A Cable-Driven Pelvic Robot: Human Gait Adaptation and Rehabilitation Studies

Walking is a state of continuous imbalance that requires a complex control strategy and cyclic activation of leg muscles to achieve successful inter‐limb coordination. Neuro‐musculoskeletal impairments, such as stroke, cerebral palsy, and spinal cord injury, affect one's ability to voluntarily contract muscles to normal amplitudes. This change in muscle activation pattern reduces the joint level torque generation and as a result impairs the ability to walk normally. Technological advances over the last two decades have resulted in the development of rigid link robotic exoskeletons that aim to improve gait deficits. These devices reduce repetitive and manual labor of therapists while providing objective measurement of the therapy during the gait rehabilitation. Despite the development of these robotic devices, no consensus has emerged about the superiority of robot-aided gait rehabilitation over the traditional methods. This may be because of the inherent complexity of the human musculoskeletal system and the constraints that rigid linked systems impose on the human movement. In this work, we present a cable-driven Active Tethered Pelvic Assist Device (A-TPAD) for gait rehabilitation that can apply a controlled external wrench to the human pelvis in any direction and at any point of the gait cycle for a specified duration. The A-TPAD does not add undesirable inertia on the user and does not constrain the user's motion during training. The A-TPAD provides a technological platform to scientifically study human adaptation in gait due to externally applied forces and moments on the pelvis. Human studies with the A-TPAD can motivate new gait rehabilitation paradigms which can potentially be used to correct gait deficits in human walking. The human nervous system is capable of modifying the motor commands in response to alterations in the movement conditions. Several studies have demonstrated the flexibility of human locomotion despite motor impairments and have shown the potential of using such paradigms for gait rehabilitation. In this work, we present a number of human experiments using the cable-driven A-TPAD to propose novel force interventions that induce adaptation in human gait kinematics and kinetics. In particular, stance phase gait interventions have been developed for gait rehabilitation of hemiparetic patients. In these interventions, the external force vector was applied to the pelvis to target weight bearing during walking and to promote longer stance durations. A single-session force training experiment with hemiparetic stroke patients was also conducted as a part of this work. It is shown that hemiparetic stroke patients improved the ground reaction force symmetry, forward propulsion effort, and stance phase symmetry during walking. In this work, the A-TPAD is also used to develop an intervention to apply external gait synchronized forces on the pelvis to reduce the user's effort during walking. The external forces were directed in the sagittal plane to assist the trailing leg during the forward propulsion and vertical deceleration of the pelvis during the gait cycle. A pilot experiment with five healthy subjects was conducted. This study provides a novel approach to study the role of external forces in altering the walking effort, such understanding is important while designing assistive devices for individuals who spend higher than normal effort during walking

Robotic Functional Gait Rehabilitation with Tethered Pelvic Assist Device

The primary goal of human locomotion is to stably translate the center of mass (CoM) over the ground with minimum expenditure of energy. Pelvic movement is crucial for walking because the human CoM is located close to the pelvic center. Because of this anatomical feature, pelvic motion directly contributes to the metabolic expenditure, as well as in the balance to keep the center of mass between the legs. An abnormal pelvic motion during the gait not only causes overexertion, but also adversely affects the motion of the trunk and lower limbs. In order to study different interventions, recently a cable-actuated robotic system called Tethered Pelvic Assist Device (TPAD) was developed at ROAR laboratory at Columbia University. The cable-actuated system has a distinct advantage of applying three dimensional forces on the pelvis at discrete points in the gait cycle in contrast to rigid exoskeletons that restrict natural pelvic motion and add extra inertia from the rigid linkages. However, in order to effectively use TPAD for rehabilitation purposes, we still need to have a better understanding of how human gait is affected by different forces applied by TPAD on the pelvis. In the present dissertation, three different control methodologies for TPAD are discussed by performing human experiments with healthy subjects and patients with gait deficits. Moreover, the corresponding changes in the biomechanics during TPAD training are studied to understand how TPAD mechanistically influences the quality of the human gait. In Chapter 2, an ‘assist-as-needed’ controller is implemented to guide and correct the pelvic motion in three dimensions. Here, TPAD applies the correction force based on the deviation of the current position of the pelvic center from a pre-defined target trajectory. This force acts on the pelvic center to guide it towards the target trajectory. A subject in the device experiences a force field, where the magnitude becomes larger when the subject deviates further away from the target trajectory. This control strategy is tested by performing the experiments on healthy subjects with different target pelvic trajectories. Chapter 3 describes a robotic resistive training study using a continuous force on the pelvis to strengthen the weak limbs so that subjects can improve their walking. This study is designed to improve the abnormal gait of children with Cerebral Palsy (CP) who have a crouch gait. Crouch gait is caused by a combination of weak extensor muscles that do not produce adequate muscle forces to keep the posture upright, coupled with contraction of muscles that limit the joint range of motion. Among the extensor muscles, the soleus muscle acts as the major weight-bearing muscle to prevent the knees from collapsing forward during the middle of the stance phase when the foot is on the ground. Electromyography, kinematics, and clinical measurements of the patients with crouch gait show significant improvements in the gait quality after the resistive TPAD training performed over five weeks. Both Chapters 2 & 3 present interventions that are bilaterally applied on both legs. Chapter 4 introduces a training strategy that can be used for patients who have impairments in only one leg which results in manifests as asymmetric weight-bearing while walking. This training method is designed to improve the asymmetric weight bearing of the hemiparetic patients who overly rely on the stronger leg. The feasibility of this training method is tested by experiments with healthy subjects, where the controller creates an asymmetric force field to bring asymmetry in weight bearing during walking. In summary, the present dissertation is devoted to developing new training methods that utilize TPAD for rehabilitation purposes and characterize the responses of different force interventions by investigating the resulting biomechanics. We believe that these methodologies with TPAD can be used to improve abnormal gait patterns that are often observed in cerebral palsy or stroke patients

