The problem of motor redundancy is well known as Bernstein's problem, named after the scientist who first posed the knotty problem in the early 1930s. At present, it is still a mystery how the central nervous system (CNS) solves the ill-posed problem of motor control. We recently made a discovery that links with influential hypotheses that the CNS 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. The concept of an A-A ratio provides a beneficial suggestion to many studies on muscle-synergy extraction and gives an important clue to solving Bernstein's problem of redundant degrees of freedom.
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