Identifying Respiratory Compensation with NIRS
As exercise load increases, muscle oxygen saturation in the working muscles will decrease. As a result, NIRS is commonly used to assess skeletal muscle oxygen kinetics during high intensity exercise.
In the picture above we have NIRS data from a Sanctional level Crossfit athlete performing a high turnover 10 round for time metcon. Before this athlete began the metcon I instructed them to start slow and increase their pace slightly every round such that each round of the workout is a little faster than the previous one. This athlete was able to pull that off, going unbroken on every round and steadily increasing their speed per rep from set to set.
What’s interesting is that even though this athletes speed increased in a near linear manner set to set, their physiologic metrics like heart rate and muscle oxygen saturation did not. As a result, we could use those metrics to identify ‘breakpoints’ in these parameters while allows us to identify when they cross different intensity thresholds.
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 deoxyhemoglobin from this crossfit athletes respiratory muscles. I chose this site to monitor since a large % of cardiac output (15–20%) is devoted to increasing respiratory muscle blood flow during high intensity exercise. Above you can see deoxyhemoglobin increasing in a near linear manner up to set #6 at which points it begins to rapidly increase. This indicates that the athlete reached their respiratory compensation point, and at this point total respiratory muscle blood flow (not featured in this picture) began to increase as a respiratory metaboreflex occurred.
After observing this trend I worked with this athlete to improve their inspiratory and expiratory muscle strength endurance, and 5 weeks later we re-tested the same 10 RFT workout. This time the deoxyhemoglobin breakpoint occurred in Rd 7, indicating that they were able to push for a longer period before crossing their compensation point.
