Intra-muscle synergies: Their role in the neural hierarchy
Open Access
- Author:
- Madarshahian, Seyedeh
- Graduate Program:
- Kinesiology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 26, 2022
- Committee Members:
- Robert Sainburg, Major Field Member
Chaleece Sandberg (she/her), Outside Unit & Field Member
Tarkeshwar Singh, Major Field Member
Mark Latash, Chair & Dissertation Advisor
Jonathan Bates Dingwell, Professor in Charge/Director of Graduate Studies
hyebin song, Minor Field Member - Keywords:
- Finger
Hand
Uncontrolled manifold
Referent coordinate
Hierarchy - Abstract:
- In motor tasks, it is important to explore the organization of motor units, as the smallest controllable elements and the last one characterized by abundance, which affords room for synergies. To this end, for the first time, we explored the organization of within-a-muscle MU-based synergies. First, we investigated the existence of stable groups (MU-modes) consisting of individual motor units with parallel scaling of their firing rates in flexor digitorium superficialis (FDS) muscle. Next, due to the compartmentalized composition of extrinsic hand muscles, we asked whether MU-modes are reflections of a subset of the four compartments of FDS. Further, we defined MU-modes simultaneously from the flexor (FDS) and extensor (extensor digitorum communis, EDC) muscles, which revealed two MU-modes with specific loading characteristics. Then, we investigated the effects of hand dominance on intra-muscle synergies as potential input into their neural origins. Lastly, we explored intra-muscle synergies during unintentional (reflex-based) force changes. To test our hypotheses, we asked subjects in each study to perform accurate cyclical force tracking tasks while pressing with the middle phalange of the fingers. The framework of the Uncontrolled manifold hypothesis (UCM) was used for quantifying synergies. The results showed the idea of synergic control is applicable to a single muscle. Also, MU-modes and compartments are not synonymous. The composition of the MU-modes allowed linking them to the reciprocal and co-activation commands within the theory of control with spatial referent coordinates. Laterality effects were only present in multi-finger synergies and not in MU-mode-based synergies. The results suggest the central role of segmental spinal mechanisms in intra-muscle synergies, along with trade-offs between synergies at different hierarchical levels.