Why Does Deoxygenation of the Vastus Lateralis Muscle Attenuate at High Intensity?

Abstract

Pulmonary O₂ uptake (VO₂) increases almost linearly during incremental cycling exercise, whereas deoxygenation of the vastus lateralis muscle (VL), measured by near-infrared spectroscopy, develops continuously and then attenuates before reaching VO₂ peak (VO_2peak). The mechanisms behind these are unclear. In previous studies, it was speculated that muscle deoxygenation reached maximal deoxygenation or that the attenuation was led by the recruitment of fast twitch muscle fibers. However, these theories were not supported by studies using arterial occlusion method or surface electromyography. In contrast, the changes of body posture and inspired O₂ fraction during the exercise influenced the intensity of attenuation of muscle deoxygenation, suggesting that the attenuation of muscle deoxygenation was partly related to muscle circulation, muscle metabolism, or both. One question was why VO₂ increases almost linearly until VO_2peak despite the plateau of muscle deoxygenation. One of the hypotheses was that deoxygenation of the working muscles other than VL might compensate for that of VL. In fact, deoxygenation of the rectus femoris muscle developed at high intensity. These results suggested that attenuation of muscle deoxygenation was partly related to muscle circulation, muscle metabolism, and the balance among deoxygenation of the other working muscl