Why Apple Watch Will Not Be Accurate on Stress Readings

With the imminent launch of the Apple Watch it is clear that everyone is looking to it for new functionality and inevitably there will be claims you can reduce stress using it. I wanted to look into whether the device could reliably deliver on that promise.

applewatch

The watch reportedly can read heart rate by taking pulse readings from the wrist using pulse oximetry, a method that uses light pulses to read heart beats by measuring the change in skin color due to different levels of blood flow. Pulse oximetry is refined enough for reading heart rate, but Heart Rate Variability (HRV) demands precision that pulse oximetry reportedly cannot deliver.

I am interested in using HRV to improve personal performance in working sessions, face to face meetings, negotiations and public speaking. I thought it would be interesting to be able to use an Apple Watch to read HRV and improve those skills. So I wanted to test pulse oximetry myself.
MIO

I don’t have an Apple Watch, so I connected a Mio Velo wrist worn band to the SweetbeatLife app on an iPhone. The Mio product claims to deliver “EKG-accurate heart rate data” and uses pulse oximetry, so this would be my proxy for an Apple Watch.

H7

To compare this setup with readings from electrical signals I connected a Polar H7 to Marco Altini’s Heart Rate Variability Logger on an iPod Touch. This would allow me to take two readings of a single heart beat and compare the methods. After wrangling settings and conflicting signalling I got them both to work.

My goal was to use my Parasympathetic Flatline method when comparing the pulse oximetry with electrical readings using a heart rate belt. This means I am looking for 10 consecutive heart beat intervals that vary less than 17 milliseconds from beat to beat. When I find these strings of beats I am measuring myself in a fight/flight state.

Researching pulse oximetry I found a research paper that said that physical movement introduced errors in readings making pulse oximetry unreliable for measuring HRV when subjects were in motion. The conclusion was that pulse oximetry “is unlikely to prove a practical alternative to the ECG in ambulatory recordings or recordings made during other activities.”

negotiating

My interest is looking at activities like negotiating, meeting and coding where there is not a lot of physical movement. With the study providing some potential for pulse oximetry to provide some value to my area of interest, it seemed reasonable that readings when relaxed would be similar and when walking very different.

I conducted sessions in a relaxed state, working by myself on the computer, in meetings and while walking. I first conducted the dual measurements while in a relaxed state for ten minutes. I sat and did not move and breathed in an even rhythm. Subjectively I think I was in fight/flight for 25% of the time because sitting motionless allowed me to think about all the things I was not getting done. Here is a graph of the two readings:

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The pulse oximetry reading was that I was in fight/flight 87% of the time and that is way overstated. The P7 said 32% and that was much closer to my experience. So the relaxed state had a completely different outcome than my hypothesis.

Next I measured myself when I was in a working session, which meant I had structured some time to work on my computer without interruption. I was working on some recruiting matters which meant screening resumes. It was very focused work and I felt relaxed. I would have said I was 10% at most in fight/flight. My session was 16 minutes long, and here are the charts:

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The pulse oximetry reading said I was in fight/flight 57% of the time. This did not remotely match my experience. The H7 reading said I experienced no fight/flight at all. There were accelerations, but none that were more than 9 beats. So though I’m not sure it was a perfect session it was clear that the H7 more closely matched my experience.

I also took readings during the first and second half of a long staff meeting. I was not the host, I was a participant. There were some controversial things being discussed so I would have subjectively said I was in fight/flight 15% of the time. Here is the chart for the first half of the meeting:

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You can see the pulse oximetry said I was at 62% fight/flight, H7 10%. Here is the chart for the second half:

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Pulse Oximetry had me at 63% and H7 at 1%. The H7 seemed low because there were a few moments where I was definitely in a heightened state, but an average under 10% is much close to the perceived 15% than a consistent reading by the Apple Watch equivalent of over 60%. That just made no sense.

I took measurements while walking. I had low expectations because I had taken readings when exercising and know that HRV is low when physically active. Here is the chart as I took my first walk to the train from work.

