# Novel Technologies Used in the Assessment of Patellofemoral Pain: A Scoping Review
## 4 Discussion
### EMG Studies  
The number of EMG studies demonstrates the importance of assessing muscle forces in PFP. These studies reported on a wide range of motor tasks and of muscle groups, also including those around the hip and the ankle joints, and both flexors and extensors. However,standard data collection methods and systems were used. Only two studies [27, 28], though the same research team reported the utilization of original high-density surface EMG (HDsEMG) electrodes; this technology enables the placement of several tens of electrodes for each single muscle. HDsEMG is a linear electrode grid array with 16 silver bar electrodes, with a 10 mm interelectrode distance (OT Bioelettronica, Torino, Italy). Each grid comprised 64 electrodes arranged in 5 columns and 13 rows with a single electrode missing in one of the corners. This electrode is claimed to provide information about innervation zones and anatomical factors for the best estimation of neuromuscular activation and regions. Using these electrodes, data from the vastus medialis and vastus lateralis were collected, since these can be placed between the edges of each muscle [27, 28]. Acknowledged advantages of HDsEMGs with respect to traditional bipolar electrodes are a topographical representation of the EMG amplitude, more selective recordings, and reliable motor unit behavior [56, 57]. Another study from the same team aimed to investigate whether Principal Component Analysis (PCA) and non-Negative Matrix Factorization (NMF) can provide similar results when used to identify regional muscle activation from HDsEMG signals and showed that a single factor from either the NMF or PCA explained on average 70% of the variance across channels [58]. They found that up to 30% of the variance was explained by regional variations in muscle activity rather than common fluctuations across the muscle, suggesting that a single bipolar electrode would not fully capture the EMG across the whole muscle.  
HDsEMGs differ from conventional sEMGs in terms of their scopes. Conventional sEMGs are mainly used in movement studies by yielding concurrent information on activity in different muscles. With HDsEMGs, it is possible to subtract information at a single motor unit, and also muscle-fiber conduction velocity can be gathered as in the needle EMG [59]. Surface electromyographic amplitude is influenced by factors such as adipose tissue thickness, normalization, cross-talk, and motor unit action potential cancellations [27]. As signals are collected from different locations of the muscles, HDsEMG helps to overcome the limitations such as describing activation of the regions within a muscle and taking into account the effect of location of the innervation zone [28]. For example, in the selected study, [PERSON] et al. showed that females with PFP have simpler VM and VL activation strategies, observed as a lower co-activation of regions between VM and VL and a lower redistribution of activation from VL to VM when the concentric and eccentric phases of the knee extension are compared by using this novel electrode [28]. HDsEMG is an electrode that needs uniformity from different perspectives since it is still in a very early stage in terms of clinical applications and the existence of different names for HDsEMGs (high-resolution EMG, multi-array EGM, matrix electrodes).  
Despite the obvious experimental issues, these novel electrodes may provide relevant information regarding femur muscle characteristics in different motor tasks.