Abstract
The act of eating serves multiple purposes. Most importantly, it provides us our daily nourishment consisting of vitamins, proteins etc. Next to that, it has a significant social function. During dinner in a restaurant, important trade agreements are settled, and people declare each other eternal love. While these actions may demand the most of our attention, at the same time we have to bring the spoon or fork to our mouth more than once, preferably without spilling. This multiple degrees-of-freedom movement seems relatively simple to co-ordinate, but is it really? In this paper we address the question to what extent the principles that are found during traditional reach and grasp studies may also apply to the co-ordination of a functional action such as eating. We will focus on two questions. First, do increased accuracy demands lead to prolonged movement times during eating as well, and to what extend is this due to a disproportional lengthening of the deceleration phase? Second, is 'freezing' of the joints implemented when more accuracy is demanded? Five hungry subjects had to eat soup and kale with a spoon and the motions were recorded with a Vicon-3D motion recording device. The results showed that the transport of water was performed with a significant longer movement time compared to transport of kale. However, this was not due to a significant lengthening of the deceleration phase. It is argued that the increased deceleration phase found in previous reach to grasp studies may be an 'artefact' of the task used, in which accuracy demands expose themselves only at the end of the movement. In the present task, accuracy demands expose themselves throughout the complete movement range, leading to symmetrical velocity profiles. Based on these results it may be argued that the symmetrical velocity profile is the basic movement profile from which all other profiles are deduced. The results further showed that eating soup was concurrent with reduced elbow excursion, enlarged shoulder elevation and increased head involvement. This indicates a redistribution of segments involved for the two substances. Subjects 'freeze' the more distal movement components of the arm and bring the soup to the mouth basically by an elevation of the shoulder and an increased involvement of head movement. The point to the freezing of degrees of freedom as a general principle underlying movement (re-)organisation and lend support to a proximo-distal direction of movement organisation.
