A noninvasive EMG technique for investigating the excitation propagation in single motor units

A new EMG recording technique which is based on a spatially filtering surface multi-electrode array enables, with its high spatial resolution, the noninvasive investigation of single motor unit (MU) excitations. This recording technique was applied to study the propagation velocity within single, superficial MUs of lower arm muscles. The conduction velocity (CV) was estimated by time shifts between action potentials (APs) recorded at different positions along the muscle fibres. Several signal characteristics to define this time shift were compared in regard of their influence on the reliability of CV values. Best results were obtained by calculating the time shift between main peak maxima, with a standard deviation found for the CV values from single APs in the range of 4 to 25%. The disturbance by signal noise could be reduced by the application of approximating cubic spline functions. The CV values showed a remarkable variability between different sites along the single MU muscle fibres.