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

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.

Robustness and Reliability of Synergy-Based Myocontrol of a Multiple Degree of Freedom Robotic Arm

“…Since the speed-accuracy tradeoff has been widely proposed as an objective measure for real-time performance [39]- [41], this result validates the use of IP, derived from the linear relationship between time and difficulty of the task, to quantify the control performance in the current study.…”

Real-time behaviour synthesis for dynamic hand-manipulation

“…An up-to-date discussion on motion capture based hand manipulation can be found in [16] B. Dimensionality reduction and synergy based control Although rich and diverse, animal forms exhibit characteristic movements that can be attributed to its morphology, neural system and habitat. Researchers have ascribed these to bio-muscular and neural factors [17] [18]. Low dimensional embedding has been found at the level of kinematics [19], instantaneous muscle activity [20], spatio-temporal muscle activity [21] and feedback control law [22].…”

Neural basis for hand muscle synergies in the primate spinal cord

“…This hypothesis is phenomenologically supported by experimental observations that a wide range of behaviors can be decomposed into basic patterns of muscle activity, namely "muscle synergies" (2)(3)(4)(5). However, there remains heated debate regarding whether these observations reflect a neurophysiological basis or are an epiphenomenon of task constraints and biomechanics (6)(7)(8). This controversy stems from a lack of direct evidence for (or against) the neural implementations of muscle synergies in the CNS (9)(10)(11).…”