Identifying Physiologic Breakpoints With NIRS

A Cheap & Non-Invasive Method for Identifying Metabolic Thresholds

As exercise load increases, muscle oxygen saturation in the working muscles will decrease. This is one of the first things you’ll observe when using a NIRS device, like a Moxy Monitor. As a result, it’s common to see ramp incremental exercise tests used to assess oxygen kinetics during exercise with NIRS.

For example, if you’re working with a rower you might put them on a concept 2 erg and have them row for 4:00 at 150 watts, then each subsequent 4:00 block you increase the power output by a fixed amount (say, 50 watts) until failure.

What’s interesting about ramp tests is that while power output increases linearly, physiologic metrics like blood lactate, oxygen consumption, and muscle oxygenation do not. As a result, we can use ramp tests to find ‘breakpoints’ in these different parameters as a means of identifying intensity thresholds. Additionally, by monitoring these break points in different muscle groups during a ramp incremental exercise tests we can take a peek behind the curtains and get a deeper understanding of how systemic loads impacts different working muscles to varying extents. We can then juxtapose this information to blood lactate measurements, and break points, which tell us about systemic load on the body.

When calculating breakpoints with NIRS data i’ll typically use deoxy hemoglobin measurements since they are less affected by blood volume under the NIRS probe. Even though the Moxy monitor does not provide a deoxy hemoglobin measurement directly, we can back calculate it from the THb and SmO2 data with the following formula: Deoxy hemoglobin = ((100 — %SmO2)/100))*THb.

In the picture above you can see Deoxy hemoglobin from a rower’s vastus lateralis muscle plotted against their power output on an incremental step test. Note the near linear increase in deoxyhemoglobin 400 watts at which points the rate of increase rapidly increases. This ‘break point’ demarcates the crossing of an intensity threshold. You’ll also notice that deoxy hemoglobin plateaus towards the end of the test. This indicates that the athelte has ‘maxed out’ their oxygen extraction capabilities, and given that they were no longer able to increase their oxygen supply, then failed shortly after, thus ended the test and causing deoxy HB to drop down rapidly after the test ends.