Noninvasive near infrared (NIR) spectroscopic measurement of muscle oxygenation requires the penetration of
light through overlying skin and fat layers. We have previously demonstrated a dual-light source design and
orthogonalization algorithm that corrects for inference from skin absorption and fat scattering. To achieve
accurate muscle oxygen saturation (SmO<sub>2</sub>) measurement, one must select the appropriate source-detector
distance (SD) to completely penetrate the fat layer. Methods: Six healthy subjects were supine for 15min to
normalize tissue oxygenation across the body. NIR spectra were collected from the calf, shoulder, lower and
upper thigh muscles with long SD distances of 30mm, 35mm, 40mm and 45mm. Spectral preprocessing with the
short SD (3mm) spectrum preceded SmO<sub>2</sub> calculation with a Taylor series expansion method. Three-way
ANOVA was used to compare SmO<sub>2</sub> values over varying fat thickness, subjects and SD distances. Results:
Overlying fat layers varied in thickness from 4.9mm to 19.6mm across all subjects. SmO<sub>2</sub> measured at the four
locations were comparable for each subject (p=0.133), regardless of fat thickness and SD distance. SmO<sub>2</sub>
(mean±std dev) measured at calf, shoulder, low and high thigh were 62±3%, 59±8%, 61±2%, 61±4%
respectively for SD distance of 30mm. In these subjects no significant influence of SD was observed (p=0.948).
Conclusions: The results indicate that for our sensor design a 30mm SD is sufficient to penetrate through a
19mm fat layer and that orthogonalization with short SD effectively removed spectral interference from fat to
result in a reproducible determination of SmO<sub>2</sub>.