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 less affected arm of twenty 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 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 on both motor execution and planning. Individuals with stroke demonstrate substantially more deviation from straight-line paths than controls, despite using more conservative strategies (i.e., leftward shift of velocity profile) and extensive feedback control (i.e., segmentation).

Inter-Joint Coordination Deficits Revealed in the Decomposition of Endpoint Jerk During Goal-Directed Arm Movement After Stroke

“…In this case, joint stiffness is a function of both the limb's position in joint space and the activations of mono-and bi-articular muscles, all of which are functions of time. Previous work 67 has demonstrated that an integrable pseudo-inverse of the Jacobian in the form # = −1 ( −1 ) −1 (7) gives a unique solution to the inverse kinematic problem of redundant robots. (7) is a form of generalized inverse where −1 enters as a quadratic form "weighting matrix".…”

“…7 The simulations were guided by experimentallymeasured reach kinematics, which we analyzed using a non-parametric inverse-normal-transform (INT; 59) Rank Transformation test (RT Type 1; 60) applied to selected structural components of the endpoint jerk. In the INT-RT1 test, the entire set of observations for a given dependent variable is first ranked from smallest to largest, with the smallest observation having rank 1, the second smallest rank 2, and so on (average ranks are assigned in case of ties).…”

“…4 By contrast, hemiparetic reaches exhibit systematic misdirection 5 and a lack of hand-path smoothness (1,6) that can coincide with an increased frequency of terminal corrective submovements. 7 Two neuromotor deficits have been implicated as contributing to the lack of hand trajectory smoothness post-stroke: abnormal management of interaction torques (5,8; c.f., 9) and an increased variability in the generation of muscle force 10 that is related to muscle weakness. 11 With recovery, hand trajectory smoothness progressively increases; 6,12 this trajectory smoothing corresponds to a gradual reduction in the third time derivative of hand displacement (i.e., hand trajectory jerk) (c.f., 13).…”

“…28,29 It has been hypothesized that stroke-related increases in neuromotor noise compromises both the planning and execution of movement. 7 In particular, single-unit recordings from muscles in the hemiparetic limb reveal disorderly motor neuron recruitment, which may also contribute to muscle weakness and increased variability of muscle force production. 30 Regardless of its origin, failure to contend with excess motor noise has potential to degrade the accuracy and smoothness of reaching movements post-stroke.…”

“…We found that the movement coordination anomalies captured by the jerk decomposition technique could be replicated by neglecting the effect of interaction torque compensation as suggested by, 5 but not merely by increasing motor noise as suggested by. 7 SECTION II.…”