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I walk briskly so I expected a 60% to 75% reading here. What you see is Oximetry at 91% and H7 at 67%. Again oximetry was high. Here is my reading for leaving the train and going to the pickup point:

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What is interesting here is my wife picked me up about a third of the way through the reading and I relaxed in the car chatting with her as she drove me home. The H7 clearly shows me moving from an accelerated state to more relaxed, which was my experience. The oximetry reading shows continued stress. Again the H7 reading matched the experience.

So what conclusion can we draw from comparing pulse oximetry as used by Apple Watch to electrical readings from chest worn heart rate belts? In the range of activity from sitting motionless to walking briskly the pulse oximetry method overstates stress.

So your Apple Watch is not the best tool to measure stress response using HRV. When you read, “Physicians and digital health experts are encouraged by the level of accuracy suggested by the Apple Watch’s sensors,” remember that pulse oximetry will overstate your stress. In a world of stress the last thing you need is to have it overstated.

Superpower Series: Why You Should Memorize Your Speeches

One superpower that some people seem to have mastered is the ability to stand in front of a group of people and give a speech. For most people it is the most stressful of events, up there with losing a job and divorce. So how can you use Heart Rate Variability (HRV) to keep your fight or flight mode from kicking in and you entering a panic state in front of the group?

Well, it does not appear you can avoid the fight or flight response standing in front of a group of people. From measurements I have taken when speaking I think that the only strategy you can employ is to have more material embedded in memory so you just speak automatically without having the engage your prefontal cortex.

The mechanics of this are straightforward. When you are in fight/flight your body optimizes blood flow to get you out of danger. Blood flows to your limbs and the back of your brain, allowing you to maximize your ability to react. When you are in this reaction mode your thinking brain is offline. You can’t wing it when your thinking brain is offline.

I took readings while speaking at two different Quantified Self meetups. This is a very sympathetic crowd. There is no pressure to perform for this crown. And I have experience speaking publicly, from corporate engagements to speaking competitions for Toastmasters. I enjoy public speaking, so I should be on the more relaxed end of the spectrum. The data shows that even experienced speakers are not immune to the stress of presenting to groups.

As a baseline for compare readings from a meeting with senior people that I am working for as I presented in an earlier post. This was a high intensity meeting where I was expected to present information. The graph shows with blue bars where I experienced the fight/flight impulse.


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Readings on a speech I did at the meetup July 28th of 2014 showed how much stress the system kicks in when on a stage. I started the measurement about five minutes before the speech and did some breathing exercises to see if that would have an effect on my measurements. Here is me giving the speech.

Qs Speech

The breathing exercises did help kick in my relaxation response for the first five minutes of the reading. Once I got to the podium, however, the fight/flight kicked in. I spoke for 14 minutes and answered questions for ten minutes. You can see the readings in this graph: The red box shows the period in which the speaking portion of the presentation took place.

Slide1

A second speech showed a similar pattern. In March of 2015 I spoke again at a Quantified Self meetup to a smaller group. Again a very sympathetic group. I knew the material and was pleased to be presenting. This was a shorter speech, five minutes speaking and five minutes of Q&A. You can see from the chart and the red box that the during speaking portion I was almost entirely in fight/flight state.

Slide2

Why this is important is  as described above the pre-frontal cortex is offline when in this state. Meaning that you can’t think through what you are going to say in real time when you are on the podium. You are in reaction mode. So rehearse the material. When a slide comes up, you react to what you have memorized. I have had experience of “watching” myself giving a speech when I have memorized the material and am bouncing along well. And I have had the experience of freezing in place when I had nothing in my deep memory to react to. And I just stared blankly at the audience.

So memorize your material before you get up to speak. Your physiology will ensure your brain is offline. If you react well throughout the speech you will give a great speech. Even this guy had to memorize his speech:

Braveheart

HRV Tutorial – Measurement Times

One important dimension of your measuring that is important to understand is how the length of time that you measure. Heart Rate Variability (HRV) is the variability of your heartbeats over a period of time. The standard readings (rMSSD, pNN50, etc) are based on some reading over a specific period.

That means if you are studying the effect of eating donuts on your HRV you need to ensure that reading 1 and reading 2 have approximately the same length (5 secs, 30 secs, 2 mins, etc). I prefer 30 seconds snapshots. Readings of different lengths are apples and oranges because you are getting an average over that time period.

