Kinematic synergies in upper-limb drawing tasks: Effects of geometric complexity on fine motor control

A quantitative assessment of fine motor abilities, as well as the design and optimisation of prosthetic and rehabilitation exoskeletons, can be supported by joint kinematic synergy analysis. However, extant research has primarily focused on discrete movements, offering limited insight into the coordination mechanisms underlying continuous fine motor tasks. This has resulted in considerable challenges with regard to practical implementation in both clinical and engineering contexts. The present study investigated the spatiotemporal characteristics of upper-limb joint synergies during drawing tasks involving targets of varying geometric compålexity. Angular velocity data from 17 upper-limb joints were collected from 15 healthy participants. Non-negative matrix factorization identified eight synergy modules that collectively accounted for over 90 % of the motion variance. An analysis of the spatial aspect reveals a correlation between increased target complexity and enhanced kinematic independence of distal joints. A parallel analysis of the temporal aspect demonstrates a close relationship between synergy activation patterns and the stability requirements of the task. Furthermore, geometric shape-rather than complexity level alone-emerged as the predominant factor influencing synergistic coordination strategies. These findings contribute to the enhancement of our understanding of continuous motor coordination and provide a data-driven foundation for clinical assessment, rehabilitation training, and the development of assistive technologies.