Intraday HRV API: Questions regarding hf and lf variables

[itismeghasyam]

As per the Intraday HRV API documentation, Get HRV Intraday by Interval (fitbit.com), both the hf and lf variables correspond to power in a certain frequency range. Could you please provide answers to the following questions:

• What are the units for hf and lf?
• What is the duration of the time window used to calculate hf and lf? Does it remain the same across various records and Fitbit models?
• What is the expected range of hf and lf?

[GordonFitbit]

Hi @itismeghasyam 

• What are the units for hf and lf?  As described in the documentation, the units is Hz.
• What is the duration of the time window used to calculate hf and lf? Does it remain the same across various records and Fitbit models?  As described in the documentation, the duration is every 5 minutes and it will adjust accordingly.  The sample response demonstrates this.
• What is the expected range of hf and lf?   As described in the documentation, the range for hf is 0.15 Hz - 0.4 Hz and the lf is 0.04 Hz - 0.15 Hz.  

[itismeghasyam]

Hi @GordonFitbit 

It seems there may have been a slight confusion regarding the question. According to the documentation, 'hf' and 'lf' denote the power within specific frequency bands, rather than the frequency bands themselves. I would recommend referring to the example response in the documentation for clarification.

To elaborate, power is defined as energy per unit time. Also see - 'units for power spectral density' which refers to power per frequency. Ideally, by summing this up over the desired frequency values, we can determine the power within that frequency range.

• For instance, 'hf' represents the power in the 0.15 Hz - 0.4 Hz band, while 'lf' pertains to the power in the 0.04 Hz - 0.15 Hz band. Could you clarify the units used here? (e.g., g^2? m^2? V^2?)
• The response rate of the HRV API is 5 minutes. Could you specify the length of the time window used to calculate the hf and lf in the provided HRV response? Is it still 5min? Time window length can affect the power in signal, depending on how power is calculated/defined here. We want to compare hf and lf across multiple records/devices and want to make sure that we are making a apples-to-apples comparison.
• Furthermore, it's important to note that 'hf' and 'lf', as outlined in the documentation, do NOT refer to frequency ranges but to the power within those ranges. The example response from the documentation shows 'hf' and 'lf' values in the hundreds, which seems inconsistent with the ranges you've mentioned above. Could you shed some light on the expected ranges for 'hf' and 'lf'? This information is crucial for our QC efforts.

Thank you for your attention to these matters. I look forward to your clarification.

- Megha

[itismeghasyam]

Hi @GordonFitbit 

I'm just following up on my previous question. Has anyone had a chance to look into this? Any insight would be greatly appreciated. Thanks!

[GordonFitbit]

Hi @itismeghasyam 

Our engineering team is researching your questions with another team.   I hope to hear back in a few business days.   Once I do, I'll update this forum post.