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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 The latest optical heart rate monitors are vastly superior, but still have many accuracy issues. The most accurate form of heart rate monitoring is to use a view of chest strap that picks up the optical sensors of electrical signal from the Basis Peakheart. While a chest strap is not perfect, [[Garmin 225]]it works remarkably well as long as it has good contact with your skin, [[TomTom Cardio Runner]]the battery is not flat, and the [[Garmin 235]] strap is not damaged. The accuracy of Optical Heart Rate Monitoring (left to rightOHRM).]]will depend on a number of factors:This approach has been used for decades, and I had an early version back in the 1980s. * The latest optical heart rate monitors are vastly superior, but still have many accuracy issueswatch needs to fit just right. The most accurate form Because of heart rate monitoring the sensor is to use a chest strap that picks up measuring the changes in blood flow with each heartbeat, too much pressure will push the electrical signal blood away from the heartsensor. While a chest strap is not perfectHowever, it works remarkably well as long as it has to lose and the watch won't get a good reading due to lack of contact with your skin, the battery is not flat. Getting this tension just right can be tricky, and itespecially if you's not malfunctioningre wrist expands or contracts over time. The accuracy of Optical Heart Rate Monitoring (OHRM) will depend on a number of factors: * The watch needs Temperature seems to fit just right. Because of the sensor is measuring the expansion of the capillaries with each heartbeatbe a huge factor, too much pressure will prevent this expansion. However, to lose and the watch won't get a good reading due to lack of contactmost systems work better in warmer conditions. Getting this tension just right can be tricky, especially if If you're wrist expands or contracts over time.* Movement seems a little chilled, your body will restrict superficial blood flow to confuse OHRM systemsretain body heat, possibly because making it changes much harder for the papillary fillingoptical HRM. Some users have noted that their OHRM systems seem to lock on to their Cadence rather than their In my experience, the issue is mostly around how warm you are when running, rather than at the absolute temperature. I've found that optical heart rate.* Temperature seems to be monitors can do better in colder conditions when I'm wrapped up warm and sweating a huge factor, and most systems work better slightly than they will do in warmer mild conditionswhere I'm running with the bear arms. If you're a little chilled* Naturally, your body will restrict blood flow because optical heart rate monitoring systems need to your capillaries to retain body heatbe against the skin, making it much harder for the optical HRM. Of course, because the system needs to be against the skin, it can be can be tricky to use them in cold conditions. I've cut a hole in a an arm warmer so that I can see the watch face while preventing frostbite to the surrounding skin.* It's possible that bright sunlight might also influence Movement seems to confuse OHRM systems, possibly because it changes the accuracy, though I've not noticed any obvious correlationpapillary filling.=Anecdotal Accuracy=Many reviews of optical Some users have noted that their OHRM systems seem to lock on to their Cadence rather than their heart rate monitors will include anecdotal comparisons such as the ones shown below. The OHRM systems use a (accelerometer) to try to filter out movement related artifacts. {| class="wikitable" |- valign="top"|[[File:Garmin235-OHR5* Changes in your heart rate tend to cause problems for OHRM systems.jpg|none|thumb|500px| During this run you see the 235 having a couple of major dropoutsMy testing indicates that running at a steady, even intensity makes things a lot easier for the sensor. For the rest of the runChanges in intensity, such as interval training, tend to cause problems. Sometimes these problems are the 235 roughly tracks the true OHRM system doesn't seem to notice the change in heart ratecontinues 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-OHR3.jpg|none|thumb|500px| Here we see the 235 giving an accurate reading, but one that is rather misleading. While * It's possible that bright sunlight might also influence the accuracy, though I frequently see the 235 displaying a heart rate that is wildly too high or too low've not noticed any obvious correlation. I typically run in shady conditions, so this may not be an issue I know I 've been exposed to. * Skin pigmentation and tattoos can ignore that informationinterfere with the light. Where the 235 My skin tone is quite pale, so uses with greater pigmentation may have far more problematic than other optical systems problems, and putting the OHRM sensor over a tattoo is that it will display a plausible but inaccurate valueunlikely to work.]]