Comfort constrains graphic workspace: Test results of a 3D forearm model

Human movement performance is subject to many physical and psychological co nstraints. Analyses of these constraints may not only improve our understan ding of the performance aspects that subjects need to keep under continuous control, but may also shed light on the possible origins of specific behav ioral preferences that people display in motor tasks. The goal of the prese nt paper is to make an empirical contribution here. In a recent simulation study, we reported effects of pen-grip and forearm-posture constraints on t he spatial characteristics of the pen tip's workspace in drawing. The effec ts concerned changes in the location, size, and orientation of the reachabl e part of the writing plane, as well as variations in the computed degree o f comfort in the hand and finger postures required to reach the various par ts of this area. The present study is aimed at empirically evaluating to wh at extent these effects influence subjects' graphic behavior in a simple, f ree line-drawing task. The task involved the production of small back-and-f orth drawing movements in various directions, to be chosen randomly under t hree forearm-posture and five pen-grip conditions. The observed variations in the subjects' choice of starting positions showed a high level of agreem ent with those of the simulated graphic-area locations, showing that biomec hanically defined comfort of starting postures is indeed a determinant of t he selection of starting points. Furthermore, between-condition rotations i n the frequency distributions of the realized stroke directions corresponde d to the simulation results, which again confirms the importance of comfort in directional preferences. It is concluded that postural rather than spat ial constraints primarily affect subjects' preferences for starting positio ns and stroke directions in graphic motor performance. The relevance of the present modelling approach and its results for the broader field of comple x motor behavior, including the manipulation of tools, is indicated briefly .