The action of trunk muscles in arm reaching tasks.

Abstract

The electromyographic (EMG) activity of trunk muscles has received little attention for tasks such as arm movements which do not explicitly direct trunk behavior. For such tasks, trunk muscles are considered to play a postural role, but only a limited set of conditions have been examined. The experiments presented in this study were designed to examine the relationship of the activity of trunk muscles at the initiation of rapid reaching movements to postural requirements as target direction and distance were broadly varied. Seated subjects performed bilateral arm reaching movements in the vertical plane to visible targets in many directions and at several distances. Surface EMG was recorded from trunk and shoulder muscles. Trunk position, trunk acceleration, and wrist acceleration were recorded. EMG patterns of the trunk muscles varied systematically with both target direction and target distance. The initial configuration of the body was varied to determine if the systematic variation with target direction was specific to one of three reference axes: the longitudinal axis of the forearm, the longitudinal axis of the trunk, or an absolute vertical axis that was external to the body. No one reference axis proved to have a stronger relationship with the activity of the trunk muscles across all target directions. Calculated muscle torques were compared to recorded activity of trunk muscles. Across all target directions and distances, trunk EMG at movement onset was not always qualitatively consistent with resisting either the calculated static torques of the final position or the segmental interactive effects of the arm on the trunk. By broadly varying target direction and distance, it was made clear that the activity of the trunk muscles was not universally related to any of the control requirements tested. Regardless, systematic changes in EMG patterns were observed for trunk muscles with target direction and target distance. The regularities of the data were more striking than the scatter. These regularities make the data a critical test for any proposed control schemes of trunk muscle activity.