Feedback Control of an Exoskeleton for Paraplegics: Toward Robustly Stable Hands-free Dynamic Walking

This manuscript presents control of a high-DOF fully actuated lower-limb exoskeleton for paraplegic individuals. The key novelty is the ability for the user to walk without the use of crutches or other external means of stabilization. We harness the power of modern optimization techniques and supervised machine learning to develop a smooth feedback control policy that provides robust velocity regulation and perturbation rejection. Preliminary evaluation of the stability and robustness of the proposed approach is demonstrated through the Gazebo simulation environment. In addition, preliminary experimental results with (complete) paraplegic individuals are included for the previous version of the controller.Comment: Submitted to IEEE Control System Magazine. This version addresses reviewers' concerns about the robustness of the algorithm and the motivation for using such exoskeleton

In silico case studies of compliant robots: AMARSI deliverable 3.3

In the deliverable 3.2 we presented how the morphological computing ap- proach can significantly facilitate the control strategy in several scenarios, e.g. quadruped locomotion, bipedal locomotion and reaching. In particular, the Kitty experimental platform is an example of the use of morphological computation to allow quadruped locomotion. In this deliverable we continue with the simulation studies on the application of the different morphological computation strategies to control a robotic system

Use of a Multi-Axis Robotic Testing Platform to Investigate the Sagittal Mechanics of the Multi-Body Lumbar Spine

A biomechanical study was performed to compare range of motion of the multi-body lumbar spine using three different protocols: pure moment, eccentric loading, and a new method called combined loading moment. The objectives of the study were to introduce a new protocol that overcomes the limitations of previous methods by applying more realistic loading conditions and to compare the range of motion of this new protocol to those of eccentric loading and pure moment protocols within the same specimen pool. The second objective of this study was to compare the data sets of these three protocols to both in vivo and in vitro data sets. Because of the clinical issues with the lumbar region of the spine, it is important to understand the normal mechanics of the lumbar spine. In vivo methods of determining normal mechanics are limited, and in vitro methods have shown to be more realistic. Pure moment methods apply only a pure moment that is constant at every level of the spine. Pure moment with the addition of follower load increases the load carrying capacity of the in vitro spine. Eccentric loading produces physiological motion but the moment loads applied at each level are unknown. The previous methods of loading do not account for shear loads applied in vivo. A new method was developed called combined loading moment and includes axial, shear, and moment loading. This study compared three methods using six harvested spines tested in flexion and extension. Eccentric loading was first applied using an existing protocol of the University of Tennessee Health Science Center (UTHSC) Joint Implant Biomechanics Laboratory. Pure moment and combined loading moment were both applied using new protocols adapted from pure moment for single motion segment units using the UTHSC Spine Robot. The intersegmental rotations were measured using a camera system with LEDs. These rotations were used to create motion profiles to compare to in vivo studies and previous in vitro studies. Variations among in vivo and in vitro motion profiles could be due to a number of factors including subject groups and test methods. Results from this study showed that follower load may not be necessary in order to apply in vivo loads to the in vitro spine. Axial load was determined to limit range of motion. Shear loads appeared to increase flexion range of motion, but appeared to only increase extension range of motion until the facets joints prevented further motion. The pure moment protocol used with the Spine Robot overcame the limitations of standard fixed pulley methods of pure moment. Future work with the new protocol should attempt to increase the magnitude of the vertical load and further explore the effects of shear load. Additionally, future work should include lateral, axial, and coupled rotations

Muscle synergies in neuroscience and robotics: from input-space to task-space perspectives

In this paper we review the works related to muscle synergies that have been carried-out in neuroscience and control engineering. In particular, we refer to the hypothesis that the central nervous system (CNS) generates desired muscle contractions by combining a small number of predefined modules, called muscle synergies. We provide an overview of the methods that have been employed to test the validity of this scheme, and we show how the concept of muscle synergy has been generalized for the control of artificial agents. The comparison between these two lines of research, in particular their different goals and approaches, is instrumental to explain the computational implications of the hypothesized modular organization. Moreover, it clarifies the importance of assessing the functional role of muscle synergies: although these basic modules are defined at the level of muscle activations (input-space), they should result in the effective accomplishment of the desired task. This requirement is not always explicitly considered in experimental neuroscience, as muscle synergies are often estimated solely by analyzing recorded muscle activities. We suggest that synergy extraction methods should explicitly take into account task execution variables, thus moving from a perspective purely based on input-space to one grounded on task-space as well

