THE INFLUENCE OF MOVEMENT SEGMENT DIFFICULTY ON MOVEMENTS WITH 2-STROKE SEQUENCE

Arm movements in the horizontal plane consisting of two segments were examined to determine whether the difficulty of the second segment inf luenced the kinematic characteristics of the first segment. The direct ion of the first segment was an elbow extension movement away from the trunk and remained constant throughout the experiment. The direction of the second segment varied between forearm extension and flexion mov ements, Based an Fitts' law, two different indexes of difficulty (ID) of the second segment were utilized by changing target size and moveme nt amplitude. The effects of changing ID were examined for two differe nt movement amplitudes, All movements were single-joint movements empl oying elbow flexion/extension and were recorded by an x-y digitizer. V ariations in the ID of the second segment produced context-dependent k inematic changes in the performance of the initial segment. Moves ment duration increased when the ID was increased by reducing target size for both extension-extension sequence and extension-flexion sequences. Peak velocity also decreased for higher ID targets in the extension-f lexion sequence. However, there was an interaction between the ID and movement amplitude in the extension-flexion sequence, In this sequence the duration of movement for the high ID/large movement amplitude con dition increased substantially compared with the low ID/small movement amplitude condition. In addition, changing ID of the second segment i nfluenced the time between the two segments (intersegment interval) in the extension-flexion sequence. Collectively, these data suggest that the planning of complex movements is based in part on the accuracy de mands of multiple segments of the sequence.