Skeletal Muscle Blood Flow and NIRS Oxygenation Kinetics as a Tool to Evaluate Adaptations to High-Intensity Exercise...

Exercise training improves maximum aerobic capacity, in part, through improvements in skeletal muscle function. This study aimed to investigate adaptations to improved aerobic capacity training through non-invasive and non-exhaustive tests of hyperemic muscle blood flow and near-infrared spectroscopy (NIRS) muscle oxygenation kinetics. An experimental study was conducted on 18 participants (age, 28.2 ± 5.3 yr; _absVO_2max, 3.60 ± 0.67 L·min^-1). Before and after the intervention of a 6-week of high-intensity interval training (HIIT), participants underwent three tests: (1) a graded cardiopulmonary exercise test; (2) a vascular occlusion test; and (3) a steady-state exercise (SSE) at 60% of PPO. Expired gas analysis, superficial femoral blood flow (occlusion test only) and SmO₂ kinetics were measured. The intervention increased maximal aerobic capacity _absVO_2max (p < 0.001, d = 0.65) and PPO (p < 0.001; d = 0.41). Moreover, steady-state _absVO₂ (p = 0.006; d = 0.37) and HR (p = 0.001; d = 0.65) were reduced. With the cuff test, the SmO₂ desaturation slope increased (p = 0.04; d = 0.52), while peak muscle blood flow (p = 0.02; d = 0.51) and the SmO₂ 10 s reoxygenation rate increased (p < 0.001 d = 1.11; 0.74 ± 0.28 to 1.17 ± 0.45%/s). During steady-state exercise, SmO₂ decreased less (p = 0.02; d = 0.43), and the 10s recovery kinetics rate was slowed (p = 0.01 d = 0.30; 0.28 ± 0.20 to 0.22 ± 0.21%/s). The improvement in VO_2max had a moderate correlation with the SmO₂ recovery rate post-steady-state exercise (p = 0.05, r = -0.54). HIIT changed maximal aerobic capacity alongside improvements in skeletal muscle hyperemic blood flow, SmO₂ post-occlusive reactive hyperemia and SmO₂ post-exercise recovery kinetics. Thus, the findings indicated that non-invasive and non-exhaustive hemodynamic kinetic profiles can monitor adaptations to improved aerobic capacity.