Investigation of the thumb and foldable palm interaction for optimum design of thumb assistive mechanisms

Nanayakkara, Visakha Kumari (2018) Investigation of the thumb and foldable palm interaction for optimum design of thumb assistive mechanisms. (PhD thesis), Kingston University, .


The unique musculoskeletal structure of the human hand brings in wider dexterous capabilities to grasp and manipulate a repertoire of objects than non-human primates. The orientation and the position of the thumb play an important role in this characteristic behaviour. The musculoskeletal arrangement at the base of the thumb facilitates grasp stability, maintains sufficient grasp force, and thumb's larger movement span compared to the fingers. Modelling these complex interactions about the mechanical axes of the joints is a challenging task. Even though there have been numerous attempts to develop anthropomorphic robotic hands, the musculoskeletal arrangement of the foldable palm with the thumb is not explored in detail in thumb kinematic modelling. Furthermore, biomechanical studies indicate that the people who do continuous and repetitive grasping and manipulation tasks are likely to develop thumb related impairments due to excessive musculoskeletal loading at the base joints of the thumb. Consequently, biologically informed wearable robotic assistive mechanisms can provide viable solutions to prevent occuring such injuries. Based on this perspective, this thesis proposes a simplified kinematic model of the thumb using a novel virtual palm-folding axis to abstract the essential musculoskeletal arrangement with the palm. The palmar arch formed when the thumb moves to reach the fingertips with respect to the palm is incorporated as the virtual joint axis in this model. The model validated using human demonstrations of precision grasping is used: 1) to find correlated activity using the thumb joint angles and 2) to test effective thumb assistive force directions when given at the kinematicallu accessible metacarpophalangeal (MCP) joint of the thumb. In this regard, this thesis hypothesizes that an external assistive force exerted at the MCP joint of the thumb will be most effective when applied perpendicular to the palm folding axis in terms of maximising the contribution at the thumb-tip as well as minimizing the projections on the vulnerable base joints of the thumb. The analytical results show strong linear relationships in certain pairs of thumb joint rotations in the torque space. The kinematic model also predicts a range of effective assistive force directions at the MCP joint of the thumb. Experiments conducted using an assistive tendon driven glove on human subjects validated the predictions made by the model showing that the assistive forces at the MCP joint, exerted perpendicular to the palm-folding axis, maximise the force gain at the thumb-tip. Finally, how to provide thumb assistance during reaching to grasp is explored. A single tendon loop configuration anchored to the MCP joint of the thumb is optimized using human thumb grasping data trajectories in order to provide an adaptive resultant assistive force vector in the thumb moving direction during reaching to grasp. In order to expand the resultant assistive force range, a double tendon loop is optimized by combining two separate tendon loops. These findings provide design guidelines for hand assistive mechanisms to maximize the efficacy of thumb external assistance.

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