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Optical Heart Rate Monitoring detects the changes in blood filling the capillaries under your skin as your heart beatsflow that occur with each heartbeat. Each time your Most optical heart beats the capillaries expand with blood, and this ebb and flow can be used to determine your heart rate. Most optical heart rate monitors for use while exercising shine rate monitors for use while exercising shine a green light into the skin and use a receptor to detect the changes in the reflected light. [[File:Optical HRM SensorsThis approach has been used for decades, and I had an early version back in the 1980s.jpg|none|thumb|300px|This is a view of the The latest optical sensors of the Basis Peakheart rate monitors are vastly superior, [[Garmin 225]], [[TomTom Cardio Runner]], and but still have many accuracy issues. The most accurate form of heart rate monitoring is to use a chest strap that picks up the electrical signal from the [[Garmin 235]] (left to right)heart.]]This approach While a chest strap is not perfect, it works remarkably well as long as it has been used for decades, and I had an early version back in good contact with your skin, the battery is not flat, and the 1980sstrap is not damaged. The latest optical heart rate monitors are vastly superior, but still have many accuracy issues. accuracy of Optical Heart Rate Monitoring (OHRM) will depend on a number of factors:* The most accurate form watch needs to fit just right. Because of heart rate monitoring the sensor is to use a chest strap that picks up measuring the electrical signal from changes in blood flow with each heartbeat, too much pressure will push the heartblood away from the sensor. While However, to lose and the watch won't get a chest strap is not perfect, it works remarkably well as long as it has good good reading due to lack of contact with your skin. Getting this tension just right can be tricky, the battery is not flat, and itespecially if you's not malfunctioningre wrist expands or contracts over time.=Anecdotal Accuracy=Many reviews of optical heart rate monitors will include anecdotal comparisons such as * Temperature seems to be a huge factor, and most systems work better in warmer conditions. If you're a little chilled, your body will restrict superficial blood flow to retain body heat, making it much harder for the ones shown belowoptical HRM. {| class="wikitable" |- valign="top"|[[File:Garmin235-OHR5In my experience, the issue is mostly around how warm you are when running, rather than at the absolute temperature.jpg|none|thumb|500px| During this run you see the 235 having I've found that optical heart rate monitors can do better in colder conditions when I'm wrapped up warm and sweating a couple of major dropoutsslightly than they will do in mild conditions where I'm running with the bear arms. For the rest of the run* Naturally, the 235 roughly tracks the true because optical heart ratemonitoring systems need to be against the skin, it can be tricky to use them in cold conditions. ]]|- valign="top"|[[File:Garmin235-OHR3.jpg|none|thumb|500px| Here we I've cut a hole in an arm warmer so that I can see the 235 giving an accurate readingwatch face while preventing frostbite to the surrounding skin.* Movement seems to confuse OHRM systems, but one possibly because it changes the papillary filling. Some users have noted that is their OHRM systems seem to lock on to their Cadence rather misleadingthan their heart rate. While I frequently see the 235 displaying The OHRM systems use a (accelerometer) to try to filter out movement related artifacts.* Changes in your heart rate tend to cause problems for OHRM systems. My testing indicates that is wildly too high or too lowrunning at a steady, even intensity makes things a lot easier for the sensor. Changes in intensity, such as interval training, I know I can ignore that informationtend to cause problems. Where Sometimes these problems are the 235 is more problematic than other optical systems is that it will display a plausible but inaccurate value.]]OHRM system doesn't seem to notice the change in heart rate continues on as if nothing had happened, and sometimes the OHRM system becomes disassociated with the real heart rate, either going up or down too far, or sometimes moving in the opposite direction.|- valign="top"|[[File:Garmin235-OHR4* It's possible that bright sunlight might also influence the accuracy, though I've not noticed any obvious correlation.jpg|none|thumb|500px| For this I typically run the 235 initially gives in shady conditions, so this may not be an inaccurate reading that is somewhat close issue I've been exposed to . * Skin pigmentation and tattoos can interfere with the real heart ratelight. My skin tone is quite pale, but then spikes so uses with greater pigmentation may have far more problems, and putting the OHRM sensor over a tattoo is unlikely to wildly too highwork. I tried several times =How to adjust the tension Best Use an Optical Heart Rate Monitor=If you have reasonably pale skin, and position of the 235you can find an optical heart rate monitor that fits your wrist effectively, but nothing helped.]]|}These comparisons can be quite useful and you're prepared to give accept a sense of the type of problems that are common with little inaccuracy, then an optical heart rate monitorsmonitor might be viable for you under specific conditions. However, they provide no quantification of My suggestion is to use the accuracy, making optical heart rate monitor when it impossible 's likely to evaluate if one device is better than another. Thereforework, I decided it would be worthwhile to undertake and use a slightly more rigorous approachchest strap heart rate monitor at other times.=Methodology=To evaluate This means avoiding the accuracy in a more quantifiable manner Ioptical system if you've analyzed the heart rate re doing any form of readings of several optical heart rate monitors compared with interval training, and only use it when you're running at a chest strap based monitorsteady, even pace.