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

It is well documented that neurological deficits after stroke can disrupt motor control processes that affect the smoothness of reaching movements. The smoothness of hand trajectories during multi-joint reaching depends on shoulder and elbow joint angular velocities and their successive derivatives as well as on the instantaneous arm configuration and its rate of change. Right-handed survivors of unilateral hemiparetic stroke and neurologically-intact control participants held the handle of a two-joint robot and made horizontal planar reaching movements. We decomposed endpoint jerk into components related to shoulder and elbow joint angular velocity, acceleration, and jerk. We observed an abnormal decomposition pattern in the most severely impaired stroke survivors consistent with deficits of inter-joint coordination. We then used numerical simulations of reaching movements to test whether the specific pattern of inter-joint coordination deficits observed experimentally could be explained by either a general increase in motor noise related to weakness or by an impaired ability to compensate for multi-joint interaction torque. Simulation results suggest that observed deficits in movement smoothness after stroke more likely reflect an impaired ability to compensate for multi-joint interaction torques rather than the mere presence of elevated motor noise

Cortical Networks for Control of Voluntary Arm Movements Under Variable Force Conditions

A neural model of voluntary movement and proprioception functionally interprets and simulates cell types in movement related areas of primate cortex. The model circuit maintains accurate proprioception while controlling voluntary reaches to spatial targets, exertion of force against obstacles, posture maintenance despite perturbations, compliance with an imposed movement, and static and inertial load compensations. Computer simulations show that model cell properties mimic cell properties in areas 4 and 5. These include delay period activation, response profiles during movement, kinematic and kinetic sensitivities, and latency of activity onset. Model area 4 phasic and tonic cells compute velocity and position commands which activate alpha and gamma motor neurons, thereby shifting the mechanical equilibrium point. Anterior area 5 cells compute limb position using corollary discharges from area 4 and muscle spindle feedback. Posterior area 5 cells use the perceived position and target position signals to compute a desired movement vector. The cortical loop is closed by a volition-gated projection of this movement vector to area 4 phasic cells. Phasic-tonic cells in area 4 incorporate force command components to compensate for static and inertial loads. Predictions are made for both motor and parietal cell types under novel experimental protocols.Office of Naval Research (N00014-92-J-1309, N00014-93-1-1364, N00014-95-l-0409, N00014-92-J-4015); National Science Foundation (IRI-90-24877, IRI-90-00530

Simulation Study on Acquisition Process of Locomotion by Using an Infant Robot

Locomotion is one of the basic functions of a mobile robot. Using legs is one of the strategies for accomplishing locomotion. The strategy allows a robot to move over rough terrain. Therefore, a considerable amount of research has been conducted on motion control of legged locomotion robots. This chapter treats the motion generation of an infant robot, wit

Robust muscle synergies for postural control

The musculoskeletal structure of the human and animal body provides multiple solutions for performing any single motor behavior. The long-term goal of the work presented here is to determine the neuromechanical strategies used by the nervous system to appropriately coordinate muscles in order to achieve the performance of daily motor tasks. The overall hypothesis is that the nervous system simplifies muscle coordination by the flexible activation of muscle synergies, defined as a group of muscles activated as a unit, that perform task-level biomechanical functions. To test this hypothesis we investigated whether muscle synergies can be robustly used as building blocks for constructing the spatiotemporal muscle coordination patterns in human and feline postural control under a variety of biomechanical contexts. We demonstrated the generality and robustness of muscle synergies as a simplification strategy for both human and animal postural control. A few robust muscle synergies were able to reproduce the spatial and temporal variability in human and cat postural responses, regardless of stance configuration and perturbation type. In addition inter-trial variability in human postural responses was also accounted for by these muscle synergies. Finally, the activation of each muscle synergy in cat produced a specific stabilizing force vector, suggesting that muscle synergies control task-level variables. The identified muscle synergies may represent general modules of motor output underlying muscle coordination in posture that can be activated in different sensory contexts to achieve different postural goals. Therefore muscle synergies represents a simplifying mechanism for muscle coordination in natural behaviors not only because it is a strategy for reducing the number of variables to be controlled, but because it represents a mechanism for simply controlling multi-segmental task-level variables.Ph.D.Committee Chair: Ting, Lena H.; Committee Member: Chang, Young-Hui; Committee Member: Lee, Robert H.; Committee Member: Nichols, T. Richard; Committee Member: Wolf, Steve L