A Robotic Neuro-Musculoskeletal Simulator for Spine Research

An influential conceptual framework advanced by Panjabi represents the living spine as a complex neuromusculoskeletal system whose biomechanical functioning is rather finely dependent upon the interactions among and between three principal subsystems: the passive musculoskeletal subsystem (osteoligamentous spine plus passive mechanical contributions of the muscles), the active musculoskeletal subsystem (muscles and tendons), and the neural and feedback subsystem (neural control centers and feedback elements such as mechanoreceptors located in the soft tissues) [1]. The interplay between subsystems readily encourages thought experiments of how pathologic changes in one subsystem might influence another--for example, prompting one to speculate how painful arthritic changes in the facet joints might affect the neuromuscular control of spinal movement. To answer clinical questions regarding the interplay between these subsystems the proper experimental tools and techniques are required. Traditional spine biomechanical experiments are able to provide comprehensive characterization of the structural properties of the osteoligamentous spine. However, these technologies do not incorporate a simulated neural feedback from neural elements, such as mechanoreceptors and nociceptors, into the control loop. Doing so enables the study of how this feedback--including pain-related--alters spinal loading and motion patterns. The first such development of this technology was successfully completed in this study and constitutes a Neuro-Musculoskeletal Simulator. A Neuro-Musculoskeletal Simulator has the potential to reduce the gap between bench and bedside by creating a new paradigm in estimating the outcome of spine pathologies or surgeries. The traditional paradigm is unable to estimate pain and is also unable to determine how the treatment, combined with the natural pain avoidance of the patient, would transfer the load to other structures and potentially increase the risk for other problems. The novel Neuro-Musculo

A Passive Pure Moment Protocol for Testing Spine Segments: Development and Application

The pure moment protocol is the accepted standard for performing in-vitro biomechanical testing of spinal devices. Published studies predominantly report range of motion and flexibility data, but information regarding the segment center of rotation is also relevant. Most current pure moment platforms are not sensitive enough to accurately calculate the instantaneous axis of rotation (IAR) for a segment throughout a bending motion. The purpose of this study was to simulate a pure moment protocol using a programmable spine robot, and use the data gathered to calculate the IAR for harvested specimen and those implanted with a constrained total disc replacement (TDR) device. Six human lumbar single-level motion segment units (MSUs) at the L4-L5 level were dissected and potted. The average age of the spines was 47 ± 11.4 years. The robot was programmed to rotate the specimen in flexion and extension and left and right lateral bending in 0.25 degree increments, minimizing shear and axial loading after each rotation, thereby finding a quasistatic rotational path of minimal loading. The specimens were rotated to 8Nm of sagittal moment during flexion and extension and 6Nm of lateral moment during lateral bending. During lateral testing, the specimens were unconstrained axially. Once harvested testing was completed, specimens were implanted with a constrained ProDisc-L implant (Synthes Inc., West Chester, PA) by a spine surgeon under fluoroscopy. All pure moment testing was repeated on the implanted specimen. Throughout testing, the specimens underwent an average off-axis force of 1.51N. With an average perpendicular distance of 0.062mm, this force value contributed 0.000094Nm to the maximum bending moment, meaning the test platform was 99.99% free of off-axis loading. During flexion and extension tests the specimens rotated an average of 9.90 ± 2.23 degrees and 3.40 ± 1.43 degrees respectively. During left and right lateral bending tests the specimens rotated an average of 6.21 ± 1.34 degrees and 5.64 ± 1.77 degrees respectively. These values are in agreement with other published studies of lumbar spinal biomechanics. Range of motion comparisons between the harvested and implanted specimen showed a significant difference in right lateral bending and combined lateral bending (one-way repeated measures ANOVA with SNK test, p\u3c0.05). No significant differences were observed for flexion or extension motions. IAR values were calculated for the harvested and implanted specimen for flexion and extension testing and normalized based on the height and anterior-to-posterior (A-P) width of the disc. These values were compared with a One-Way ANOVA with Dunn\u27s comparison test between locations of X and Y coordinates for each IAR within and between conditions (p\u3c0.05). All comparisons save for the position of Y-coordinates in harvested testing between flexion and extension showed significance. Future work will be to allow for a user-inputted axial load to simulate a net muscle vector, use of the protocol with other constrained as well as unconstrained TDR devices, and use of the protocol within multi-body studies

