movesense RR series above. There were no artifacts.
- I was wearing the Polar H10 at the same time this was done (note the Polar H10 date from the above) which showed 195 watts/135 bpm heart rate at the DFA a1 .75 value.
- The numbers were close.
Movesense Medical module - Waveform in Kubios:
Here is a closeup. The waveform is smooth and has a sharper peak than the Heart rate plus unit.
Ability to identify arrhythmia or artifact:
- This is where having the actual ECG comes in handy. Both benign (like the above APC) or potentially harmful arrhythmia can be identified in an athlete's tracing. In addition, a missed beat due to noise could be corrected manually.
Usage under extreme body motion:
I decided to torture test the Medical module. In this scenario, I performed 3 sets of body weight push ups, 45 seconds per set, only 15 seconds rest between sets (total 3 minutes). Since the overlying muscles will be very active with their own EMG related electrical activity, this should provide some information on how good the signal filtering mechanism is in the Med module.
Here is the start of the series of push ups (the orange line, red arrow)
Notice how the background noise appears on the baseline signal:
But despite this, the entire series does not have any missed beat artifact:
- I have tried this with a Polar H10 and the series is essentially unusable due to missed beat artifact.
For instance, here is a 3 minute cycling interval at VO2 max power, followed by some easy pedaling:
Back to the Medical module during push ups - Arrhythmia identification
There was one APC in the series, easily identified:
- Both the Movesense Medical module and Heart rate plus are able to record a single lead ECG. From the standpoint of arrhythmia identification, both are comparable.
- Simple RR transmission to a Garmin watch or other headunit is done with both devices using a sample rate of 125 Hz. I have been told that the Medical module may be able to go higher with a firmware change. Both seem to track DFA a1 well during one of my typical ramps, making them potentially useful for zone 1 identification. I was extremely impressed with the lack of missed beat artifacts compared to the Polar H10.
- For bare bones HRV using RR values only, the Heart rate plus is a reasonable choice at a very attractive price.
- The Medical module operating in ECG mode at 500 Hz, will record a single lead ECG with good precision, excellent DFA a1 tracking and is extremely resistant to body motion artifact. Even significant usage of the chest wall muscle group does not markedly interfere with the ECG nor HRV. The Medical module can also transmit simple RR values to a bluetooth head unit (even a smartphone or trainer) with what appears to be good precision, despite a sample rate of 125 Hz. Ideally, a sample rate of at least 250 Hz would be preferable. The Medical module with it's current form factor, range of abilities, artifact rejection and price are unbeatable. The development team has been extremely helpful and informative. I have very high hopes for the role of the Medical module in HRV research during exercise.
- One last comment - each bluetooth device can only transmit to one other receiver unit at a time. For instance, recording the Medical module RRs to a Garmin watch will disable the ability to record the ECG series to a smartphone. You would need to stop the watch recording session, then open the smartphone app for ECG recording.