Consequences of increased neuromotor noise for reaching movements in persons with stroke

Abstract

Understanding stroke-induced changes to the motor control of the more affected arm of people with stroke may lead to more effective rehabilitation interventions that improve function. Reaching movements of the more affected arm in persons with stroke are slow, segmented, and indirect. Such changes may be related to a reduced capacity to transmit motor commands in the presence of neuromotor noise. In tasks requiring both speed and accuracy, transmission capacity can be characterized by the linear relationship between movement time and task difficulty (Fitts' law). This study quantified Fitts' slope and intercept coefficients in stroke during reaching tasks and their relationship to kinematic measures of path accuracy (directness), trajectory corrections (segmentation), and planning strategy (skewness). We compared Fitts' slope and intercept and kinematics among the more and the less affected arm of 20 persons with stroke and the nondominant arm of ten healthy persons. Slope and intercept were significantly increased in the more affected arm of the group with stroke and related to clinical measurements of motor impairment and tone. For both the more and the less affected arm of the group with stroke, increased slopes and intercepts were correlated to more indirect, segmented, and positively skewed movement. Our findings suggest that stroke results in greater neuromotor noise, which has consequences for both motor execution and planning. Individuals with stroke demonstrate substantially more deviation from straight-line paths than do controls, despite using more conservative strategies (i.e., leftward shift of velocity profile) and extensive feedback control (i.e., segmentation).

Figure 1

Target Locations and Sizes. Targets were placed on top of a table at 10, 20, and 30 (or 40) cm distances from the start position. Reaching movements were to targets of widths 2,4,8, and 16 cm.

Figure 2

Kinematic descriptors of reaching movements. The velocity profile can have changes in segmentation (2a) and skewness (2b). The reaching path can have changes in its directness (2c). Note: all the descriptors of the kinematic profile are independent of distance and velocity.

Figure 3

Movement Time versus Index of Difficulty for the less (empty circles and dashed line) and more (solid circles and line) affected arm of persons with stroke. (3a) Low Fugl-Meyer score (SR01 – FM=19). (3b) High Fugl-Meyer Score (SR20 – FM=55). Notice that as the impairment reduces, the slope and intercept of the more and less affected lines become more similar.