Introduction to NIRS: Turning Over Classic Paradigms of Energy System Testing
NIRS stands for near infrared spectroscopy. At the simplest level, a NIRS device consists of a light source emitting two or more wavelengths of light in the near-infrared range of 650-1000 nm and a detector placed at a known distance from the light source. Near infrared light is able to penetrate biological tissues with less scattering and absorption than visible light and consequently offers advantages for imaging and quantitative measurements. Additionally, NIRS measurements can be used to reflect the balance of oxygen delivery to the working muscles and oxygen consumption in the capillary beds. This makes NIRS a very useful tool for assessing two of the major determinants exercise capacity, which are oxygen delivery and utilization respectively. It also gives us a practical way to test classic theories of exercise performance. Below you'll find a quote from a prominent energy system training course: "As a rule of thumb, the closer an event is to one minute the lower the aerobic contribution to overall performance will be. The opposite is also true: the longer the event, the more dominant the aerobic system will be".
In light of classic views of energetics, this may seem like it makes sense. Particularly when you believe that sprinting is 'anaerobic' and marathons are 'aerobic' for example. Yet, that doesn't line up with the contemporary scientific literature, or what we can observe in live time.
In the picture above you'll see an athletes response to an all out sprint on the assault bike. Notice that oxygen starts being utilized the second they begin sprinting, once once O2 bottoms out performance stalls (wattages drop).
How might this information change the dominant training paradigm? If oxygen is always a part of energy production and lactate is always present, how can you ever do 'anaerobic a-lactic' training ?