Organization of drinking: postural characteristics of arm-head coordination.

Postural characteristics of components of a natural movement-taking a sip from a mug filled with water-were studied. The authors hypothesized that important postural characteristics of each of the components involved in natural drinking can be described with a single or a few significant parameters. Seated participants (N = 7) took a small sip from a mug (selected from a set of 4) placed in front of them on the table. Movement components selected for analysis were transporting the mug, tilting the mug, moving and tilting the head, and opening the mouth. The ratio of mug diameter to the distance from the level of water to the rim of the mug was introduced as an index of difficulty (ID). ID showed significant logarithmic relationships to a number of parameters that described different components of the movement, including movement time (MT). Significant logarithmic relationships between ID and MT were observed during movements performed under visual control and with eyes closed. The authors tentatively suggest that those relationships are consequences of Fitts' law and that Fitts' law be expanded to tasks that are characterized by permissible error margin, not necessarily related to the size of a spatial target, and that are not constrained by the typical instruction "as fast and accurate as possible". A possible interpretation of the present observations is that the task constraints imposed on a number of movement parameters during the task of drinking can be described with a single parameter.