The purpose of this paper is to present the computational model of muscles' group describing the movements of flexion/extension at the elbow joint in the sagittal plane of the body when the forearm is being kept in the fixed state of supination/pronation. The method of evaluating the muscle forces is discussed in detail. This method is the basis for the quantitative and qualitative verification of the proposed computational model of muscles' group. Applying this computational model, the forces of real muscles belonging to the muscles' group can be evaluated without using any optimization technique.