# NIRS-EMG for Clinical Applications: A Systematic Review
## 3 Near Infrared Spectroscopy
### Different NIRS Techniques  
In this section, we would like to give the reader a brief introduction to the different NIRS techniques available. They are based on different hardware configurations and the different kinds of parameters that can be extrapolated [23]. Continuous Wave NIRS (CW-NIRS) employs LEDs or lasers with constant intensity as light sources, and the main parameter acquired is the light attenuation caused by biological tissues. Its simple approach allows CW-NIRS based systems to be user-friendly, low cost, and, in many cases, portable, wearable, and wireless [54]. These characteristics make them the most widespread devices on the market for monitoring muscle oxygenation and metabolism in both sport and medical applications. However, this technique presents a limited depth sensitivity and an inability to decouple the absorption from the scattering contribution. This means that we can retrieve just the variations of O\({}_{2}\)Hb and HHb with respect to a baseline (arbitrarily set to zero) and not an absolute value [55]. Since, in CW measurements, it is not possible to retrieve the absolute value of the reduced scattering coefficient, a constant a priori DPF has to be taken from literature. However, the DPF depends on many factors, such as the wavelength, the source-detector distance, and the inter-subject variability [23]. Another limitation is that it is not possible to calculate SO\({}_{2}\) with this method. In order to overcome these limitations, it is necessary to perform measurements at different source-detector distances (Spatially Resolved NIRS, SR-NIRS) which allows to obtain, by doing some approximations (i.e., scattering linearly dependent on wavelength), a parameter called the Tissue Oxygenation Index (TOI) or Tissue Saturation Index (TSI) [56]. Furthermore, in the CW measurements, the acquired signal is contaminated by the more superficial layers: The contributions of the skin, superficial capillary bed, and fat layer cannot be decoupled with a single measurement. In order to overcome this issue, it is necessary to perform more measurements--for example, at different wavelengths or different inter-fiber distances [57]--making the hardware more complex or increasing the total measurement time. A comprehensive list of CW-NIRS devices can be found in the review from [PERSON] and [PERSON] [5]. Some approaches, such as Frequency Domain (FD)- or Time Domain (TD)-NIRS, overcome the disadvantages of the CW-NIRS measurements. In particular, these approaches can be exploited to retrieve absorption and scattering coefficients independently, allowing the calculation of the absolute values of O\({}_{2}\)Hb, HHb, and DPF. FD-NIRS is based on amplitude-modulated light sources, with modulation frequencies ranging from 100 MHz up to 1 GHz. The signal exiting the tissue is demodulated, in order to obtain amplitude and phase as a function of the frequency. In fact, while travelling into the tissue, light amplitude and phase, at different modulation frequencies, depend on the absorption and scattering properties [58]. These instruments are quite complex and not widespread, both in research laboratories and on the market. To our knowledge, the only available FD devices specifically developed for muscle applications are the OxiplexTSTM[5] and its clinical version OxiplexTS200 TM, made by the ISS company [59]. TR-NIRS instruments typically employ short laser pulses with duration of tens of picoseconds as light sources. The injected pulses are delayed and broadened in time, since scattering causes photons to follow different randomized paths inside the tissue [60]. Furthermore, the pulses are attenuated due to tissue absorption. Overall, the signal collected at a certain source-detector distance is given by the photon Distribution of Time Of Flight (DTOF) during their travel inside the diffusive medium.