When multiple become one: understanding the coordination of degrees of freedom in synergies in goal-directed pointing movements

In our daily lives, we make hundreds of movements with our body to achieve the daily goals we set for ourselves. Although we perform such movements with relative ease, underlying this performance is the complex control of a tremendous number of elements, called degrees of freedom (DOF). For instance, thousands of neurons, dozens of muscles, and several joint-angles are involved when grasping a cup of coffee from our desk. Fascinatingly, in our daily life movements, we are rarely aware of all these DOF, and how we should control them to perform a movement. We do not focus on which muscles need to be activated to displace the arm—we just do it.

In my thesis, I examined the idea that DOF in our body are functioning in groups, i.e., coordinated in so-called synergies. Interestingly, there is an ongoing debate about how this coordination of DOF in synergies should be regarded. One approach suggests that DOF are always coordinated in the same fixed groups, while another approach claims DOF coordination is dynamic and synergies can change, e.g., across conditions. In the thesis, I examined these different characterizations of synergies to assess how DOF are coordinated during goal-directed pointing movements in different conditions. Results consistently showed that different synergies were used for pointing movements in different conditions, suggesting that the coordination of DOF in synergies is dynamic. Future research should further examine how these dynamic synergies come about, which might open up insights to improve current training programs, e.g., in rehabilitation practice.