The Role of Object Contact in Pointing and Grasping

We grasp and lift objects many times a day. Most of us perform such a task without any difficulty. However even just grasping the packet of chocolate flakes at the breakfast table is in fact a complex task. Our brain has to – among others - coordinate and select multiple muscles, select the appropriate grasping points and guide the movement in such a way that any unwanted obstacles are avoided. As soon as our arm, hand or fingers do not function properly, it becomes clear that grasping an object is not so simple. Fundamental research on how the human body performs these daily tasks may therefore be important for designing effective diagnostic procedures and rehabilitation therapies. Knowledge on how grasping behaviour is controlled may for instance be useful for designing and optimizing prosthetic arms or hands. Next to the esthetical value of a prosthetic arm, nowadays prosthetic arms are designed to be capable of grasping and manipulating objects in a close to natural manner. To be able to design such prostheses, the knowledge of how humans naturally use their hands is useful. Therefore defining the requirements of a grasping task (difficulty of the movement, selecting the appropriate grasping points) could be helpful. Knowledge of grasping behaviour is not only useful for prosthetics. The reverse is also true. Prosthetic hand design gives us an opportunity for understanding prehension better. Grasping behaviour can be modelled and tested directly on the mechanical hand, showing the results of intervention immediately. In this thesis, we explored what and how healthy humans control when reaching with the hand and grasping with the fingers. We did so by using the model of Smeets and Brenner (1999).