Segment interdependency and difficulty in two-stroke sequences.

We previously demonstrated that velocity and movement time for the initial segment for a two-stroke movement are scaled in relation to the difficulty of the second segment. The interdependent kinematic changes were interpreted as evidence that movement planning/organization processes consider the movement parameters of both segments when determining the movement characteristics of the entire sequence. In this experiment we examined two-stroke movements where the difficulty of the first segment had either a low or high level of difficulty to determine if the interdependent kinematic changes are diminished when parameter specification is high for the initial segment. Two-stroke arm movements toward defined targets were made in the horizontal plane on an x-y digitizer. The direction of the first segment was an elbow extension movement away from the trunk. The direction of the second segment varied between forearm extension and flexion movements. Two different indexes of difficulty (IDs) of the first segment and two of the second segment were created by varying target size. In the low ID condition for the first segment, movement duration of the initial segment lengthened and peak velocity decreased when the ID of the second segment was increased, and this pattern was found for both the extension-extension and extension-flexion sequences. In contrast, when the level of difficulty was high for the first segment, the interdependencies disappeared for the extension-extension sequence: movement duration and peak velocity were unaffected by the difficulty of the second segment. For the extension-flexion sequence, however, the interdependencies were found in the movement time of the initial segment but were eliminated in the peak velocity, i.e., movement time increased, but the peak velocity did not change. Furthermore, for both the extension-extension and extension-flexion sequences, the intersegment interval was lengthened as the level of difficulty increased. These findings suggest that difficulty of the initial segment affects how the motor planning/organization processes treat adjacent segments of the sequence. In particular, the data support the hypothesis that when the initial movement segment has a high index of difficulty, motor planning/organization processes appear to treat the adjacent segments separately as two discrete actions.