Acting with the feet and hands: Does one effector system dominate the other?

Abstract

To act effectively, one must select actions that satisfy performance criteria. The more important the criteria, the more important it is to satisfy them. This idea leads to the suggestion that a goal of research on action selection should be determining the relative importance of different performance criteria. We pursued this aim by focused on a physical action task: walking to a table, picking up a box with two hands, and moving the box back and forth between two positions. In earlier work (Ozana et al., 2023), university students who did this task stood as they wished when they arrived at the table. It was found that participants adjusted the separation between their feet depending on the inter-target distance and required back-and-forth rate, timed with a metronome. This result was taken to suggest that hand-move distance and rate were prioritized over foot separation in this context. In the two experiments reported here, we asked participants to adopt wide or narrow foot stances. In Experiment 1, we asked participants, with wide or narrow foot stances, to move the box back and forth at a high or low displacements rate specified by a metronome, covering whatever distance they wished. We found that the freely chosen hand-move distances depended on the hand-move rate but hardly depended on the foot separation. In Experiment 2, we asked new participants to adopt the wide or narrow foot spread and to move the box over two required distances, freely choosing the box-move rate. In this case, we found that participants chose hand-move rates that depended on the required box-move distance but hardly depended on the foot separation. We interpret the results of both the previous and current study to suggest that constraints imposed by the postural control system are relatively less important than those related to manual dexterity in whole-body object manipulation tasks. Instead, it appears that the balance and postural control system functions as an agile support to the manual control system.