Muscle Oxygenation Trends of Deep Versus Superficial Muscles

Identifying Changes in Muscle Coordination & Recruitment During Maximal Effort Exercise

The other day I had one of my rowers (lets call him Steve) do a 2k row for time at a fixed stroke rate (32–36 SPM). He rowed the 2k in 6:22, and in the picture above you can see the muscle oxygenation trends from his vastus lateralis and rectus femoris muscles. You’ll notice that the desaturation curves are quite different between these two heads of the quadricep, which may seem perplexing at first glance.

In order to understand why this is the case we need to appreciate the fact that NIRS allows us to sample muscle oxygenation in both deep and superficial muscles. Deeper muscles tend to have greater capillarity and blood flow than superficial muscles, and they also tend to be more oxidative. As a result, deeper muscles present with slower rates of oxygen desaturation than superficial muscles at a given power output, and they also have higher nadirs.

Additionally, deep and superficial muscles have different oxygen transport strategies. Deep muscles rely more of perfusive transport and superficial muscles rely more on diffusive transport. This is an often under appreciated aspect of oxygen transport and metabolic control, and can account for much of these differences between the vastus lateralis (superficial muscle) and rectus femoris (deep muscle) trends above.

In addition to different active muscles having different oxygen kinetic profiles, we also need to consider how changes in coordination and recruitment impact muscle oxygenation trends as fatigue set in. For example, a rower may start with more of a knee flexion dominant pattern and as the knee flexors fatigue they may rely more on hip extension to power their stroke. This would result in less oxygen extraction in the VL as fatigue onsets and more oxygen extraction in the rectus femoris. As a result, monitoring oxygen saturation in just one working muscle can potentially misrepresent the status of overall systemic energy reserves.

So, going back to Steve’s muscle oxygenation trends, we see that deoxygenation occurs in both the VL and RF as soon as the activity starts. Within the first ~600m muscle oxygen saturation in both the VL and the RF reach a low point (though the VL plateaus at a lower SmO2), and this low SmO2 (muscle oxygen saturation) level is maintained up to ~1300m, indicating that Steve is working at a maximum steady state between ~600m to ~1300m. However, once Steve hits the 1300m mark we see that muscle oxygen saturation in the VL begins to climb and while SmO2 in the RF begins to drop down further. This indicates a change in coordination and recruitment which is a unconscious strategy employed to keep Steve powering through his event while his ‘primary’ working muscles begin to fatigue. By observing this we can better understanding the energetic constraints of Steve’s event and how he copes with those demands, which will allow for greater precision in his training, even preparation, and pacing on subsequent events.