In Vitro Biomechanical Testing and Computational: Modeling in Spine

Two separate in vitro biomechanical studies were conducted on human cadaveric spines (Lumbar) to evaluate the stability following the implantation of two different spinal fixation devices interspinous fixation device (ISD) and Hybrid dynamic stabilizers. ISD was evaluated as a stand-alone and in combination with unilateral pedicle rod system. The results were compared against the gold standard, spinal fusion (bilateral pedicle rod system). The second study involving the hybrid dynamic system, evaluated the effect on adjacent levels using a hybrid testing protocol. A robotic spine testing system was used to conduct the biomechanical tests. This system has the ability to apply continuous unconstrained pure moments while dynamically optimizing the motion path to minimize off-axis loads during testing. Thus enabling precise control over the loading and boundary conditions of the test. This ensures test reliability and reproducibility. We found that in flexion-extension, the ISD can provide lumbar stability comparable to spinal fusion. However, it provides minimal rigidity in lateral bending and axial rotation when used as a stand-alone. The ISD with a unilateral pedicle rod system when compared to the spinal fusion construct were shown to provide similar levels of stability in all directions, though the spinal fusion construct showed a trend toward improved stiffness overall. The results for the dynamic stabilization system showed stability characteristics similar to a solid all metal construct. Its addition to the supra adjacent level (L3- L4) to the fusion (L4- L5) indeed protected the adjacent level from excessive motion. However, it essentially transformed a 1 level into a 2 level lumbar fusion with exponential transfer of motion to the fewer remaining discs (excessive adjacent level motion). The computational aspect of the study involved the development of a spine model (single segment). The kinematic data from these biomechanical studies (ISD study) was then used to validate a finite element model

Injury and Skeletal Biomechanics

This book covers many aspects of Injury and Skeletal Biomechanics. As the title represents, the aspects of force, motion, kinetics, kinematics, deformation, stress and strain are examined in a range of topics such as human muscles and skeleton, gait, injury and risk assessment under given situations. Topics range from image processing to articular cartilage biomechanical behavior, gait behavior under different scenarios, and training, to musculoskeletal and injury biomechanics modeling and risk assessment to motion preservation. This book, together with "Human Musculoskeletal Biomechanics", is available for free download to students and instructors who may find it suitable to develop new graduate level courses and undergraduate teaching in biomechanics

Development of a Physiologic In-Vitro Testing Methodology for Assessment of Cervical Spine Kinematics

In-vitro biomechanical testing has been critical in the design and evaluation of spinal surgical instrumentation, however determination of realistic physiologic loading levels has proven difficult outside of the in-vivo setting. Unconstrained pure moment testing combined with the hybrid testing method is currently the gold standard test protocol for evaluation of motion preservation technology and adjacent level effects. Pure moment testing is well suited for making relative comparisons between treatments, but is currently not based on or representative of in-vivo spine motion, bringing the clinical relevance into question. The human cervical spine supports substantial compressive load in-vivo arising from muscle forces and the weight of the head. However, traditional in-vitro testing methods rarely include compressive loads, especially in investigations of multi-segment cervical spine constructs. Therefore, a systematic comparison of standard pure moment testing without compressive loading versus published and novel compressive loading techniques (follower load, axial load, and combined load) was performed. To achieve a pure moment test, a robot/UFS testing system was programmed with hybrid control, which combined load and displacement control to overcome the limitations of either control methodology alone. A follower load system was developed with actively controlled linear actuators and integrated into the robot/UFS testing system’s control algorithm. Thorough investigation of the integrated system ensured that the pure moment assumption was upheld and enabled characterization of the kinetics resulting from the application of follower load. In contrast, axial load was applied perpendicular to superior most vertebral body using the robot end-effector; it did not maintain the pure moment assumption resulting in alterations of the segmental motion patterns. The pure moment testing protocol without compression or follower load was not able to replicate the typical in-vivo segmental motion patterns throughout the entire motion path. Axial load or a combination of axial and follower load was necessary to mimic the in-vivo segmental contributions at the extremes of the extension-flexion motion path. It is hypothesized that dynamically altering the compressive loading throughout the motion path is necessary to mimic the segmental contribution patterns exhibited in-vivo—a novel concept that will be explored in future investigations