Challenges and New Approaches to Proving the Existence of Muscle Synergies of Neural Origin

TLDR

Muscle coordination studies consistently reveal low‑dimensional muscle activation patterns, and these basis vectors—muscle synergies—are thought to reflect neurally defined functional groupings, yet proving or falsifying this hypothesis has proven challenging. The study employs cadaveric experiments and computational modeling to conduct a thought experiment that offers an alternative explanation for the emergence of muscle synergies without invoking neural grouping. By modeling limb biomechanics and assuming each muscle independently resists length change, the authors demonstrate that low‑dimensional EMG synergies can arise purely from mechanical constraints. These results demonstrate that limb biomechanics inherently constrain musculotendon length changes to a low‑dimensional subspace, that independent muscle resistance generates EMG synergies without neural coupling, and that similar dimensionality constraints exist in isometric force production, thereby suggesting new experimental avenues to test the muscle‑synergy hypothesis.

Abstract

Muscle coordination studies repeatedly show low-dimensionality of muscle activations for a wide variety of motor tasks. The basis vectors of this low-dimensional subspace, termed muscle synergies, are hypothesized to reflect neurally-established functional muscle groupings that simplify body control. However, the muscle synergy hypothesis has been notoriously difficult to prove or falsify. We use cadaveric experiments and computational models to perform a crucial thought experiment and develop an alternative explanation of how muscle synergies could be observed without the nervous system having controlled muscles in groups. We first show that the biomechanics of the limb constrains musculotendon length changes to a low-dimensional subspace across all possible movement directions. We then show that a modest assumption—that each muscle is independently instructed to resist length change—leads to the result that electromyographic (EMG) synergies will arise without the need to conclude that they are a product of neural coupling among muscles. Finally, we show that there are dimensionality-reducing constraints in the isometric production of force in a variety of directions, but that these constraints are more easily controlled for, suggesting new experimental directions. These counter-examples to current thinking clearly show how experimenters could adequately control for the constraints described here when designing experiments to test for muscle synergies—but, to the best of our knowledge, this has not yet been done.