The summation of the motor units action potentials (MUAPs) in the rat medial gastrocnemius muscle was studied. Experiments were performed on anesthetized animals and single motor units (MUs) were functionally isolated by electrical stimulation of thin fi laments of ventral roots. MUAPs were recorded with two silver wires inserted into the muscle. Algebraic sum of the MUAPs recorded from two to four individual MUs were compared to the action potentials recorded during their simultaneous stimulation. The peak-to-peak amplitude, the longitudinal center of the signal square, the number of phases and turns, correlation coeffi cient and mean-square error were measured. In all studied cases of summation the number of phases and turns were the same in the two compared signals. When summation effects of two MUs were compared, there were no signifi cant differences in the peak-topeak amplitude. When 3 and 4 MUs were co-activated signifi cant differences between a sum of individual MUAPs and the recorded action potential were noted in some cases. The most variable parameter was the longitudinal center of the signal square which was the major source of differences. It was also shown that in a case of contractions of low force MUs the studied parameters of the algebraic sum of their MUAPs was similar to parameters of action potentials recorded during simultaneous stimulation.
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