# NIRS-EMG for Clinical Applications: A Systematic Review
## 5 Discussion and Conclusions  
Performing a search on PubMed Central by employing \"NIRS and EMG\" as keyword combinations, a list of 491 studies, from the year 2000 onwards, was found. This number indicates that the combined use of the two techniques has been an explored field over the last 20 years. However, from the critical analysis of the results of this review, as shown in Section 4, it has emerged that the combined use of EMG and NIRS has been only partially exploited in clinical practice for the assessment and evaluation of patients with muscular dysfunction. In fact, only 17 studies matched the research criteria. The first one was dated 2005, but only in the last 7-8 years, the number of papers combining the two techniques started to increase, as is shown in the road-map in Figure 1. This is due, in our opinion, to the fact that NIRS is a relatively recent technique and it is difficult to find, in the clinic, both NIRS and EMG instruments. Figure 1: Road map of the papers found for the NIRS and EMG, NIRS \(>\) EMG and NIRS and EMG HW research. Furthermore, the use and interpretation of data coming from both techniques requires the employment of technical staff, which is often not available in the clinical environment. In this perspective, groups of specialized personnel have to be recruited and employed with a consequent investment, which is not always bearable; especially within public structures. The development of instruments embedding both type of sensor has started only recently (the first instrument was released in 2015) and, if the research and the market go ahead in this direction, we expect the number of studies to increase remarkably. The fact that dedicated hardware has been released indicates that the joint use of NIRS and EMG will be probably exploited in the near future, in research and in clinical practice. Nevertheless, from a hardware point of view, the combined used of EMG and NIRS has been well-assessed and the integration of the two techniques is not compromised by issues related to signal interference, considering the differences in the signal nature; electrical and optical, respectively. The same level of integration has not been found in the data analysis so far. In fact, according to the studies screened in this review, while many parameters have been separately extrapolated from both techniques, no merged outcome measures have been proposed. In the literature, we can find very few examples such as that of [PERSON] et al., who proposed a feature, named NIRSRMS, which combines a weighted NIRS signal with the RMS value of EMG signals [87; 93]. In our opinion, the biggest challenge for the future of the clinical application of combined NIRS-EMG sensors will be the interpretation of the combination of these two signals, in order to provide a clear picture of the oxidative and electrical status of the muscle to medical professionals. This process will take place in two main lines: First, evaluating how to merge the parameters extracted by the two techniques; or, secondly, by establishing evidence of the relationships between NIRS- and EMG-based measures. The combination of these two techniques provides some issues. First, there is still a numerical decoupling, in terms of number of employed sensors. The EMG electrodes are smaller than the actual NIRS optodes. Furthermore, both techniques are based on \"integral\" measures, which do not guarantee that the same volume of tissue is mapped, even if effort is dedicated to the careful placement of sensors. This means that, in general, the mapping of the muscle with a certain number of EMG probes might not be completely consistent with same number and positioning of NIRS probes. Recently, with the advent of the 3D-print technologies, it is possible to create custom probes for the measurements allowing for a better organization of the optodes, but the physical dimensions of the optical fibers still limit the number of NIRS probes which can be used at the same time.