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The aim of this study was to clarify the influence of subcutaneous adipose tissue (AT) thickness on nearinfrared (NIR) optical density and the penetration depth of light in muscle tissue in vivo. The thickness of adipose tissue in the leg was measured using ultrasonography in 12 young subjects. Optical densities (OD) at 775, 807, and 827 nm were measured when the distance between the light source and the detector was increased from 20 to 100 mm in 10 mm steps. Ultrasonography showed that AT thickness ranged from 4 to 10 mm. The OD increased with increasing distance between the light source and the detector in all subjects. At the same distance between the light source and the detector (30 mm), the OD values correlated negatively with AT thickness (r 5 20.79, −0.82, and −0.79 at 775, 807, and 827 nm, respectively). Ultrasonography also showed that only the extensor hallucis longus muscle (EHL), which is under the extensor digitrum longus muscle (EDL), was activated during the flexion of the big toe. In order to evaluate the penetration depth of NIR light, the depth of EHL was measured and the OD observed before and during flexion. When the distance between the light source and detector was set at 30 mm, the OD values before exercise ranged from 0.36 to 3.18 at 775 nm, from 0.19 to 2.43 at 807 nm, and from 0.15 to 1.60 at 827 nm. Changes in OD during exercise were detectable for all subjects, and the EHLs of the subjects were located 10.0 to 20.2 mm under the detector. However, when the light source-detector distance was set at 20 mm, changes in OD during exercise were detectable for only 2 subjects, whose AT thicknesses were 4.0 or 5.0 mm, and the EHLs of the subjects were 10.0 or 11.7 mm deep. At a distance of 40 mm, 9 out of 12 subjects showed changes in OD, and their AT thicknesses and EHL locations ranged from 6.4 to 10.0 mm and from 11.4 to 20.4 mm deep, respectively...
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