Intra-muscle Synergies Stabilizing Reflex-mediated Force Changes

We used the framework of the uncontrolled manifold hypothesis to explore force-stabilizing synergies and motor equivalence in the spaces of individual motor unit (MU) firing frequencies. Healthy subjects performed steady force production tasks by pressing with one finger or three fingers of a hand. Surface EMG was used to identify individual MU action potentials. MUs formed stable groups (MU-modes) with parallel scaling of the firing frequency in both flexor digitorum superficialis (FDS) and extensor digitorum communis (EDC) that allowed identifying them with the reciprocal and coactivation commands. Smooth lifting of the fingers by an “inverse piano” device led to an unintentional, reflex-based force increase. There was significantly larger motion in the space of MU-modes that kept the force unchanged (motor equivalent) compared to motion that changed force (non-motor equivalent). The force change was stabilized by co-varying contributions of the MU-modes defined separately for FDS and EDC. In contrast, analysis of the three-finger task in the space of individual finger forces showed no synergies stabilizing total force change. Effects of hand dominance were seen on multi-finger synergies but not intra-muscle synergies. We conclude that spinal mechanisms, such as recurrent inhibition and reflex loops from proprioceptors, contribute significantly to intra-muscle synergies, while multi-finger synergies reflect supra-spinal processes. These results provide methods to explore the contributions of spinal vs supraspinal circuitry to changed motor synergies in populations with a variety of neurological disorders.