Mechanisms underlying muscle recruitment in response to postural perturbations

The neural and sensory mechanisms underlying appropriate muscle recruitment in response to balance challenges remains elusive. We asked whether the decerebrate cat preparation might be employed for further investigation of postural mechanisms. First, we evaluated the muscular activation patterns and three-dimensional whole limb forces generated by a modified premammillary decerebrated cat. We hypothesized that directionally appropriate muscle activation does not require the cerebral cortices. Furthermore, we hypothesized that the muscle responses would generate functionally appropriate and constrained force responses similar to those reported in the intact animal. Data confirmed both of our hypotheses and suggested important roles for the brainstem and spinal cord in mediating directionally appropriate muscular activation. Second, we investigated how individual muscle activation is translated to functional ground reaction forces. We hypothesized that muscles are selectively activated based upon their potential counteractive endpoint force. Data demonstrated that the endpoint force generated by each muscle through stimulation was directed oppositely to the principal direction of each muscle's EMG tuning curve. Further, muscles that have variable tuning curves were found to have variable endpoint forces in the XY plane. We further hypothesized that the biomechanical constraints of individual muscle actions generate the constrained ground reaction forces created in response to support surface perturbations. We found that there was a lack of muscles with strong medial-lateral actions in the XY plane. This was further exaggerated at long stance conditions, which corresponds to the increased force constraint present in the intact animal under the same conditions. Third, we investigated how loss of cutaneous feedback from the footpads affects the muscle recruitment in response to support surface perturbations. We utilized our decerebrate cat model as it allows 1) isolation of the proprioceptive system (cutaneous and muscle receptor) and 2) observation of the cutaneous loss before significant compensation by the animal. We hypothesized that muscle spindles drive directionally sensitive muscle activation during postural disturbances. Therefore, we expected that loss of cutaneous feedback from the foot soles would not alter the directional properties of muscle activation. While background activity was significantly diminished, the directionally sensitive muscular activation remained intact. Due to fixation of the head, the decerebrate cat additionally does not have access to vestibular or visual inputs. Therefore, this result strongly implicates muscle receptors as the primary source of directional feedback. Finally to confirm that muscle receptors, specifically muscle spindles, are capable of generating feedback to drive the directionally tuning, we investigated the response properties of muscle spindles to horizontal support surface perturbations in the anesthetized cat. As previously stated, we hypothesized that muscle spindles provide the feedback necessary for properly directed muscular responses. We further hypothesized that muscle spindles can relay feedback about the perturbation parameters such as velocity and the initial stance condtion. Results confirmed that muscle spindle generate activation patterns remarkably similar to muscular activation patterns generated in the intact cat. This information, along the knowledge that cutaneous feedback does not substantially eliminate directional tuning, strongly suggests that muscle spindles contribute the critical directional feedback to drive muscular activation in response to support surface perturbations.Ph.D.Committee Chair: T. Richard Nichols; Committee Member: Lena Ting; Committee Member: Shawn Hochman; Committee Member: Thomas Burkholder; Committee Member: Timothy Cop

On the intrinsic control properties of muscle and relexes: exploring the interaction between neural and musculoskeletal dynamics in the framework of the equilbrium-point hypothesis

The aim of this thesis is to examine the relationship between the intrinsic dynamics of the body and its neural control. Specifically, it investigates the influence of musculoskeletal properties on the control signals needed for simple goal-directed movements in the framework of the equilibriumpoint (EP) hypothesis. To this end, muscle models of varying complexity are studied in isolation and when coupled to feedback laws derived from the EP hypothesis. It is demonstrated that the dynamical landscape formed by non-linear musculoskeletal models features a stable attractor in joint space whose properties, such as position, stiffness and viscosity, can be controlled through differential- and co-activation of antagonistic muscles. The emergence of this attractor creates a new level of control that reduces the system’s degrees of freedom and thus constitutes a low-level motor synergy. It is described how the properties of this stable equilibrium, as well as transient movement dynamics, depend on the various modelling assumptions underlying the muscle model. The EP hypothesis is then tested on a chosen musculoskeletal model by using an optimal feedback control approach: genetic algorithm optimisation is used to identify feedback gains that produce smooth single- and multijoint movements of varying amplitude and duration. The importance of different feedback components is studied for reproducing invariants observed in natural movement kinematics. The resulting controllers are demonstrated to cope with a plausible range of reflex delays, predict the use of velocity-error feedback for the fastest movements, and suggest that experimentally observed triphasic muscle bursts are an emergent feature rather than centrally planned. Also, control schemes which allow for simultaneous control of movement duration and distance are identified. Lastly, it is shown that the generic formulation of the EP hypothesis fails to account for the interaction torques arising in multijoint movements. Extensions are proposed which address this shortcoming while maintaining its two basic assumptions: control signals in positional rather than force-based frames of reference; and the primacy of control properties intrinsic to the body over internal models. It is concluded that the EP hypothesis cannot be rejected for single- or multijoint reaching movements based on claims that predicted movement kinematics are unrealistic