|- valign="top"How to Best Use an Optical Heart Rate Monitor=|[[File:Garmin235-OHR4.jpg|none|thumb|500px| For this run the 235 initially gives If you have reasonably pale skin, and you can find an inaccurate reading optical heart rate monitor that fits your wrist effectively, and you're prepared to accept a little inaccuracy, then an optical heart rate monitor might be viable for you under specific conditions. My suggestion is somewhat close to use the real optical heart ratemonitor when it's likely to work, but then spikes to wildly too highand use a chest strap heart rate monitor at other times. I tried several times to adjust This means avoiding the tension optical system if you're doing any form of interval training, and position of the 235only use it when you're running at a steady, but nothing helpedeven pace.]]|}These comparisons can be quite useful to give [[File:Optical HRM Sensors.jpg|center|thumb|300px|This is a sense view of the type optical sensors of problems that are common with optical heart rate monitors. Howeverthe Basis Peak, they provide no quantification of the accuracy[[Garmin 225]], making it impossible to evaluate if one device is better than another. Therefore[[TomTom Cardio Runner]], I decided it would be worthwhile and the [[Garmin 235]] (left to undertake a slightly more rigorous approachright).]]=MethodologyTesting Optical Heart Rate Monitors=To evaluate the accuracy in a more quantifiable manner I've analyzed the heart rate of readings of several optical heart rate monitors compared with a chest strap based monitor. My first step was to verify I've spent months gathering over 1 million data points, and developing analysis software defined patterns in the data. I found two factors that influence the chest strap based monitor is reasonably accurateaccuracy; temperature and rate of change of heart rate (delta HR, or ΔHR). To do this, I ran with two different 'm defining ΔHR as the difference between the highest and lowest heart rate values recorded using the chest strap systemsin the preceding 60 seconds, so a Garmin Ant+ (HRM4) and ΔHR of 5 could come from a Polar Bluetooth (H7)heart rate range of 130 to 135. I compared over 10,000 * About 78% of readings and found that across all of the optical heart rate monitors is within 3 BPM of the two systems matched extremely wellcorrect reading. I have uploaded the results below, using two different visualizationsThat's pretty grim in my opinion. The first graph shows * When the heart rate measured by temperature is warm (between 68f/20c and 85f/20c) about 83% of readings across all of the Garmin HRM4 on optical heart rate monitors is within 3 BPM of the horizontal against the Polar H7 on the verticalcorrect reading. If * When the two systems match exactly then ΔHR is 5 BPM or less, about 85% of readings across all of the point will be on optical heart rate monitors is within three BPM of the green line of equalitycorrect reading. You can see a few places where * When the two systems don't line up perfectlytemperature is warm (between 68f/20c and 85f/20c) and ΔHR is 5 BPM or less, but the vast majority about 90% of readings across all of the readings align optical heart rate monitors is within a couple three BPM of heartbeatsthe correct reading. I've used transparent points ==Optical Heart Rate Monitors In The Warm==Because optical heart rate monitoring relies on blood flow to give a better impression of the density of dataskin, with black areas having at least 10 data points lining upair temperature is an important factor in accuracy. There In practice, the relationship is a blue regression line, which little more complex than you might expect. Warmer temperatures will be aligned with add heat stress to a runner, and blood will flow to the green line if skin to help control temperature. However, the system is accurate. I've also included two red lines that are 25 bpm away from relationship between air temperature and heat stress on the true valuerunner will depend on exercise intensity, as high intensity will produce more heat to be dispersed. The second graph shows Heat stress will also depend on humidity, as the distribution evaporative cooling of differences between the two systemssweat is less effective. Another factor is BMI, as higher BMI values represent less effective surface area to volume ratio. Finally, and again we can see that the vast majority amount of data points are within a couple of beatsclothing you wear will have an effect on both heat stress and localized blood flow. For those