Noninvasive measurements of skeletal muscle hemodynamics using frequency-domain near-infrared spectroscopy: contributions from adipose and bone tissues.

SIGNIFICANCE: Noninvasive optical assessments of skeletal muscle perfusion and metabolism are crucial for studying muscle physiology. However, measurements can be affected by contributions from overlying adipose and underlying bone tissues.

AIM: We aim to investigate the influence of bone tissue on frequency-domain near-infrared spectroscopy (FD-NIRS) measurements of the human forearm using different data types (intensity and phase), source-detector separations (25 and 37 mm), and acquisition modes (single-distance and dual-slope).

APPROACH: FD-NIRS data were collected on the forearms of 14 healthy subjects using a custom optical probe designed to regulate probe-skin contact pressure. Tissue blood flow and oxygen consumption were measured with venous and arterial occlusion protocols, respectively. In vivo findings were compared with diffusion theory simulations for two- and three-layer media. Ultrasound imaging quantified adipose tissue thickness (1.5 to 9.5 mm) and bone depth (14 to 26 mm).

RESULTS

Experimental results for bone depths below 20 mm could not be reproduced by two-layer simulations, whereas they aligned well with three-layer simulations mimicking adipose, muscle, and bone tissues. Adipose tissue and bone contributions depend on data type, measurement mode, and source-detector separation.

CONCLUSIONS

Both adipose and bone tissues may contribute to noninvasive FD-NIRS measurements of skeletal muscle, depending on tissue anatomy and data collection methods.