Blood flow occlusion-related O2 extraction reserve is present in different muscles of the quadriceps but greater in deeper regions after ramp-incremental test

It was recently demonstrated that an O-2 extraction reserve, as assessed by the near-infrared spectroscopy (NIRS)-derived deoxygenation signal ([HHb]), exists in the superficial region of vastus lateralis (VL) muscle during an occlusion performed at the end of a ramp-incremental test. However, it is unknown whether this reserve is present and/or different in magnitude in other portions and depths of the quadriceps muscles. We tested the hypothesis that an O-2 extraction reserve would exist in other regions of this muscle but is greater in deep compared with more superficial portions. Superficial (VL-s) and deep VL (VL-d) as well as superficial rectus femoris (RF-s) were monitored by a combination of low- and high-power time-resolved (TRS) NIRS. During the occlusion immediately post-ramp-incremental test there was a significant overshoot in the [HHb] signal (P < 0.05). However, the magnitude of this increase was greater in VL-d (93.2 +/- 42.9%) compared with VL-s (55.0 +/- 19.6%) and RF-s (47.8 +/- 14.0%) (P < 0.05). The present study demonstrated that an O-2 extraction reserve exists in different pools of active muscle fibers of the quadriceps at the end of a ramp exercise to exhaustion. The greater magnitude in the reserve observed in the deeper portion of VL, however, suggests that this portion of muscle may present a greater surplus of oxygenated blood, which is likely due to a greater population of slow-twitch fibers. These findings add to the notion that the plateau in the [HHb] signal toward the end of a ramp-incremental exercise does not indicate the upper limit of O-2 extraction. NEW & NOTEWORTHY Different portions of the quadriceps muscles exhibited an untapped O-2 extraction reserve during a blood flow occlusion performed at the end of a ramp-incremental exercise. In the deeper portion of the vastus lateralis muscle, this reserve was greater compared with superficial vastus lateralis and rectus femoris. These data suggest that the O-2 extraction reserve may be dependent on the vascular and/or oxidative capacities of the muscles.