Precise assessment of HD-sEMG grid misalignment using nonlinear correlation

The aim of this paper is to perform a precise assessment of the misalignment of high-density surface electromyogram (HD-sEMG) grid according to its rotation with the muscle fibers. For this purpose, a generic and quantitative method was used in this study, namely the nonlinear correlation coefficient (h
²
) that is widely applied in connectivity and directionality of stochastic and complex signals. This approach is applied on simulated data through an (8×8) simulated electrode matrix placed on the simulated Biceps Brachii using a multilayered cylindrical model (muscle, fat tissue and skin tissue). Simulations with 5 different anatomies and 5 different motor unit recruitment patterns were computed at three constant contractions levels 20%, 40%, and 60% of the maximal voluntary contraction (MVC) for grid rotation from -10° to 10°, with a step of one degree according to muscle fibers. The obtained results show an important sensitivity of the nonlinear correlation parameter, namely the mean of the image matrix value, to the grid rotation angle. Furthermore, this parameter seems to be also sensitive to the contraction level and muscle anatomy.