Interlimb differences of directional biases for stroke production

Abstract

Directional preferences during center-out horizontal shoulder–elbow movements were previously characterized for the dominant arm. These preferences were attributed to a tendency to actively accelerate one joint, while exploiting largely passive motion at the other joint. Since the non-dominant arm is known for inefficient coordination of inter-segmental dynamics, here we hypothesized that directional preferences would differ between the arms. A center-out free-stroke drawing task was used that allowed freedom in the selection of movement directions. The task was performed both with and without a secondary cognitive task that has been shown to increase directional biases of the dominant arm. Mirror-symmetrical directional preferences were observed in both arms, with similar bias strength and secondary task effects. The preferred directions were characterized by maximal exploitation of interaction torques for movement production, but only in the dominant arm. The non-dominant arm failed to benefit from interaction torques. The results point to a hierarchical architecture of control. At the higher level, a movement capable to perform the task while satisfying preferences in joint control is specified through forward dynamic transformations. This process is mediated for both arms from a common neural network adapted to the dominant arm and, specifically, to its ability to exploit interaction torques. Dynamic transformations that determine actual control commands are specified at the lower level of control. An alternative interpretation that strokes might be planned evenly across directions, and biases emerge during movement execution due to anisotropic resistance of intrinsic factors that do not depend on arm dominance is also discussed.

Load emphasizes muscle effort minimization during selection of arm movement direction

“…The association of the directional preferences with the tendency to minimize active control of interaction torque can be interpreted as a tendency to minimize “neural effort” for control of inter-segmental dynamics. This interpretation was supported by marked strengthening of directional preferences caused by cognitive load [31,40]. …”

“…In our previous studies of directional preferences revealed during the free-stroke drawing task, it was found that movements in the preferred directions were performed by rotating either the shoulder or the elbow actively and by moving the other joint largely passively, by interaction torque [18,19,31,40]. This finding was predicted by the leading joint hypothesis (LJH) that suggests that during shoulder-elbow movements, muscle torque at one (leading) joint serves to generate motion and muscle torque at the other (subordinate) joint serves to control the effect of interaction torque caused by the leading joint motion [23,24].…”

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

“…The matrix # can be inverted using basic linear algebra techniques to obtain a Jacobian that is a function of the joint stiffness matrix = ( −1 )� −1 �. (8) It is immediate to see that by substituting (8) into (2), the derivatives of the Jacobian matrix with respect to time (e.g., G 1 and G 2 ) are functions of the change in joint stiffness. Therefore, it is possible that the variations in jerk components we observed experimentally might also be explained in part by an abnormal modulation of joint stiffness, which is often observed in stroke survivors.…”

“…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).…”

“…19 Failure of the nervous system to properly compensate for multi-joint interaction torques can lead to increased movement curvature and the desynchronization of motions at the shoulder and elbow joints. 8 After stroke, systematic deficits in the coordination of agonist / antagonist muscle activations arise in some regions of the arm's workspace due to limitations in the ability of the central nervous system to regulate stretch reflex thresholds in the flexor and extensor muscles in the arm. 20 It is understandable therefore, that abnormal management of interaction torques has been implicated in the degradation of hand path smoothness after stroke.…”