with a statistical background, You'll notice that the standard deviation accuracy at 40f/4c is fractionally better than at 50f/10c, and this is 1.55 BPM because I wear an arm warmer with a bias of -0hole cut in it at lower temperatures.06 BPM(I have experimented with wearing an arm warmer in moderate conditions, but the discomfort makes this a little impractical for me.) All this means that the temperatures shown below should not be taken as absolute, but as a general indication that temperature is an important factor, and optical heart rate monitors are probably impractical in cooler conditions for most runners. {| class="wikitable" |- valign="top"|[[[[File:VerifyScatterOHRM_ALL_ByTemp.pngjpg|nonecenter|thumb|500px| The distribution accuracy of readings between all optical heart rate monitoring systems combined by temperature.]]Here's the Polar Bluetooth system vertically and table of how the Garman Ant+ system horizontallywatches do under warm conditions, but with varying levels of ΔHR. This is how the distribution should look if the system being tested is accurate. ]]|- valign{{:Optical Heart Rate Monitoring-warm}}=="top"|[[File:VerifyDistribution.png|none|thumb|500px| Here Accuracy and Rate Of Change Of Heart Rate==Another patent in my data is another view of the same datathat optical heart rate monitors tended to do quite well if my heart rate is steady, showing the most readings were exactly the samebut don't track changes in heart rate very well, and can often become completely lost. This means that I can slow up and the optical heart rate monitor reading can jump wildly high, or differing by just I can start a couple of beatshigh intensity interval and the reading can drop.]]|}=Garmin 235 Accuracy=The graphs below highlight I'm using ΔHR to mean the accuracy problems off at difference between the Garman 235highest and lowest heart rate values recorded using the chest strap in the preceding 60 seconds. Naturally, ΔHR and air temperature interact, so let's look at how optical heart rate monitormonitors cope with changes in heart rate under ideal (68f/20c to 85f/20c) conditions. Of the 20,000 readings, slightly more than 10% were out by more than 25 BPM (demarked You'll notice a fairly rapid decline in accuracy as heart rate becomes a changeable.[[File:OHRM_ALL_BydHR.jpg|center|thumb|500px|The accuracy of all optical heart rate monitoring systems combined by ΔHR.]]The graph below breaks down the red lines on the first graphaccuracy by specific device.) You can see a distinct cloud of points clustered high above the true readings. ItSome devices clearly do better than others, but I's possible that d urge caution in interpreting this cloud represents the optical system becoming confused by the impact of my feet landingdata, as they are vaguely in different devices might fit different people better or worse. Looking at the vicinity of my typical [[CadenceGarmin Fenix 5X]]. Other runners have reported , which does far worse than any other device, but I strongly suspect that the due to the phenomenon weight of optical heart rate monitors displaying wildly high valuesthe device. You can also see this set of high values in the distribution graphThis means the Fenix 5X is moving around rather more than a lighter device, way it is displayed as a small bump to increasing the right hand side number of bad readings. Even the main spike. The average error (standard deviation) best devices here only work reasonably well when my heart rate is 19fairly steady.1 BPM, with an average reading that was 5Even moderate changes in heart rate cause a fairly significant drop in accuracy.7 BPM too high[[File:OHRM_Device_BydHR. If you're an experienced runner that has a good idea what your jpg|center|thumb|500px| The accuracy of some optical heart rate should be, then you may be able to ignore values that are out monitoring systems by more than 25 bpmΔHR. In that case, ]]Here's the table of how the standard deviation drops to 4.2 BPM watches do with an average that is 0.03 too low. Of course, if you know your steady heart rate to within 25 (<5 BPM, then the 235 will only get you slightly closer. ΔHR) but a variety of temperatures.{| class{:Optical Heart Rate Monitoring-easy}}==Accuracy Under Ideal Conditions="wikitable" |- valign="top"|[[File:ORHM-Garmin235-ScatterThe table below shows how accurate I've found the OHRM to be under the best possible conditions.png|none|thumb|500px| The distribution of readings between the Garmin 235 OHRM vertically and the Garman Ant+ system horizontallyThis means warm temperatures (68f/20c to 85f/20c) AND with my Heart Rate steady.]]|- valign="top"|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 [[File:ORHM-Garmin235-DistributionGarmin 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.]]|}