If you eat a donut and read your HRV for 5 minutes the reading will an average for the 5. Eat the second donut a day later and read for 35 minutes you will get the average for 35 minutes. If your blood sugar changes more during the 35 minutes simply because your body has more time to digest it will look like a lower number on day 2 and you may draw wrong conclusions based on an error of measurement.

If all other variables are kept constant and you vary the measurement times you will get incompatible readings. Make sure you keep an eye on your timer and know that the similar timing of your readings are your most important source of useable measurements.

HRV Tutorial – Setting Up What You Study

After you have your basic kit you can start taking readings and compare numbers. To save time and avoided taking readings that don’t compare well to other readings it is important to do a good job defining what you are looking to study. This helps keep what you are reading consistent and easier to implement.

Primarily you will be looking at the difference in your Heart Rate Variability (HRV) as you vary conditions in your environment. For example, if you decide to look at the effect of sleep on your HRV you will have to pick a time, conditions and length of time that you will measure your HRV.

For example, if you read your HRV each morning as part of your sleep study, you have to understand that reading HRV immediately after waking is different than 30 minutes after you wake up. Your SNS and PNS are in a different state during those two periods. Additionally, if you walk upstairs to retrieve you heart rate belt on one morning and you didn’t the day before, the two readings will not be comparable.

So it is best to define precisely what you are going to do when you take your readings. In your sleep study example write out “Two minutes after I wake up I take my reading with my heart rate belt reading to the app for 5 minutes as I lay on my bed.” Each morning that you meet those conditions include that reading in those you compare. If one condition is violated, unfortunately, you will have set those readings aside.

Find out more by reading about Measurement Times.

HRV Tutorial – Basic Equipment Setup

Because of the limitations of pulse oximetry I recommend you start with a Polar H7 heart rate belt. It is affordable (about $79) and available on Amazon or a local sporting goods store. If you don’t choose an H7, ensure your heart rate belt is Bluetooth Low Energy (BLE) so it syncs with the updated iPhone or Android device. Garmin uses a different standard (Ant+) and won’t work with what you want to do.

There are many, many apps that will take the readings from your Polar H7 and convert the data. The three best are iThlete, SweetwaterHRV and Marco Altini’s Heart Rate Variability Logger. iThlete is limited in that it is for a daily, short reading to determine your state of physical training. SweetwaterHRV is a good app made by very nice people and it is a bit difficult to extract data (you download and email it). It has a lot of interpretations of the data in the app.

Marco Altini’s app gives nice readings in real time and has a dead simple Dropbox or iCloud download feature. You take a reading, press a button on your device and the .csv file is in your file folder. That simple. Once you have a .csv file you can import it to excel and play with the numbers. You can also see the derivations and start looking there.

Find out more by reading about Setting Up What You Study.

HRV Tutorial – Sensors

You will have numerous sensor available to you and they break two types. One type uses pulse oximetry, which shoots light through your skin to sense the heartbeat by the change in color of your skin based on your blood flow. When you use a watch (Apple Watch, Mio, Basis, etc), an ear clip (Heartmath), or a finger clip (as in those common in hospitals), they all use pulse oximetry.

This type of sensor is good for measuring heart rate, but NOT as good for Heart Rate Variability (HRV). This is because the way it measures heart beats will have a lot of errors when you move your body. There are medical studies on these error rates. Nothing wrong with your Apple Watch telling you your heart rate on a run or during the day. But if you are trying to get precise HRV information from that same watch while you are moving around you will have a problem. So you can use pulse oximetry if you plan to be motionless for your studies, but if you want to look at anything where you move your body you will need to use a different method.

The way to accurately measure your individual heart beats and then your HRV is based on reading the electrical impulses that make your heart beat. These sensors are heart rate belts or more expensive adhesive patches. These give you the required accuracy because they are reading the electrical output from your heart when it beats. I have tested pulse oximetry (Heartmath, Mio wrist band) vs Polar heart rate belt and the differences in accuracy when in motion are noticeable.

