The features of superposition of central motor commands (CMCs) have been studied during generation of the “two-joint” isometric efforts by hand. The electromyogram (EMG) amplitudes which were recorded from the humeral belt and shoulder muscles have been used for estimation of the CMCs intensity. The forces were generated in the horizontal plane of the work space; the position of arm was fixed. Two vectors of equal amplitudes and close direction and their geometrical sum were compared. The hypothesis of the CMCs’ superposition in the task of the force vector summation has been examined. The directions of the constituent and resulting forces with satisfactory superposition of the CMCs were defined. Differ­ences in the co-activation patterns for flexor and extensor muscles of both joints were shown. The high level of the flexor muscles activity has been observed during extension efforts, while the flexion directions demonstrated much weaker activa­tion of the extensor muscles.