QUALITATIVE DISCREPANCIES BETWEEN TRUNK MUSCLE-ACTIVITY AND DYNAMIC POSTURAL REQUIREMENTS AT THE INITIATION OF REACHING MOVEMENTS PERFORMEDWHILE SITTING

Reaching movements are associated with widespread, nonfocal muscle act ivity. That activity is often assumed to play a postural role. We test ed this assumption for the trunk muscles at the initiation of reaching movements with the following question. Does initial trunk muscle acti vity play a dynamic postural role by resisting the segmental interacti ve effects of the arm movement on the trunk? Seated subjects performed bilateral reaching movements while target direction was systematicall y varied. Muscle activity was recorded from flexors and extensors of t he trunk and shoulder. Trunk muscle activity was compared with trunk t orques calculated from simulations of reaching movements in which the trunk was modeled to stay still. Recorded trunk muscle activity was in qualitative agreement with torque predictions for only some target di rections, suggesting that the nervous system does not activate trunk m uscles across all target directions to counteract postural disturbance s at the initiation of reaching movements.