Despite many years of research, the issue of load sharing among muscles acting on the same joint is still unresolved and very controversial. There is a monotonic relationship between the EMG amplitude of a specific muscle and its force. Such relationship depends on the specific anatomical (subcutaneous thickness, fiber orientation, etc.) and detection conditions (electrode location, inter-electrode distance) and recruitment modality of motor units (random, superficial to deep, deep to superficial). The force produced by a specific muscle cannot be measured and what is measured is the total force provided by all the active muscles acting on a joint, including agonists and antagonists The first part of this work (ch 3) addresses the issue of load sharing by proposing two possible approaches and testing them. The second part (ch. 4 and 5) addresses two applications of surface EMG focusing on the study of a) muscle relaxation associated to Yoga sessions and b) the activation of muscle of the back and shoulder of musicians playing string instruments (violin, viola and cello).

In both parts the element of innovation is the use of two dimensional electrode arrays and of techniques based on EMG Imaging. The objectives of this work are presented and explained in chapter 1 while the basic concepts of surface EMG are summarized in chapter 2. Different EMG-based muscle force models found in the literature are explained and discussed. Two renowned amplitude indicators in surface EMG (sEMG) studies are the average rectified value (ARV) and the root mean square (RMS). These two amplitude indicators are computed over a defined time window of the recorded signals to represent the muscle activity. In the second chapter, ARV and RMS are defined in both time (1D) and space (2D). The advantages and disadvantages of RMS and ARV are compared and discussed for a simple sinusoid as well as for more complex signals (simulated motor unit