We used frequency-domain (FD) near-infrared spectroscopy (NIRS) in a dual-slope (DS) configuration to non-invasively assess skeletal muscle hemodynamics in the human forearm muscle. Our objective is to leverage FD-NIRS data (intensity and phase) collected in DS mode (a combination of single-distance and multi-distance measurements) to differentiate superficial and deep tissue hemodynamics. FD-NIRS signals feature contributions from adipose tissue (AT), muscle tissue (MT), and possibly bone tissue, in cases of relatively thin AT and MT layers. We performed measurements of blood flow (BF) and oxygen consumption (OC) using venous occlusion and arterial occlusion protocols, respectively. Additionally, we performed theoretical simulations based on diffusion theory to guide the interpretation of our experimental results. First, we were able to show that our experimental results are consistent with a top layer (adipose tissue) that is more scattering than a bottom layer (muscle) and that absorption changes are greater in the top layer during venous occlusion whereas they are greater in the bottom layer during arterial occlusion, in agreement with previous results on different human subjects [C. Fernandez et al., J. Biomed. Opt. 28, 125004 (2023)]. Second, we started measurements on subjects featuring a range of thicknesses of adipose and muscle tissue to explore the feasibility of discriminating superficial and deeper hemodynamics using the full information content of the data collected with DS FD-NIRS.
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