Follow-Up: Garmin Vivoactive HR & Polar H10: Which measures heart rate more accurately?

Heart Rate Measurement Using Garmin & Polar Wearables

A study was made of the Garmin Vivoactive HR and Polar H10 chest strap in terms of comparative heart rate assessments. The units are shown in Figure 1 below. The two units involved included a wrist-based sensor (Garmin Vivoactive HR) and a chest strap (Polar H10).

Polar h10 chest strap and garmin vivoactive hr smart watch were used in the comparison

This follow-up focuses on 20 minutes of water rowing using both units in an effort to assess the heart rate measurement consistency and reliability. Both watch and chest strap were properly attached with no movement between these devices and the skin. Data were collected and then downloaded and processed through a Microsoft Excel spreadsheet. The data were time-synchronized so that corresponding data points from each device were associated in time. A summary of the analysis is provided here.

Time-Based Plots of Heart Rate

Overlay scatter plots of heart rate measurements versus time were made and are as shown in Figure 2.

Figure 2: Heart rate measurement while water-rowing approximately 20 minutes. Shown are overlays of Garmin Vivoactive HR and polar h10 heart rate versus time.

A general observation from the data is that the heart rate measurements from the two devices seem to overlap reasonably well as viewed by the naked eye. But there are key drops in measurements, particularly with the wrist-based heart rate sensor, that show as deviations in the overlap of the two signals. This can be seen more readily via the correlation curve shown in Figure 3. The correlation coefficient of 0.91 was determined between the two sets of measurements. It should be noted that the wrist-based sensor was snug with no movement on the wrist. Ambient temperature was approximately 80F.

As I showed in a previous post, there was a serious issue with the wrist-based sensor in which there were data dropouts with some significant time lags between measurements. In the case of the wrist-based sensor for the associated measurements here, this was also experienced. For comparison, I show histogram plots of the time intervals between measurements for both the wrist-based sensor (Figure 4) and the chest strap (Figure 5). The wrist-based sensor experiences a significant number of events in which the time between actual measurements are greater than one second. Indeed, from the figure, only 83 measurements during this interval were obtained within one-second of one another! There were a significant number of measurements in which the interval was > 1 second, with one as high as 40 seconds. The overall quantity of measurements was thus reduced to approximately 430 during the workout. On the other hand, the chest strap consistently measured at one-second intervals for a total of approximately 1320 measurements.

Figure 3: Scatter plot of heart rate as measured between the wrist-based Garmin device and the chest-strap Polar H10. A correlation coefficient of 0.91 was determined between the measurements. Perfect correlation is shown by the diagonal line.
Figure 4: Historgram of time between measurements for Garmin wrist-based sensor. Note the significant quantities of measurements in which the interval is greater than 1 second (the advertised measurement interval). For example, there were 20 instances in which the measurement interval was 6 seconds, and one instance in which the measurement interval was 40 seconds! Note that only 83 measurements were in the one-second interval width!
Figure 5: Historgram of time between measurements for the chest strap polar H10 sensor. All measurements (of which there were more than 1300) were reliably at one-second intervals.

Conclusions

Chest straps are much more reliable for heart rate measurement versus wrist-based sensors. Users of wrist-based sensors for heart rate measurement should be advised that measurements can be in question, as results illustrate here. This is not to say that chest straps are the gold-standard. Clearly, ECG measurement similar to those obtained through stress-testing are of diagnostic quality. Yet, for rate measurement chest straps are quite adequate and seemingly reliable.

Garmin Vivoactive HR & Polar H10: Which measures heart rate more accurately?

Figure 1: Polar h10 chest strap and garmin vivoactive hr smart watch were used in the comparison.

Heart Rate Measurement Using Garmin & Polar Wearables

A study was made of the Garmin Vivoactive HR and Polar H10 chest strap in terms of comparative heart rate assessments. Three different types of tests were conducted while the author wore these devices. The units are shown in Figure 1. The Garmin unit is able to be used with a number of sports, including rowing, and provides measurements of heart rate, stroke rate, distance per stroke, split times, and also provides for location tracking during the workout. Data can be uploaded to http://connect.garmin.com/and are also available for download in TCX (an XML format) as well as splits downloads in CSV format. The Polar H10 is strapped around the chest just below the level of the breast bone. This unit, too, can upload data to the http://flow.polar.com/web site, where data can be downloaded in TCX format, as well.  In order to provide some variety, I considered three different activities:

  • General workout, involving weight lifting, sit-ups, squats;
  • Walking for 1 mile; and,
  • Indoor rowing for 15 minutes.

