Topographical charakterization of the upper facial musculature revealed by means of high-density surface electromyography

This study's aim was to gain topographical information of the upper facial musculature on a motor unit level, i.e. detection of motor units’ (MU) motor endplate position and muscle fiber orientation. High density surface electromyography (HDsEMG) data of 15 healthy subjects were analyzed, which were recorded during selective contraction of four upper facial muscles (Orbicularis oculi (OOC), corrugator supercilii (CSC), procerus (PRO) and frontalis (FRO) muscle). The EMG signals were decomposed into the contributions of single MUs by the CKC-method (convolutional kernel compensation). The decomposed motor unit action potentials revealed the localization of motor endplates as well as fiber directions. Afterwards, the results were visually respresented in a schematic mean face, which was created by averaging 3D-face scan data. Besides the MUs of the four deliberatly contracted muscles, MUs of depressor supercilii muscle (DSC) were identified as well. The results of the recorded MUs' muscle fiber directions conform to descriptions and drawings in anatomical textbooks and literature. The analysis shows motor endplates to be mainly clustered in eccentric positions towards the muscles' origin. However, OOC is an exception, as its endplates are distributed as clusters over the entire muscle. These results contribute to basic anatomical and neurophysiological knowledge of the complex facial muscle system and may advance surgical, non-surgical and aesthetic therapies in the area of the forehead and glabella.