[[File:Optical HRM Sensors. My first step was to verify that the chest strap based monitor jpg|center|thumb|300px|This is reasonably accurate. To do this, I ran with two different chest strap systemsa view of the optical sensors of the Basis Peak, a [[Garmin Ant+ (HRM4) and a Polar Bluetooth (H7). I compared over 10225]], [[TomTom Cardio Runner]],000 readings and found that the two systems matched extremely well[[Garmin 235]] (left to right). I have uploaded ]]=Testing Optical Heart Rate Monitors=To evaluate the results below, using two different visualizations. The first graph shows accuracy in a more quantifiable manner I've analyzed the heart rate measured by the Garmin HRM4 on of readings of several optical heart rate monitors compared with a chest strap based monitor. I've spent months gathering over 1 million data points, and developing analysis software defined patterns in the horizontal against data. I found two factors that influence the Polar H7 on the verticalaccuracy; temperature and rate of change of heart rate (delta HR, or ΔHR). If I'm defining ΔHR as the two systems match exactly then difference between the point will be on highest and lowest heart rate values recorded using the chest strap in the green line preceding 60 seconds, so a ΔHR of equality. You can see 5 could come from a few places where the two systems don't line up perfectly, but the vast majority heart rate range of 130 to 135. * About 78% of the readings align within a couple across all of heartbeatsthe optical heart rate monitors is within 3 BPM of the correct reading. IThat've used transparent points to give a better impression of s pretty grim in my opinion. * When the density temperature is warm (between 68f/20c and 85f/20c) about 83% of readings across all of the optical heart rate monitors is within 3 BPM of datathe correct reading.* When the ΔHR is 5 BPM or less, with black areas having at least 10 data points lining up. There about 85% of readings across all of the optical heart rate monitors is a blue regression line, which will be aligned with within three BPM of the correct reading.* When the green line if the system temperature is warm (between 68f/20c and 85f/20c) and ΔHR is accurate. I've also included two red lines that are 25 bpm away from 5 BPM or less, about 90% of readings across all of the true value. The second graph shows the distribution optical heart rate monitors is within three BPM of differences between the two systems, and again we can see that correct reading.==Optical Heart Rate Monitors In The Warm==Because optical heart rate monitoring relies on blood flow to the vast majority of data points are within a couple of beats. For those with a statistical backgroundskin, air temperature is an important factor in accuracy. In practice, the standard deviation relationship is 1a little more complex than you might expect.55 BPM with Warmer temperatures will add heat stress to a bias of -0runner, and blood will flow to the skin to help control temperature.06 BPM. {| class="wikitable" |- valign="top"|[[File:VerifyScatterHowever, the relationship between air temperature and heat stress on the runner will depend on exercise intensity, as high intensity will produce more heat to be dispersed.png|none|thumb|500px| The distribution Heat stress will also depend on humidity, as the evaporative cooling of readings between the Polar Bluetooth system vertically and the Garman Ant+ system horizontallysweat is less effective. This Another factor is how BMI, as higher BMI values represent less effective surface area to volume ratio. Finally, the distribution should look if amount of clothing you wear will have an effect on both heat stress and localized blood flow. You'll notice that the system being tested accuracy at 40f/4c is fractionally better than at 50f/10c, and this is accuratebecause I wear an arm warmer with a hole cut in it at lower temperatures. ]]|- valign="top"|[[File:VerifyDistribution(I have experimented with wearing an arm warmer in moderate conditions, but the discomfort makes this a little impractical for me.png|none|thumb|500px| Here ) All this means that the temperatures shown below should not be taken as absolute, but as a general indication that temperature is another view of the same dataan important factor, showing the most readings were exactly the same, or differing by just a couple of beatsand optical heart rate monitors are probably impractical in cooler conditions for most runners.]]|}=Garmin 235 Accuracy=[[File:OHRM_ALL_ByTemp.jpg|center|thumb|500px|The graphs below highlight the accuracy problems off at the Garman 235's of all optical heart rate monitormonitoring systems combined by temperature. Of ]]Here's the table of how the 20watches do under warm conditions,000 readings, slightly more than 10% were out by more than 25 BPM (demarked by the red lines on the first graph.) You can see a distinct cloud of points clustered high above the true readings. It's possible that this cloud represents the optical system becoming confused by the impact of but with varying levels of ΔHR.