We made a discovery that links with influential hypotheses that the central nervous system may produce movements by combining units of motor output. This paper introduces the key concept we call the "A-A ratio," which is the EMG ratio of agonist-antagonist muscle pairs. The statistical analysis based on the A-A ratio specifies that (1) human lower limb movement during walking is explained as the superposition of a few modular units, and that (2) decomposed modules encode the kinematic information of body movement. The results also clarifies that various hypotheses, such as the muscle-synergy hypothesis, the population-vector hypothesis, and the convergent-force-fields hypothesis, are different interpretations of a common equation derived from our analysis. © 2010 IEEE.