Bio-inspired robotic joint and manipulator : from biomechanical experimentation and modeling to human-like compliant finger design and control

textOne of the greatest challenges in controlling robotic hands is grasping and manipulating objects in unstructured and uncertain environments. Robotic hands are typically too rigid to react against unexpected impacts and disturbances in order to prevent damage. The human hands have great versatility and robustness due, in part, to the passive compliance and damping. Designing mechanical elements that are inspired by the nonlinear joint compliance of human hands is a promising solution to achieve human-like grasping and manipulation. However, the exact role of biomechanical elements in realizing joint stiffness is unknown. We conducted a series of experiments to investigate nonlinear stiffness and damping of the metacarpophalangeal (MCP) joint at the index finger. We designed a custom-made mechanism to integrate electromyography sensors (EMGs) and a motion capture system to collect data from 19 subjects. We investigated the relative contributions of muscle-tendon units and the MCP capsule ligament complex to joint stiffness with subject-specific modeling. The results show that the muscle-tendon units provide limited contribution to the passive joint compliance. This findings indicate that the parallel compliance, in the form of the capsule-ligament complex, is significant in defining the passive properties of the hand. To identify the passive damping, we used the hysteresis loops to investigate the energy dissipation function. We used symbolic regression and principal component analysis to derive and interpret the damping models. The results show that the nonlinear viscous damping depends on the cyclic frequency, and fluid and structural types of damping also exist at the MCP joint. Inspired by the nonlinear stiffness of the MCP joint, we developed a miniaturized mechanism that uses pouring liquid plastic to design energy storing elements. The key innovations in this design are: a) a set of nonlinear elasticity of compliant materials, b) variable pulley configurations to tune the stiffness profile, and c) pretension mechanism to scale the stiffness profile. The design exhibits human-like passive compliance. By taking advantage of miniaturized joint size and additive manufacturing, we incorporated the novel joint design in a novel robotic manipulator with six series elastic actuators (SEA). The robotic manipulator has passive joint compliance with the intrinsic property of human hands. To validate the system, we investigated the Cartesian stiffness of grasping with low-level force control. The results show that that the overall system performs a great force tracking with position feedback. The parallel compliance decreases the motor efforts and can stabilize the system.Mechanical Engineerin

Neuromechanical Tuning for Arm Motor Control

Movement is a fundamental behavior that allows us to interact with the external world. Its importance to human health is most evident when it becomes impaired due to disease or injury. Physical and occupational rehabilitation remains the most common treatment for these types of disorders. Although therapeutic interventions may improve motor function, residual deficits are common for many pathologies, such as stroke. The development of novel therapeutics is dependent upon a better understanding of the underlying mechanisms that govern movement. Movement of the human body adheres to the principles of classic Newtonian mechanics. However, due to the inherent complexity of the body and the highly variable repertoire of environmental contexts in which it operates, the musculoskeletal system presents a challenging control problem and the onus is on the central nervous system to reliably solve this problem. The neural motor system is comprised of numerous efferent and afferent pathways with a hierarchical organization which create a complex arrangement of feedforward and feedback circuits. However, the strategy that the neural motor system employs to reliably control these complex mechanics is still unknown. This dissertation will investigate the neural control of mechanics employing a “bottom-up” approach. It is organized into three research chapters with an additional introductory chapter and a chapter addressing final conclusions. Chapter 1 provides a brief description of the anatomical and physiological principles of the human motor system and the challenges and strategies that may be employed to control it. Chapter 2 describes a computational study where we developed a musculoskeletal model of the upper limb to investigate the complex mechanical interactions due to muscle geometry. Muscle lengths and moment arms contribute to force and torque generation, but the inherent redundancy of these actuators create a high-dimensional control problem. By characterizing these relationships, we found mechanical coupling of muscle lengths which the nervous system could exploit. Chapter 3 describes a study of muscle spindle contribution to muscle coactivation using a computational model of primary afferent activity. We investigated whether these afferents could contribute to motoneuron recruitment during voluntary reaching tasks in humans and found that afferent activity was orthogonal to that of muscle activity. Chapter 4 describes a study of the role of the descending corticospinal tract in the compensation of limb dynamics during arm reaching movements. We found evidence that corticospinal excitability is modulated in proportion to muscle activity and that the coefficients of proportionality vary in the course of these movements. Finally, further questions and future directions for this work are discussed in the Chapter 5

Studie zur neuromuskulären Stabilisierung des Sprunggelenkkomplexes anhand ausgewählter Muskeln

Das erfolgreiche Interagieren mit der Umwelt ist bezüglich der motorischen Kontrolle des Bewegungsapparates eine komplexe Aufgabe. Bei sich ändernden äußeren Anforderungen muss eine situationsadäquate Regulation der involvierten motorischen Strukturen erfolgen. Dazu müssen neuromuskuläre Prozesse im Sinne der Aufgabenerfüllung aufeinander abgestimmt werden, um beispielsweise ein externes Objekt sicher kontrollieren zu können. Wird die Stabilität während der Aufgabenerfüllung nicht durch die Umwelt (bzw. das Interaktionsobjekt) gesichert, muss das neuromuskuläre System diese Ausgabe übernehmen