In all cases, both the Vivoactive HR and the H10 were attached, with the Vivoactive HR snuggly affixed to the left wrist. Both watch and chest strap were properly attached with no movement between these devices and the skin. Data were collected and then downloaded and processed through a Microsoft Excel spreadsheet. The data were time-synchronized so that corresponding data points from each device were associated in time. Plots of the measurements were made.

Time-Based Plots of Heart Rate

Overlay scatter plots of heart rate measurements versus time were made of all three activities, shown in Figure 2 through Figure 4. Data were downloaded from the Garmin & Polar cloud sites and were uploaded into MS Excel. The data were then time synchronized using visual basic to align the measurements.

Figure 2: Heart rate measurement while walking 1 mile. Scatter overlay of Garmin Vivoactive HR and polar h10 heart rate versus time.
Figure 3: Heart rate measurement during general exercise activity. Scatter overlay of Garmin Vivoactive HR and polar h10 heart rate versus time.
Figure 4: Heart rate measurement while  rowing indoors on Concept 2 ergometer. Scatter overlay of Garmin Vivoactive HR and polar h10 heart rate versus time.

Heart Rate Comparison: Walking

Measurements of heart rate were taken during a one mile walk. The heart rates were plotted against one another and the correlation coefficient was computed between the two sets of measurements. In the case of the comparison shown in Figure 5, the correlation among measurements was rather poor: the correlation coefficient was determined to be -0.54. Perfect correlation is given by the diagonal line in the figure. Interesting to note is that the data points taken from the Vivoactive HR time variance. In the case of the Vivoactive HR, in some instances, the time between measurements was as high as 47 seconds with 62 measurements in the 12-14 second interval range, whereas in the case of the Polar H10, all measurements were 1 second interval. Thus, the number (quantity) of measurements taken by the Polar H10 were far denser than those of the Vivoactive HR.

Figure 5: Scatter plot of heart rate measured while walking one mile using the Polar H10 versus Garmin Vivoactive HR. The correlation coefficient of -0.54 was determined between the two sets of measurements. Perfect correlation is shown by the diagonal line.

Heart Rate Comparison: General Activity

In the case of general activity, which included some weight lifting, sit-ups, leg raises and standing exercises, the heart rate comparison is as shown in Figure 6. The correlation coefficient among these measurements is a bit higher at 0.60. The variation in measurement collection time associated with the Garmin HR was even higher here, with one measurement interval as high as 88 seconds!

Figure 6: Scatter plot of heart rate measured while performing weight lifting, sit-ups and general standing exercises using the Polar H10 versus Garmin Vivoactive HR. The correlation coefficient of 0.60 was determined between the two sets of measurements. Perfect correlation is shown by the diagonal line.

I have hypothesized that the wide variation in data collection time may be due to arm motion that is not experienced to the degree in walking. I also have hypothesized that the improved correlation may be due to the higher heart rate, which is more easily detected by the Vivoactive HR. We will see some supporting evidence of this in the final section on indoor rowing.

Heart Rate Comparison: Indoor Rowing

Rowing on the Concept 2 PM5 unit while wearing both the Vivoactive HR and the Polar H10 produced the results as illustrated in Figure 7. The correlation between the Vivoactive HR and the Polar H10 is much higher here, with a correlation coefficient of 0.95. Several items of note: the variation in measurements with the Vivoactive HR is much lower, with only two measurements 19 seconds apart and most measurements having 1-2 second intervals. This complies much more closely with the 1-second measurement intervals of the Polar H10. Furthermore, heart rate measurements are much higher here: some measurements as high as 165 beats/minute (during sprints). In general, corroboration between the two units is better as heart rate measurement is higher. This could be due to more accurate peripheral measurement.

Figure 7: Scatter plot of heart rate measured while performing indoor rowing using the Polar H10 versus Garmin Vivoactive HR. The correlation coefficient of 0.95 was determined between the two sets of measurements. Perfect correlation is shown by the diagonal line.

Conclusions

Based on the limited sampling and workouts thus far, the general conclusion regarding heart rate measurement “trust” is that the Polar H10 is more reliable based on several observations: (1) data collection time variation remains consistent at 1 second; and, (2) data density remains high with no dropouts in any of the workouts. This is not a surprise in general as the conventional wisdom is that chest straps are much more reliable. Yet, I wanted to quantify this reliability using some objective measures. It should be noted that while heart rate remains somewhat questionable with the Vivoactive HR, I have found that stroke rate measurement in comparison with the Concept 2 PM5 measurement is dead on accurate (at least based on the data I have observed).

Rowing Data Analytics: Reducing and Studying the Rowing Workout

In my last post (“Rowing Data…”) I discussed the steps associated with downloading the Garmin Vivoactive HR data from Garmin Connect to an Excel spreadsheet. In this post, I’m going to take the reader through the analysis of the data as a tutorial and guide for assessing certain elements of these data.

