Not all brain regions are created equal for improving bimanual coordination in individuals with chronic stroke.

OBJECTIVE: The now standard cortical stimulation approach of inhibiting contralesional primary motor cortex (cM1) disrupts bimanual coordination while facilitating ipsilesional M1 (iM1) fails to enhance paretic arm function, in severely impaired individuals. We propose an alternative target, enhancing contralesional dorsal premotor cortex (cPMd) to improve bimanual coordination and compare its effects to iM1. METHODS: Fourteen participants with stroke received 5-Hz repetitive transcranial magnetic stimulation (rTMS) on cPMd or iM1 in a repeated cross-over design. Bimanual force/neuromuscular coordination and cortical excitability were assessed. We also examined the relationship of baseline motor function/interhemispheric inhibition (IHI) to participant's responses to each stimulation target. RESULTS: We identified two patterns of responses. Participants with more severe impairment and weaker IHI improved bimanual force/neuromuscular coordination, ipsilesional activations and reduced IHI after cPMd-rTMS; whereas, those with milder impairment and stronger IHI improved only after iM1-rTMS. CONCLUSIONS: Cortical stimulation protocols could be tailored to the types of tasks and to individuals' severity of impairment. Facilitation of cPMd may improve bimanual coordination especially for individuals with limited arm/hand function. SIGNIFICANCE: Our study is the first to identify cortical stimulation strategies for improving bimanual coordination for individuals with different level of severity of stroke.

Neural motor control differs between bimanual common-goal vs. bimanual dual-goal tasks.

Coordinating bimanual movements is essential for everyday activities. Two common types of bimanual tasks are common goal, where two arms share a united goal, and dual goal, which involves independent goals for each arm. Here, we examine how the neural control mechanisms differ between these two types of bimanual tasks. Ten non-disabled individuals performed isometric force tasks of the elbow at 10% of their maximal voluntary force in both bimanual common and dual goals as well as unimanual conditions. Using transcranial magnetic stimulation, we concurrently examined the intracortical inhibitory modulation (short-interval intracortical inhibition, SICI) as well as the interlimb coordination strategies utilized between common- vs. dual-goal tasks. Results showed a reduction of SICI in both hemispheres during dual-goal compared to common-goal tasks (dominant hemisphere: P = 0.04, non-dominant hemisphere: P = 0.03) and unimanual tasks (dominant hemisphere: P = 0.001, non-dominant hemisphere: P = 0.001). For the common-goal task, a reduction of SICI was only seen in the dominant hemisphere compared to unimanual tasks (P = 0.03). Behaviorally, two interlimb coordination patterns were identified. For the common-goal task, both arms were organized into a cooperative "give and take" movement pattern. Control of the non-dominant arm affected stabilization of bimanual force (R2 = 0.74, P = 0.001). In contrast, for the dual-goal task, both arms were coupled together in a positive fashion and neither arm affected stabilization of bimanual force (R2 = 0.31, P = 0.1). The finding that intracortical inhibition and interlimb coordination patterns were different based on the goal conceptualization of bimanual tasks has implications for future research.

Dynamic Balanced Reach: A Temporal and Spectral Analysis Across Increasing Performance Demands.

Standing balanced reach is a fundamental task involved in many activities of daily living that has not been well analyzed quantitatively to assess and characterize the multisegmental nature of the body's movements. We developed a dynamic balanced reach test (BRT) to analyze performance in this activity; in which a standing subject is required to maintain balance while reaching and pointing to a target disk moving across a large projection screen according to a sum-of-sines function. This tracking and balance task is made progressively more difficult by increasing the disk's overall excursion amplitude. Using kinematic and ground reaction force data from 32 young healthy subjects, we investigated how the motions of the tracking finger and whole-body center of mass (CoM) varied in response to the motion of the disk across five overall disk excursion amplitudes. Group representative performance statistics for the cohort revealed a monotonically increasing root mean squared (RMS) tracking error (RMSE) and RMS deviation (RMSD) between whole-body CoM (projected onto the ground plane) and the center of the base of support (BoS) with increasing amplitude (p < 0.03). Tracking and CoM response delays remained constant, however, at 0.5 s and 1.0 s, respectively. We also performed detailed spectral analyses of group-representative response data for each of the five overall excursion amplitudes. We derived empirical and analytical transfer functions between the motion of the disk and that of the tracking finger and CoM, computed tracking and CoM responses to a step input, and RMSE and RMSD as functions of disk frequency. We found that for frequencies less than 1.0 Hz, RMSE generally decreased, while RMSE normalized to disk motion amplitude generally increased. RMSD, on the other hand, decreased monotonically. These findings quantitatively characterize the amplitude- and frequency-dependent nature of young healthy tracking and balance in this task. The BRT is not subject to floor or ceiling effects, overcoming an important deficiency associated with most research and clinical instruments used to assess balance. This makes a comprehensive quantification of young healthy balance performance possible. The results of such analyses could be used in work space design and in fall-prevention instructional materials, for both the home and work place. Young healthy performance represents "exemplar" performance and can also be used as a reference against which to compare the performance of aging and other clinical populations at risk for falling.

An Engineering Model of Human Balance Control-Part I: Biomechanical Model.

