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Double camera, double trouble [fixed]

10/18/2016

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This post is a quick overview of the changes implemented in HRV4Training to support the latest iPhones. While the iPhone 7 required no changes in our algorithms, the 7+ is equipped with two cameras, and a few users reported having trouble getting a stable reading. We finally received our testing devices this week, and implemented the necessary changes to support the iPhone 7+.

As some of you reported, two main issues were present; 1) heart rate was higher than it should have been 2) the phone was using the "wrong" camera, the one quite far from the flash, making it difficult to get a reading at all. 

The good news is that all issues have been fixed, and an update will be available in 1-2 weeks to extend support of the camera based algorithms for the iPhone 7+ as well. ​We would like to thank everyone for your patience and feedback as this input was very useful in quickly identifying the main sources of trouble, and fixing them. First, we switched the camera used by the app, as shown below:
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By using the camera closest to the flash, we ensure proper lighting which is necessary to capture changes in skin color due to blood flow. You can't really get it wrong, as covering the other camera will be obvious in the camera view in the app.

Secondly, we had to make some additional adjustments to ensure that the sampling frequency and interpolation were working as in previous versions. Once we were able to make these adjustments, we ran a preliminary validation using our trusted Polar H7, and a prototype app we developed for our clinical studies. This special app can collect data from both the camera and the bluetooth sensor (the same app described here). Data cannot be perfectly synchronized because it is not timestamped by the bluetooth sensors, however we can log time and then split data in windows based on when it was collected, then compute HRV features based on RR intervals included in these windows. With this procedure we are typically off by one beat maximum, hence a small variation over a minute of data.

First, let's look at an example of the PPG data collected using the app (12 minutes of data), where you can also see discarded beats in red, these ones are typically due to an initial stabilization phase and artifacts found during the measurements, for example intervals that are too close or too far apart and might be due to noise, ectopic beats, and so:
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After collecting accurate PPG, we need to extract peak to peak intervals, basically our RR intervals. Below we show the usual time series, for segments of data collected under different conditions (rest, deep breathing, etc.) - you can see how the RR intervals match very well the ones from the Polar sensor, the figures also report the HRV value (rMSSD in ms) for the entire segment. The first segment is the one that comes from the PPG data above:​
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You can easily spot the deep breathing segment above, where oscillations due to RSA are quite obvious (minute 3 to 6). Here is another segment for self-paced breathing (just lying down relaxing):
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And finally another one also containing deep breathing:
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The data above shows quite clearly how RR intervals can be extracted accurately using the iPhone 7+ as well. Below you can see rMSSD and heart rate data for 1 minute windows, as this is our main use case in HRV4Training:
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Data shown in this post comes from one person only, and we will be collecting more to cover a broader range of HR and rMSSD values in the incoming weeks.

​However, the main point here was simply to find (and fix) the issues that came up due to the double camera present in the latest iPhone 7+, and we believe these issues have been addressed. Hence, we will be releasing an update in the next 1-2 weeks so that you can start using the app with the iPhone 7+ as well (updates will come for both HRV4Training and Camera HRV).
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    This blog is curated by
    Marco Altini, founder of HRV4Training


    ​Blog Index
    ​
    The Ultimate Guide to HRV
    1: Measurement setup
    2: Interpreting your data
    3: Case studies and practical examples

    How To
    1. Intro to HRV
    ​2. How to use HRV, the basics
    3. HRV guided training
    ​4. HRV and training load
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    5. HRV, strength & power
    6. Overview in HRV4Training Pro​
    7. HRV in team sports
    ​

    HRV Measurements
    Best Practices

    1. Context & Time of the Day
    2. Duration
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    3. Paced breathing
    4. Orthostatic Test
    5. Slides HRV overview
    6. Normal values and historical data
    ​7. HRV features
    ​
    Data Analysis
    1a. Acute Changes in HRV
    (individual level)

    1b. Acute Changes in HRV (population level)
    ​
    1c. Acute Changes in HRV & measurement consistency
    1d. Acute Changes in HRV in endurance and power sports​
    2a. Interpreting HRV Trends
    2​b. HRV Baseline Trends & CV
    3. ​Tags & Correlations​
    4. Ectopic beats & motion artifacts
    5. HRV4Training Insights
    6. HRV4Training & Sports Science
    7. HRV & fitness / training load
    ​8. HRV & performance
    9. VO2max models
    10. Repeated HRV measurements
    11. VO2max and performance
    12. HR, HRV and performance
    13. Training intensity & performance​
    14. Publication: VO2max & running performance
    ​
    15. Estimating running performance
    16. Coefficient of Variation
    17. More on CV and the big picture
    ​​​​​18. Case study marathon training
    19. Case study injury and lifestyle stress
    20. HRV and menstrual cycle
    21. Cardiac decoupling
    22. FTP, lactate threshold, half and full marathon time estimates
    ​23. Training Monotony
    ​
    Camera & Sensors
    1. ECG vs Polar & Mio Alpha
    2a. Camera vs Polar
    2b. Camera vs Polar iOS10
    2c. iPhone 7+ vs Polar
    2d. Comparison of PPG sensors
    3. Camera measurement guidelines
    4. Validation paper
    ​5. Android camera vs Chest strap
    ​6. Scosche Rhythm24
    ​7. Apple Watch
    8. CorSense
    ​
    9. Samsung Galaxy
    ​
    App Features
    ​1. Features and Recovery Points
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    4. Sleep tracking
    5. Training load analysis
    ​6a. Integration with Strava
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    6c. Integration with SportTracks
    6d. Integration with Genetrainer
    ​
    6e. Integration with Apple Health
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    ​6f. Integration with Todays Plan
    7. Acute HRV changes by sport
    8. Remote tags in HRV4T Coach
    9. VO2max Estimation
    ​
    10. Acute stressors analysis
    11. Training Polarization
    ​
    12. Lactate Threshold Estimation
    13. Functional Threshold Power(FTP) Estimation for cyclists
    14. Aerobic Endurance analysis
    15. Intervals Analysis
    ​​​16. Training Planning
    17. Integration with Oura
    18. Aerobic efficiency and cardiac decoupling
    ​
    Other
    1. HRV normal values​
    ​2. HRV normalization by HR
    ​
    3. HRV 101

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