Review of early development of near-infrared spectroscopy and recent advancement of studies on muscle oxygenation and oxidative metabolism.

Near-infrared spectroscopy (NIRS) has become an increasingly valuable tool to monitor tissue oxygenation (T) in vivo. Observations of changes in the absorption of light with T have been recognized as early as 1876, leading to a milestone NIRS paper by Jöbsis in 1977. Changes in the absorption and scatting of light in the 700-850-nm range has been successfully used to evaluate T. The most practical devices use continuous-wave light providing relative values of T. Phase-modulated or pulsed light can monitor both absorption and scattering providing more accurate signals. NIRS provides excellent time resolution (~ 10 Hz), and multiple source-detector pairs can be used to provide low-resolution imaging. NIRS has been applied to a wide range of populations. Continued development of NIRS devices in terms of lower cost, better detection of both absorption and scattering, and smaller size will lead to a promising future for NIRS studies.