We developed a balance measurement tool (the balanced reach test (BRT)) to assess standing balance while reaching and pointing to a target moving in three-dimensional space according to a sum-of-sines function. We also developed a three-dimensional, 13-segment biomechanical model to analyze performance in this task. Using kinematic and ground reaction force (GRF) data from the BRT, we performed an inverse dynamics analysis to compute the forces and torques applied at each of the joints during the course of a 90 s test. We also performed spectral analyses of each joint's force activations. We found that the joints act in a different but highly coordinated manner to accomplish the tracking task-with individual joints responding congruently to different portions of the target disk's frequency spectrum. The test and the model also identified clear differences between a young healthy subject (YHS), an older high fall risk (HFR) subject before participating in a balance training intervention; and in the older subject's performance after training (which improved to the point that his performance approached that of the young subject). This is the first phase of an effort to model the balance control system with sufficient physiological detail and complexity to accurately simulate the multisegmental control of balance during functional reach across the spectra of aging, medical, and neurological conditions that affect performance. Such a model would provide insight into the function and interaction of the biomechanical and neurophysiological elements making up this system; and system adaptations to changes in these elements' performance and capabilities.

Design and evaluation of prosthetic shoulder controller.

We developed a 2-degree-of-freedom (DOF) shoulder position transducer (sensing shoulder protraction-retraction and elevation-depression) that can be used to control two of a powered prosthetic humerus' DOFs. We also developed an evaluation protocol based on Fitts' law to assess the performance of our device. The primary motivation for this work was to support development of powered prosthetic shoulder joints of a new generation of prosthetic arms for people with shoulder disarticulation and very high-level transhumeral amputation. We found that transducers that provided resistance to shoulder movement performed better than those providing no resistance. We also found that a position control scheme, where effector position is proportional to shoulder position, performed better than a velocity control scheme, where effector velocity is proportional to shoulder position. More generally, our transducer can be used to control motion along any two DOFs under a proportional control scheme. It can also be used in a more general 4-DOF control scheme by sequentially controlling two DOFs at a time. The evaluation protocol has general applicability for researchers and practitioners. Researchers can employ it to compare different prosthesis designs and control schemes, while practitioners may find the evaluation protocol useful in evaluating and training people with amputation in the use of prostheses.

Clinical application of a modular ankle robot for stroke rehabilitation.

BACKGROUND: Advances in our understanding of neuroplasticity and motor learning post-stroke are now being leveraged with the use of robotics technology to enhance physical rehabilitation strategies. Major advances have been made with upper extremity robotics, which have been tested for efficacy in multi-site trials across the subacute and chronic phases of stroke. In contrast, use of lower extremity robotics to promote locomotor re-learning has been more recent and presents unique challenges by virtue of the complex multi-segmental mechanics of gait. OBJECTIVES: Here we review a programmatic effort to develop and apply the concept of joint-specific modular robotics to the paretic ankle as a means to improve underlying impairments in distal motor control that may have a significant impact on gait biomechanics and balance. METHODS: An impedance controlled ankle robot module (anklebot) is described as a platform to test the idea that a modular approach can be used to modify training and measure the time profile of treatment response. RESULTS: Pilot studies using seated visuomotor anklebot training with chronic patients are reviewed, along with results from initial efforts to evaluate the anklebot's utility as a clinical tool for assessing intrinsic ankle stiffness. The review includes a brief discussion of future directions for using the seated anklebot training in the earliest phases of sub-acute therapy, and to incorporate neurophysiological measures of cerebro-cortical activity as a means to reveal underlying mechanistic processes of motor learning and brain plasticity associated with robotic training. CONCLUSIONS: Finally we conclude with an initial control systems strategy for utilizing the anklebot as a gait training tool that includes integrating an Internal Model-based adaptive controller to both accommodate individual deficit severities and adapt to changes in patient performance.

Design and evaluation of a prosthetic shoulder controller.

The first goal of this study was to develop a 2 degree of freedom (DOF) upper limb controller utilizing shoulder elevation/depression and protraction/retraction. Its primary purpose is to control powered prosthetic shoulder joints being incorporated into a new generation of prosthetic arms for shoulder disarticulation and very high transhumeral amputees. More generally, however, such a controller can be applied to simultaneously control any 2 DOF's under a direct, proportional control scheme. There is no generally accepted method to objectively and quantitatively evaluate prosthesis and prosthesis control performance, and the second goal of this study was to develop one based on Fitts Law. The evaluation protocol has quite general applicability as well, and can be used to compare different reaching/ pointing devices, or to compare variations in the design or operation of a particular device.

Influence of moving visual environment on sit-to-stand kinematics in children and adults.

The effect of visual field motion on the sit-to-stand kinematics of adults and children was investigated. Children (8 to12 years of age) and adults (21 to 49 years of age) were seated in a virtual environment that rotated in the pitch and roll directions. Participants stood up either (1) concurrent with onset of visual motion or (2) after an immersion period in the moving visual environment, and (3) without visual input. Angular velocities of the head with respect to the trunk, and trunk with respect to the environment, w ere calculated as was head andtrunk center of mass. Both adults and children reduced head and trunk angular velocity after immersion in the moving visual environment. Unlike adults, children demonstrated significant differences in displacement of the head center of mass during the immersion and concurrent trials when compared to trials without visual input. Results suggest a time-dependent effect of vision on sit-to-stand kinematics in adults, whereas children are influenced by the immediate presence or absence of vision.