Abstract— the aim of the present study is to propose a subject-specific screening approach of High Density surface EMG (HD-sEMG) Probability Density Function (PDF) shape evolution in experimental conditions following a ramp exercise from 0% to 50% of the Maximum Voluntary Contraction (MVC) during 25 seconds of isometric contractions of the Biceps Brachii from six healthy subjects. This method uses High Order Statistics (HOS), namely the kurtosis and the skewness for PDF shape screening examined on selectively positioned Laplacian sEMG channels obtained on an 8x8 HD- sEMG grid. For each subject, the position of the Laplacian channels was chosen based on the level of muscle activation obtained from the Signal to Noise Ratio (SNR) matrix computed for the 64 sEMG signals of the grid in order to obtain independent Laplacian configurations localized in areas with high SNRs indicating high muscle activation. Afterwards, we used the Principal Component Analysis (PCA) to obtain the principal trend of the kurtosis and the skewness computed from the selected Laplacian signals according to force level variation. The obtained results show a globally common increasing HOS trend according to force increase from 0% to 50% MVC for all the subjects regardless of the anatomical, instrumental and physiological variability that usually strongly influences these trends. I. INTRODUCTION The surface electromyography (sEMG) is a noninvasive technique to measure the electrical currents generated in muscles during contraction following a neural command[1]. This complex electrophysiological signal depends on various parameters: anatomical, neural, instrumental…[1]. Another instrumental variability is the electrode arrangement (monopolar, bipolar) and location that have a strong influence on the measured sEMG signals. In recent years, HD-EMG recording techniques have emerged as an alternative for classical bipolar electrodes, thus, taking into account aspects of spatial distribution of potentials and eliminating the electrode position variability by covering the active muscle zone with a grid of electrodes with minimal inter-electrode spacing (high spatial resolution). Classical sEMG amplitude descriptors are generally based on the first and second order moments[1]. However, these parameters are not sufficient to study PDF shape variation of This work was carried out and funded in the framework of the Labex MS2T. It was supported by the French Government, through the program “Investments for the future” managed by the National Agency for Research (Reference ANR-11-IDEX-0004-02) M. AL Harrach, S. Boudaoud, D. Gamet, J.F. Grosset and F. Marin are with Université de Technologie de Compiègne, CNRS UMR 7338 BMBI, 60200 Compiègne, France, (e-mail: mariam.al-harrach@utc.fr, sofiane.boudaoud@utc.fr, didier.gamet@utc.fr, jean-francois.grosset@utc.fr frederic.marin@utc.fr). the sEMG signals since the assumption of the Gaussianity has been proven invalid in many contexts (contraction level, fatigue, electrode arrangement)[1][2]. Recent studies focused on the proficiency of High Order Statistics (HOS) in tracking the shape variation of the sEMG PDF and consequently these parameters have been used in EMG classification [3], force classification [1], muscle contraction determination [4]. Concerning the HOS/force relationship definition, even if some studies obtained promising results [1],[2],[5] in both simulation and experimentation, there is not yet a clear consensus on the nature of this relationship due to the described types of variability. This study describes a subject-specific method, including both Laplacian channel selection and PCA decomposition, for the evaluation of the HOS/force relationship on an experimental database collected from six subjects, using an 8x8 HD-EMG grid under isometric contractions during ramp exercise (0-50% MVC). II. MATERIALS AND METHODS A. Experimental Data Recording and Processing Six healthy male subjects (mean± std, age: 22.2±1.3 years; stature: 176.7±5.5 cm; body mass: 71.1±5.8 kg) participated in the study. The sEMG signals obtained from a two dimensional adhesive array consisting of 64 electrodes of circular shape were recorded from the Biceps Brachii (BB), during isometric voluntary contractions with the elbow flexed at 90°. The adhesive array was placed either proximally (~1.5 cm) from the main IZ (Innervation Zone) location supposed to be in the middle of the BB muscle (Fig. 1). The reference and the ground electrodes were placed on the elbow bone and around the wrist respectively. The 64 monopolar sEMG signals were acquired using TMSI REFA acquisition system (Netherlands), and recorded with specific software with a 2048 Hz sampling frequency. A force signal was measured simultaneously with the sEMG signals by a strain gauge, and displayed in real-time on an oscilloscope for visual feedback purpose to the subjects. The protocol begins by measuring the reference Maximum Voluntary Contraction (MVC) of the subjects after recording three MVC elbow flexion contractions lasting five seconds and choosing the highest as the reference. Then the subjects were requested to perform a series of isometric force ramps from 0%MVC to 50%MVC lasting 25 seconds by following the reference force ramp on the oscilloscope screen. Few ramps were performed before the beginning of the protocol to train the subjects to track the ramp target. Evaluation of HD-sEMG Probability Density Function Deformations in Ramp Exercise M. Al Harrach, S. Boudaoud, D. Gamet, J.F. Grosset and F. Marin 978-1-4244-7929-0/14/$26.00 ©2014 IEEE 2209