On Human Motor Coordination: The Synergy Expansion Hypothesis

The search for an answer to Bernsteins degrees of freedom problem has propelled a large portion of research studies in human motor control over the past six decades. Different theories have been developed to explain how humans might use their incredibly complex neuro-musculo-skeletal system with astonishing ease. Among these theories, motor synergies appeared as one possible explanation. In this work, the authors investigate the nature and role of synergies and propose a new theoretical framework, namely the "expansion hypothesis", to answer Bernsteins problem. The expansion hypothesis is articulated in three propositions: mechanical, developmental, and behavioral. Each proposition addresses a different question on the nature of synergies: (i) How many synergies can humans have? (ii) How do we learn and develop synergies? (iii) How do we use synergies? An example numerical simulation is presented and analyzed to clarify the hypothesis propositions. The expansion hypothesis is contextualized with respect to the existing literature on motor synergies both in healthy and impaired individuals, as well as other prominent theories in human motor control and development. The expansion hypothesis provides a novel framework to better comprehend and explain the nature, use and evolution of human motor skills. Significance StatementUnderstanding how humans effortlessly control coordinated movements has been a long-standing challenge in neuroscience. This research introduces the "expansion hypothesis", a new framework to explain how we develop, learn, and use motor synergies - coordinated activities of multiple features such as joints and muscles - that simplify movement control. By breaking down the nature of these synergies into mechanical, developmental, and behavioral aspects, this study offers novel insights into how our brains and bodies work together to achieve fluid motion. This work not only advances the scientific understanding of human motor control but also has potential implications for improving rehabilitation strategies for individuals with movement impairments and for developing more dexterous human-inspired robotic control techniques.