[Tyler]

Heart rate vs. heart rate variability

RR-I means the time in millisecond between two consequent heart beats.
That is the only input that heart rate monitors takes out from the heart.
Instead of showing the interval time in milliseconds, users are more familiarized watching the heart rate in beats/min value. So yes, every HR monitors measure RR-I.

However, RR-I provides more information than just the pure heart rate.
When analyzing the changes in RR-I times (heart rate variability, HRV), it is possible to detect different frequency bands (high, low, very low).
These spectral components are the “subunits of the variation in RR-intervals and they provide information on respiration rate, ventilation, oxygen consumption etc.

Why to go beyond heart rate?

Additional information to heart rate level can enhance the accuracy of the intensity estimation. The accuracy can be especially increased in interval type training, because heart rate doesn’t react linearly with real oxygen consumption. To see how much the accuracy can be increased by using the HRV derived information in addition to HR, please check validation
studies: Firstbeat Technologies - Download Research Papers

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