Muscle Oxygenation, Muscle Fiber Composition, & Talent Selection

Can You Assess Muscle Fiber Composition With NIRS? If So, What Are The Implications?

In a paper titled, “Influence of muscle fibre composition on muscle oxygenation during maximal running” investigators sought to find the relationship between muscle oxygen saturation (SmO2) levels during maximal effort running and muscle fiber composition. Using a combination of NIRS and muscle biopsies from the vastus lateralis muscle, they found a significant correlation between minimum SmO2 during max effort running and muscle fiber composition. Specifically, they found that athletes with higher % ‘s of oxidative fibers maintained higher SmO2 levels at the point of failure, and that athletes with a low % of oxidative fibers had much lower SmO2 levels at failure.

One reason for this observation may be that oxidative fibers have greater capillary density, and vascular conductance, and as a result improved oxygen delivery compared to glycolytic fibers. On the flip side, glycolytic fibers have a lower metabolic efficiency, and greater oxygen extraction compared to oxidative fibers, which may be why atheltes with a higher % of glycolytic fibers had lower SmO2 levels at failure.

Of course, there are some limitations with this study since the investigators did not look into the differences in muscle capillarization, oxidative enzyme capacity, etc between participants, and the sample size in the study was relatively small as well. However, we can still conclude that higher SmO2 levels at exhaustion during maximal effort running are at least *correlated* with a higher proportion of oxidative fibers, and as a result muscle fiber composition may potentially be [ball park] estimated non-invasively from SmO2/ O2HB levels during maximal steps tests to failure.

This may be useful from a talent identification standpoint. For example, we see that the athelte with 75% oxidative fibers has a big oxygenated hemoglobin break point at ~85% of their maximum running speed and once they cross that speed threshold their oxygen utilization outstrips their oxygen supply and ΔO2HB becomes very negative (red arrows pointing this trend out). Based on this limited information, we can infer they won’t likely excel in a marathon, whereas the athletes with a higher % of oxidative fibers are able to maintain a higher % of their maximum running speed before they start to outstrip their oxygen supply (demonstrated for the 86.3% oxidative fiber athlete with blue arrows).

If we can quickly screen atheltes and identify what distances they may excel in most, then we can look to identify what their energetic and sport specific limitations are as well, so rather than trying to fight mother nature and force them into an event they’re not suited for we can augment the traits they already possess and train them in a more strategic and targeted fashion.