Raw data in Excel format are shown in Figure 1. I am going to focus on distance (column M), speed (column N), and heart rate (column O).

Figure 1: Downloaded rowing worksheet from Garmin Connect

I normally like to study discrete, time-based data by translating the time component from the Zulu time (column L) into a relative time from the start of the workout. Furthermore, I like to translate these into units of seconds as the base unit.

To do so, we can take advantage of some powerful capabilities contained within formulas inside of Microsoft Excel. For example, the start time listed in column L begins with the entry:

2017-07-08T14:09:31.000Z

The next entry is:

2017-07-08T14:09:34.000Z

These are “Zulu” time or absolute time references. We wish all future times to be keyed or made in reference to the first time. In order to do so, we need to translate this entry into a time in seconds. We can do so by parsing each element of the entry. These entries are listed sequentially in column L2 and L3, respectively.

Each element is translated into seconds by parsing the hours, minutes and seconds using the following formula:

=MID($A$2,12,2)*60*60+MID($A$2,15,2)*60+MID($A$2,18,2)

The first component extracts the time in hours and translates into seconds. The second component extracts the “minutes” and translates into seconds. The third component extracts the “seconds” element by itself. The total time is the superposition of all three individual components.

Thus, what I normally do is to copy the contents of the initial spreadsheet into a new sheet adjacent to the original and then begin working on the data. Presently, I am in the process of developing an application that will perform this function automatically. Yet, here I am “walking the track” associated with analyzing the data in order to chronicle the mathematics surrounding the process.

The hour, minute and second can be extracted as separate columns. Let us copy the contents of column L in the original spreadsheet into a new sheet within the existing workbook and place the time in column A of that new sheet. Thus, the entries in this sheet would appear as follows:

ns1:Time Absolute Time (seconds) Relative Time (seconds)
2017-07-08T14:09:31.000Z 50971 0
2017-07-08T14:09:34.000Z 50974 3
2017-07-08T14:09:35.000Z 50975 4

The Absolute time in the middle column is the time in seconds represented by the left-hand column relative to Midnight Zulu time. The right-hand column is the time relative to the first cell entry in the middle column. Thus, zero corresponds to 50971-50971. The entry for three seconds corresponds to the difference between 50974 (second entry) and 50971 (first entry), and so on.

I also created some columns to validate parameter entries. For instance, the reported total distance and speed (in units of meters and meters per second, respectively), in column M and N and the heart rate, in column O, are referred to next. I created a new column O in the new spreadsheet to provide a derived estimate of total distance, which I computed as the integral of speed over time. The incremental distance, dS, is equal to the speed at that time, dV, multiplied by the time differential between the current time and the previous time stamp, dt. Then, the total distance is the integral, or the summation of this incremental distance and all prior distances. I reflect this as column G in the new worksheet, shown in Figure 2.

Figure 2: Modified rowing spreadsheet with derived time, range and longitude-latitude calculations.

What follows now are plots of the raw and derived data. First, the heart rate measurement over time is shown in Figure 3. Note that the resting rate is shown at first. Once the workout intensifies, heart rate increases and remains relatively high throughout the duration of the workout.

Figure 3: Workout heart rate (pulse) versus time.

The total distance covered over time is shown in Figure 4. This tends to imply a relatively constant speed during the workout due to the linear behavior over the 8700+ meters.

Figure 4: Workout range versus time. Note linear behavior, indicating relatively constant speed.

The reported speed, as measured via GPS, shows variability but is typically centered about 1.85 meters per second. The speed over time is shown in Figure 5.

Figure 5: Workout measured speed versus time. Average is 1.85 meters per second.

The GPS coordinates are also available through the Excel data. I have subtracted out the starting location in order to provide a relative longitude-latitude plot of the workout, shown in Figure 6.

Figure 6: relative longitude and altitude of the workout.

In my next post I will focus on the athletic aspects of the workout related to training.

Rowing Data: Accessing Heart Rate, Distance and GPS Location from Garmin Vivoactive HR

An Introduction to Garmin Connect

For those who use the Garmin Connect Dashboard (https://connect.garmin.com), to synchronize their Garmin fitness devices, there is a fairly straightforward method for downloading higher-frequency data from the workout relative to heart rate, distance and GPS location that can be directly imported into Microsoft Excel for further analysis.

Getting Started with the Download

From inside of Garmin Connect (Figure 1), select a specific activity. In this example, I am picking my latest rowing workout, shown by the red arrow.

Garmin Connect: Accessing workout data within your Garmin Vivoactive HR.

Once you have selected the specific workout, click on it and this will take you to the details of that workout. Once there, navigate over to the gear on the right-hand side, as shown by the red arrow.