{{:Optical Heart Rate Monitoring-warm}}==Accuracy and Rate Of Change Of Heart Rate==Another patent in my data is that optical heart rate monitors tended to do quite well if my feet landingheart rate is steady, as they are vaguely but don't track changes in the vicinity of my typical [[Cadence]]. Other runners have reported the phenomenon of optical heart rate monitors displaying wildly high valuesvery well, and can often become completely lost. You This means that I can slow up and the optical heart rate monitor reading can also see this set of jump wildly high values in the distribution graph, way it is displayed as or I can start a small bump high intensity interval and the reading can drop. I'm using ΔHR to mean the right hand side of difference between the highest and lowest heart rate values recorded using the chest strap in the main spikepreceding 60 seconds. The average error Naturally, ΔHR and air temperature interact, so let's look at how optical heart rate monitors cope with changes in heart rate under ideal (standard deviation68f/20c to 85f/20c) is 19conditions.1 BPM, with an average reading that was 5You'll notice a fairly rapid decline in accuracy as heart rate becomes a changeable.7 BPM too high[[File:OHRM_ALL_BydHR. If you're an experienced runner that has a good idea what your jpg|center|thumb|500px|The accuracy of all optical heart rate should be, then you may be able to ignore values that are out monitoring systems combined by ΔHR.]]The graph below breaks down the accuracy by more specific device. Some devices clearly do better than 25 bpm. In that caseothers, the standard deviation drops to 4but I'd urge caution in interpreting this data, as different devices might fit different people better or worse.2 BPM with an average that is 0.03 too low. Of course, if you know your heart rate Looking at the [[Garmin Fenix 5X]], which does far worse than any other device, but I strongly suspect that the due to within 25 BPM, then the 235 will only get you slightly closerweight of the device. This means the Fenix 5X is moving around rather more than a lighter device, increasing the number of bad readings. Even the best devices here only work reasonably well when my heart rate is fairly steady. Even moderate changes in heart rate cause a fairly significant drop in accuracy. {| class="wikitable" |- valign="top"|[[File:ORHM-Garmin235-ScatterOHRM_Device_BydHR.pngjpg|nonecenter|thumb|500px| The distribution accuracy of readings between the Garmin 235 OHRM vertically and the Garman Ant+ system horizontallysome optical heart rate monitoring systems by ΔHR.]]|- valign="top"Here's the table of how the watches do with steady heart rate (<5 BPM ΔHR) but a variety of temperatures.|[[File{{:ORHM-Garmin235-DistributionOptical Heart Rate Monitoring-easy}}==Accuracy Under Ideal Conditions==The table below shows how accurate I've found the OHRM to be under the best possible conditions. This means warm temperatures (68f/20c to 85f/20c) AND with my Heart Rate steady. This is as easy as it gets for an optical heart rate monitor, and you can see that some of the watches do reasonably well. If you only ever do steady, easy running and live in Key West, then the [[Garmin 235]] might work well for you. {{:Optical Heart Rate Monitoring-warm-easy}}==Verifying Chest Strap Gold Standard==My first step was to verify that the chest strap based monitor is reasonably accurate. To do this, I ran with two different chest strap systems, a Garmin Ant+ (HRM4) and a Polar Bluetooth (H7). I compared over 10,000 readings and found that the two systems matched extremely well. I have uploaded the results below, using two different visualizations. The first graph shows the heart rate measured by the Garmin HRM4 on the horizontal against the Polar H7 on the vertical. If the two systems match exactly then the point will be on the green line of equality. You can see a few places where the two systems don't line up perfectly, but the vast majority of the readings align within a couple of heartbeats. I've used transparent points to give a better impression of the density of data, with black areas having at least 10 data points lining up. There is a blue regression line, which will be aligned with the green line if the system is accurate. I've also included two red lines that are 25 bpm away from the true value. The second graph shows the distribution of differences between the two systems, and again we can see that the vast majority of data points are within a couple of beats. For those with a statistical background, the standard deviation is 1.55 BPM with a bias of -0.06 BPM. <gallery widths=300px heights=300px class="center">File:VerifyScatter.png| The distribution of readings between the Polar Bluetooth system vertically and the Garman Ant+ system horizontally. This is how the distribution should look if the system being tested is accurate. File:VerifyDistribution.png| Here is another view of the same data, showing the most readings were exactly the same, or differing by just a couple of beats.</gallery>=Optical Heart Rate Monitors Compared With Pulse Oximeters=''Main article: [[Pulse Oximeter]]s''Pulse oximeters are the small devices that are clipped to your fingertip to measure your pulse and oxygen saturation. Like optical heart rate monitors, pulse oximeters measure your pulse using a similar approach of looking for changes in how light is absorbed. While pulse oximeters provide a reliable and sufficiently accurate measure of your heart rate, they are extremely sensitive to movement, and your finger needs to be completely still. =How Optical Heart Rate Monitors Work=Optical Heart Rate Monitors for use when exercising have two sensors, one for detecting light, and the other for detecting movement. They shine a light into the skin, and then measure how much is reflected back to the light detector. Some of this variation is due to the beating of the heart, but some is due to a motion, especially motion related to each step. The optical heart rate monitors use the accelerometer to detect movement and will attempt to account for this. The difficulty of extracting the wanted signal from the noise is a primary reason why optical heart rate monitors have so many accuracy problems. The issue of step based noise is particularly problematic as the frequencies tend to be similar. It's been noted that optical heart rate monitoring may be less accurate when cycling than when running. One possible explanation that occurs to me is that the noise may be related to muscular contractions, rather than the impact of landing.png|none|thumb|500px| Here is another view of the same data, showing the greater error.]]|}