Under the Covers of mySASY: Time vs. Frequency Domain

What is the difference between monitoring heart rate variability (HRV) using time and frequency domains?


Under the covers of mySASY: time vs. frequency domain

What is the difference between monitoring heart rate variability (HRV) using time and frequency domains? What seems like a scientific game with words and a slight nuance in methodology is a key difference that sets mySASY apart from the competition.

We will try to give you a basic idea using the analogy of GPS car navigation:

The time domain shows whether to turn right or left right now.

Frequency will also show you where you are now and where you are going in the long term.

A big topic both in terms of scientific research and also among the general public thanks to the increasing popularity of applications athletes of all levels. Scientists have long been clear in this regard * (source). But what about the average Joe? Here the situation is different. Everyone has an opinion and it is difficult to take a stance without the necessary knowledge of the issue. We want to introduce you to our opinion and give you the insight into the scientific part. We will try to be brief and correct.

mySASY 3D graph


Key difference in measurement methodology

In principle, HRV focuses on monitoring the state of the autonomous nervous system (ANS) controlling the body, which has 2 equally important branches (parts):

  • parasympathetic (regeneration / replenishment)
  • sympathetic (activation / consumption)

If we want to correctly describe its activity, we have to talk about:

  • total system performance (Total Power,cumulative pulses flowing through the nervous system)
  • parasympathetic activity (regeneration / replenishment)
  • sympathetic activity (activation / consumption)
  • In this context, what is the difference between time domain and spectral analysis is shown in the following table:
Parameter Time Frequency mySASY
Total Power (approximate?) yes size of the circle
Parasympatikus yes yes X axis
Sympatikus no yes Y axis


Bottom Line. Time domain gives about 50% information about ANS, frequency 95% (a certain degree of uncertainty must always be perceived - we follow the biological system, not the machine). The question is: Can the time domain make any sense? Or is 50% sufficient information? The answer is: it depends for whom and for what purpose. We will try to show it better by comparing the results of monthly measurements with mySASY of three athletes.


Case Study

Athlete 1 Athlete 2 Athlete 3

Sportovec 1

Sportovec 2

Sportovec 3

Before evaluating the individual examples, let us recall what we would see in mySASY using only the time domain: circle size + very orientational movement along the X axis.

Using spectral analysis (mySASY) we will see:

Athlete 1 responds to the load primarily by a dramatic decrease in overall performance (circle size) associated with a change in parasympathetic activity (energy replenishment - movement along the X axis). It is all quite a negative signal, which "already" can be detected by the time domain. Using the time domain is not able to capture long-term (very desirable) profile adjustment (spectrum).

Athlete 2 has already relatively stable performance ANS, regenerative branch is almost always active (along the X-axis we see only minimal movement), the dominance of operation is sympathetic (movement along the Y-axis). Operation is almost ideal. The time domain would already give a limited amount of information. Especially in the field of long-term development.

Athlete 3 very stable and high overall ANS performance, very high stable activity of the regeneration branch. Due to the movement of the results, the time domain does not solve much in this case (it will be rather misleading) both in the current proceedings and in the longer term.


Monitoring of long-term development and changes in ANS activity by using Training Profile and population standards

Most (certainly we do not know all systems in the world) evaluation approaches using time domains are based solely on comparing an individual's actual results with their previous results. The results say – today it is better than usual, today it is worse than usual. This can also be done using the frequency domain (for mySASY, this is represented by the colour system and the % of compensation level).

However, the interpretation using the available population standard also informs the user about the overall quality of the management and, above all, whether the results are moving in the right direction in the long term = your Training Profile. The examples and their comparison show how positively the adaptation capacity could develop in Athlete 1 (compared to other profiles and development). On the other hand, on the example of Athlete 3, it is possible to demonstrate, in particular, what profile would indicate a minimization of the risk of chronic deterioration, since this athlete can hardly improve his/her adaptation ability (not to be confused with actual performance).

We believe that we have provided you with enough information to decide for yourself how to manage and evaluate your training using HRV. mySASY brings you a unique tool using the spectral analysis (ie frequency domain) to see your training efficiency with maximum accuracy. You get both immediate response and also the long-term perspective, thanks to the Training Profile.


Advanced note: we calculate the time domain parameters and have a clear idea about what kind of information this approach can and can never provide.


*Malik, M., Camm, A. J. Heart rate variability Wiley-Blackwell; 1 edition. Heart rate variability, Standards of measurement, physiological interpretation, and clinical use Task Force of The European Society of Cardiology and The North American Society of Pacing and Electrophysiology, European Heart Journal (1996) 17, 354–381.

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