Detailed view of workout within Garmin Connect.

The drop-down box from the arrow shows a number of export options. Select “Export to TCX”, shown by red arrow in Figure 3.

Accessing export function within Garmin Connect.

Upon selection, the file will be downloaded, as shown in Figure 4. On a Windows platform, this will be downloaded by default to the user’s downloads folder.

File download in .TCX format from Garmin Connect.

Once the file is downloaded, go to the file directory and locate the file you just downloaded, as shown in Figure 5.

Locating .TCX file in downloads directory on Windows platform

Then, change the suffix from .TCX to .XML, as shown in Figure 6. Accept the change when prompted.

Renaming .TCX file to .XML.

Now, open Microsoft Excel. Select the Data tab, as shown in Figure 7.

Microsoft Excel: selecting Data tab in preparation for .XML file import.

On the left-hand side, select Get External Data “From Other Sources”, and scroll down to “From XML Data Import”, as shown in Figure 8.

Importing the .XML file into Microsoft Excel using the import
XML file option.

A dialog box will open. Navigate to your newly-created XML file. Select it, and click the series of “OK” buttons in the dialogs that come up, including the one placing the location of the start in cell $A$1. Once completed, the contents of the file will be imported into your spreadsheet. Heart rate data will be contained in column O, as shown in Figure 9. Distance & speed are contained in columns M & N, respectively. GPS latitude & longitude are contained in columns P & Q, respectively. Average speed is contained in column R.

Viewing available columns of data within Microsoft Excel.

In my next post on the subject I will describe how to manipulate these data for further analysis.

Garmin Vivoactive HR for Rowing & Sculling

Vivoactive HR

Sculling and Rowing

I am a rower and sculler. I first cut my teeth in the sport over 30 years ago while at college rowing on the Charles River. I had been looking for the longest time for a device that I could use to track my heart, stroke rate, and also support GPS mapping of my workout while on the water. There are professional devices that track stroke rate and the like, such as Speed Coach GPSStroke Coach and Coxmate GPS. These are all excellent pieces of equipment, by the way. But, I am not in varsity rowing any more and I was looking for a piece of equipment that could support my rowing “habit” both for indoor and outdoor rowing (aside: I also possess a Concept 2 ergometer, which I love) while also serving the utilitarian purpose of being a good watch that can track heart rate full time.

When I row, however, I am really interested in being able to map the analytics to the motion. The Vivoactive HR enables me to do this as well as to post-process the data. I am into data. As a Chief Analytics Officer in the healthcare field for a medical device and real-time patient surveillance company, it is important to me to be able to access and understand the information collected during an activity. The connectivity and access to data provided by the Vivoactive HR are phenomenal.

Data view from Garmin Connect web site.

 

 

 

 

The figure above details an example analytics screen, which shows the map of the workout, heart rate, stroke rate, distance traveled at each measurement point, and allows tracking the entire workout with a cross-hair that is dynamic and interactive on the web screen. The unit supports many other types of workouts, including running, biking, pool, golf, walking, indoor rowing on ergometer, SUP rowing, XC skiing, indoor walking, indoor biking, and indoor running, and tracks sleep. The unit can be submerged in water and the battery life is amazing. I normally live with the unit on my wrist, and after 3 days of continuous use, battery is down to, perhaps 80%. I will take it off for an hour or so to charge, and it is good-to-go. I highly recommend this unit for the avid professional or veteran rower (like myself).

Update June 29th, 2017: Comparison among NK, Coxmate, Minimax

Robin Caroe of RowPerfect kindly left me a comment to this post last evening and provided an updated article on comparison among the NK, Coxmate GPS and Catapult Minimax which contains quite valuable data on performance related to these products. I have provided the hyperlink to the article above. Technological differences in sampling rate (e.g.: 5 Hz for NK versus 10 Hz for Coxmate) are important for accuracy. I must say that I was very close to purchasing the Coxmate GPS prior to investigating the Garmin. Upon reading the brochure for the Minimax S4, I am intrigued. The Minimax offers an update rate on the GPS that provides for precision in terms of location. In the Rowperfect article, of the key measures of performance identified, (1) heart rate & heart rate variability; (2) force and length of stroke; and, (3) GPS update rate are important measures for the elite athlete. In the case of the Minimax, GPS update on the order of 100 times per second (10 milliseconds) can reveal boat pitch, roll & yaw. Highly impressive. I would agree, though, that this level of accuracy and precision would be important for the competitive athlete. Yet, in my case (non-competitive, casual athlete), I still love my Garmin. I am able to see and track my position very accurately, monitor my stroke and heart rate, and in terms of heart rate variability, I can write an algorithm in R or Matlab to monitor that measure fairly directly.