Find out more by reading about the Basic Equipment Setup.

HRV Tutorial – How To Get Your Numbers

You capture the Heart Rate Variability (HRV) measurement as expressed in RR intervals by using a sensor and having it read its output to a software application with which you can see a display of the data or download it for viewing and manipulation.

To get your HRV readings from your body you can use a heart rate belt or other type of sensor and read the output on an app. To actually play with the data you will have to find an app that downloads the data in a format such as .csv or .txt that can be ported to a program like excel or R.

To learn how HRV works, I highly recommend you start by looking at the RR intervals so you can get familiar with the source data. Using apps that have already derived some measure is easier once you understand the underlying mechanics of the RR intervals. I wasted a lot of time trying to understand the apps before the underlying mechanics. You can avoid that.

Find out more by reading about Sensors.

HRV Tutorial – RR Intervals

Heart Rate Variability (HRV) is measured by looking at the time interval between successive heart beats over a specific period. That means that if you examine ten heart beats, there will be a difference in intervals between each successive beat and those differences are averaged into some number that indicates there was either a lot of variability between all of the beats or very little variability during the period the ten beats occurred.

The way HRV is captured is by a string of numbers called “RR Intervals.” The RR Intervals are measured in milliseconds. For perspective, a resting heart rate of 60 beats per minute means that there is an average of one heart beat per second. The difference between two of those beats will be a percentage of that once second, so milliseconds are the best unit of measure.

All of the complex measurements that you will encounter  (Fourier transformations, High Frequency/Low Frequency, rMSSD, pNN50, etc) are all derived from this string of numbers. It can be hugely frustrating looking at academic papers and articles/blog posts without understanding that because they all dive right into these derivations and at first it looks like HRV has scores of different measurements. It is one measure with a lot of interesting interpretations. And you will develop your own favorite over time.

Find out more by reading about How To Get Your Numbers.

HRV Tutorial – Variable Interval Heart Beats

Your heart beats at varying intervals based on the state of your nervous system. If you are nervous, scared, or exercising your heart will beat regularly and fast. If you are relaxed your heart will beat at irregular intervals and more slowly.

This is how that works. Your Autonomous Nervous System is made up of two subsystems, the Sympathetic and Parasympathetic Nervous Systems. The Sympathetic Nervous System (SNS) is your accelerator. It raises your heart rate and pushes you into fight/flight mode. When you are in fight/flight mode your Heart Rate Variability (HRV) is low as your heart is pumping blood regularly and fast in order to get you through the immediate danger.

Your Parasympathetic Nervous System (PNS) acts as a brake on your SNS. When there is no danger, the PNS is braking your SNS, slowing the beats and the results is a lower heart rate and higher HRV. When this is happening you are resting and digesting.

The variability in your heart rate is the interplay of your body’s accelerator (SNS) and brake (PNS). As you start examining your HRV readings, it will be a bit awkward because low HRV numbers mean you are stressed and high HRV numbers mean you are relaxed. HRV does not read stress, it measures your heart’s variability and the measures are inversely correlated with stress. High variability is low stress, low variability is high stress.

Find out more by reading about RR Intervals.

HRV Tutorial – Introduction

Heart Rate Variability (HRV) is the most accessible measurement of stress and health for someone who is not in the medical profession, and there are a shortage of plain English explanations of what it is, how to capture it, and how to use it to improve your life.

It took me almost a year to playing with HRV measurements using numerous devices and different apps before I felt I understood it enough to explain it to others. Along that way I had some very generous help from colleagues and friends who shared key concepts with me. And I went down many unproductive paths that wasted a fair bit of time. My goal here is to help you past those unproductive paths and give you the summary benefit of the great advice I have gotten.

With the purchase of some inexpensive items and reading through this summary you should be able to have some fun taking readings that are specific to your physiology. The interesting element of measuring your own HRV is that the baselines and tolerances are uniquely yours. Aggregate data from others gives you tips on where to look for those readings that are yours and yours alone.

The best thing to do is get some simple kit and start playing with it. You’ll learn as you go and I am happy to share as many tips as possible here. Enjoy!

Find out more by reading about Variable Interval Heart Beats.

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