Concentric-ring electrode systems for noninvasive detection of single motor unit activity

New recording techniques for detecting surface electromyographic (EMG) sign als based on concentric-ring electrodes are proposed in this paper. A theor etical study of the two-dimensional (2-D) spatial transfer function of thes e recording systems is developed both in case of rings with a physical dime nsion and in case of line rings. Design criteria for the proposed systems a re presented in relation to spatial selectivity. It is shown that, given th e radii of the rings, the weights of the spatial filter can be selected in order to improve the rejection of low spatial frequencies, thus increasing spatial selectivity. The theoretical transfer functions of concentric syste ms are obtained and compared with those of other detection systems. Signals detected with the ring electrodes and with traditional one-dimensional and 2-D systems are compared. The concentric-ring systems show higher spatial selectivity with respect to the traditional detection systems and reduce the problem of electrode loca tion since they are invariant to rotations. The results shown are very prom ising for the noninvasive detection of single motor unit (NW) activities an d decomposition of the surface EMG signal into the constituent NW action po tential trains.