Acute HRV changes in response to training: analysis of HRV4Training users data

Blog post by Marco Altini.

In a previous post I've covered acute HRV changes, i.e. changes in HRV following training. I've shown how we can use differences in HRV measurements between training day and the day following training to determine the impact of the training session on our physiology. What we typically expect in this situation is a reduced HRV on days following intense aerobic trainings. This method is one of the most common and reliable ways to use HRV, and is the principle behind the daily HRV advice provided by apps like HRV4Training.

In my previous post on acute HRV changes I showed as an example my data over a period of 2 months and highlighted how this type of analysis was included in HRV4Training to allow you to better understand how your trainings are affecting your physiology and recovery.

In this post, I will take the previous considerations a step further, and analyze data from about 400 HRV4Training users to determine if at the population level we are seeing the expected outcomes (i.e. HRV reductions on days following training).

Dataset

For this analysis, I used the last 2 months (September and October 2015) of measurements collected from HRV4Training users. I filtered out all users with less than 20 measurements and 10 trainings annotated during the selected time period, to make sure I had enough data to make meaningful considerations. This procedure left me with data from 375 users, summing up to 19249 HRV measurements. As we can see from the plots below, the great majority of users measured using the camera, 1 minute recordings and 8 breaths per minute (this is the same setup I also use):

Here are some histograms on the main characteristics of the included users:

From the plots above we can see how HRV4Training users tend to be people that work out a lot (3-4 trainings per week or daily), are mainly male with relatively low BMI, and cover a very broad age range.

Acute HRV changes

Acute HRV changes were computed as follows; I first determined day to day differences in HRV, and then contextualized the HRV change with respect to training. Then, for each user I averaged HRV changes on days following training, and on days not following training, to determine if there was any difference. 

In other terms, the questions we try to answer are the following; can we spot reductions in HRV on days following training? If so, can we spot greater reductions in HRV on days following more intense trainings?

Here is an example of the data used as input for this analysis. I randomly selected 20 users and plotted their rMSSD values and manually annotated trainings and training intensities:

​This plot was just to give an idea of the data at hand. Now let's finally look at the results.

​Similarly to what I was showing in the previous post on acute HRV changes, I computed HRV changes for different conditions:

• Rest day vs training day. Here we expect to see reductions in HRV on days following training.
• Training intensity. We expect to see greater reductions in HRV on days following more intense trainings.

From the plots below we can see that the results are as expected, with reductions in rMSSD on days following training, and greater reductions for more intense trainings:​

We can see how rest days typically trigger a positive change in HRV, i.e. HRV is higher on the day following a day of rest. We can also see how easy trainings (plot on the right, second bar) are also resulting in an increase in HRV the following day. Easy trainings (e.g. a recovery run) can have a stimulatory effect on parasympathetic activity, therefore explaining the higher rMSSD value. However, for both average and intense trainings we have clear reductions, i.e. negative HRV changes. The reduction is HRV is clearly more marked for intense trainings, highlighting how we can use reductions in HRV as a marker of training load.

Summary

In this post I analyzed the relation between acute changes in HRV (rMSSD) and training intensity, on almost 400 HRV4Training users and 20 000 measurements. By looking at acute changes longitudinally over 2 months, we can get some better understanding of how we cope with training. 

​The data clearly shows that trainings of higher intensity result in a reduction in HRV the following day, highlighting once again how HRV can be used as a valuable tool to measure training load and recovery. The analysis shown above is also available inside the HRV4Training app, under Menu - Insights - Acute HRV Changes.

While this kind of relationship between training load and rMSSD was already shown multiple times in literature, I believe it is very interesting to be able to capture it in totally uncontrolled settings, outside of the laboratory environment. The sample of users used for this analysis almost completely relied on 60 seconds PPG measurements. The fact that we can easily spot the reductions in HRV following trainings of different intensities highlights the validity of 60 seconds PPG measurements and HRV4Training (however, don't forget to follow the best practices, otherwise making sense of the data will be much harder).

Validating known relationships is a first important step in trying to better understand complex relations between not only training, but also lifestyle and other stressors affecting our physiology, recovery and performance. The more data, the more tags/annotations, the more we will be able to build on clinical studies by outsourcing data collection to a broader population, using tools like HRV4Training.
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HRV4Training on Facebook

HRV4Training is on Facebook, I plan to use this page as a centralized place for feedback, discussions, feature requests, bug reports and to introduce new features and changes in the app. 

Especially for the ones that are not much into Twitter, feel free to use the FacebookHRV4Training page as a place to open discussions around HRV, training, performance and more. I'll try to be responsive so that the whole community can benefit.
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