Dynamic modelling and simulation of a cable-driven parallel robot for rehabilitation applications

The aim of this work, in collaboration with the ROAR Lab of the Columbia University in the city of New York, is to build a simulation model of a new cable-driven parallel robot for rehabilitation applications, being able to compute the effort given by the patient while the system is working on him/her. The model was built on a multi-body dynamic software called Adams, which is able to simulate the behavior of the mechanism. Some theoretical issues about cable-driven parallel robots will be described, in order to familiarize with the application and introduce the state of the art of the topic. General foundations, dealing with kinematics, statics, dynamics will be detailed and a short introduction to control will be given. In the second chapter, a brief overview of the state of the art regarding rehabilitation cable-driven robotics will be outlined, first dealing with general applications possible to be found in literature, and then introducing the Columbia University work about this particular topic, with several examples and cutting edge devices. The third chapter is about the design description of the Stand Trainer, a 8-cable-driven parallel robot used for rehabilitation. Its mechanical system is introduced, while dealing especially with the issue of computing the cable tensions and the way it can be done in terms of sensors positioning. A new way of tension measurement will be explained. It will take the place of the previous one, bringing several advantages to the system. The last chapter deals with the dynamic simulations on Adams. After having introduced all the simplifications regarding three different models, an accurate description of them will be given and their comparison with the real device will be outlined. The post-process activity will be carried out explaining and discussing the final results. Finally, different points for future developments will be discussed, showing the novelty of this approach for rehabilitative treatments and applications

Robotic Strategies to Characterize and Promote Postural Responses in Standing, Squatting and Sit-to-Stand

In people with neuromotor deficits of trunk and lower extremities, maintaining and regaining balance is a difficult task. Many undergo rehabilitation to improve their movement capabilities, health, and overall interactions with their environment. Rehabilitation consists of a set of interventions designed to improve the individual’s mobility and independence. These strategies can be passive, active or task-specific and are dependent on the type of injury, how the individual progresses, and the intensity of the activity. Some of the common rehabilitation interventions to strengthen muscles and improve coordination are accomplished either by the manual assistance of a physical therapist, bodyweight suspension systems or through robotic-assisted training. There are several types of rehabilitation robotic systems and robotic control strategies.However, there are few robotic studies that compare their robotic device’s control strategy to common rehabilitation interventions. This dissertation introduces robotic strategies centered around rehabilitation ones and characterizes human motion in response to the robotic forces. Two cable-driven robotic systems are utilized to implement the robotic controllers for different tasks. Further details of the two cable-driven systems are discussed in Chapter 1. The validation and evaluation of these robotic strategies for standing rehabilitation is discussed in Chapter 2. A case study of a robotic training paradigm for individuals with spinal cord injury is presented in Chapter 3. Chapter 4 introduces a method to redistribute individuals’ weight using pelvic lateral forces. Chapter 5 and 6 characterizes how young and older groups respond to external perturbations during their sit-to-stand motion. This dissertation presents robotic strategies that can be implemented as rehabilitation interventions. It also presents how individuals’ biomechanics and muscle responses may change depending on the force control paradigm.These robotic strategies can be utilized by training individuals to improve their reactive and active balance control and thus reduce their risk of falling

Trunk Rehabilitation Using Cable-Driven Robotic Systems

Upper body control is required to complete many daily tasks. One needs to stabilize the head and trunk over the pelvis, as one shifts the center of mass to interact with the world. While healthy individuals can perform activities that require leaning, reaching, and grasping readily, those with neurological and musculoskeletal disorders present with control deficits. These deficits can lead to difficulty in shifting the body center of mass away from the stable midline, leading to functional limitations and a decline in the quality of activity. Often these patient groups use canes, walkers, and wheelchairs for support, leading to occasional strapping or joint locking of the body for trunk stabilization. Current rehabilitation strategies focus on isolated components of stability. This includes strengthening, isometric exercises, hand-eye coordination tasks, isolated movement, and proprioceptive training. Although all these components are evidence based and directly correlate to better stability, motor learning theories such as those by Nikolai Bernstein, suggest that task and context specific training can lead to better outcomes. In specific, based on our experimentation, we believe functional postural exploration, while encompassing aspects of strengthening, hand-eye coordination, and proprioceptive feedback can provide better results. In this work, we present two novel cable robotic platforms for seated and standing posture training. The Trunk Support Trainer (TruST) is a platform for seated posture rehabilitation that provides controlled external wrench on the human trunk in any direction in real-time. The Stand Trainer is a platform for standing posture rehabilitation that can control the trunk, pelvis, and knees, simultaneously. The system works through the use of novel force-field algorithms that are modular and user-specific. The control uses an assist-as-needed strategy to apply forces on the user during regions of postural instability. The device also allows perturbations for postural reactive training. We have conducted several studies using healthy adult populations and pilot studies on patient groups including cerebral palsy, cerebellar ataxia, and spinal cord injury. We propose new training methods that incorporate motor learning theory and objective interventions for improving posture control. We identify novel methods to characterize posture in form of the “8-point star test”. This is to assess the postural workspace. We also demonstrate novel methods for functional training of posture and balance. Our results show that training with our robotic platforms can change the trunk kinematics. Specifically, healthy adults are able to translate the trunk further and rotate the trunk more anteriorly in the seated position. In the standing position, they can alter their reach strategy to maintain the upper trunk more vertically while reaching. Similarly, Cerebral Palsy patients improve their trunk translations, reaching workspace, and maintain a more vertical posture after training, in the seated position. Our results also showed that an Ataxia patient was able to improve their reaching workspace and trunk translations in the standing position. Finally, our results show that the robotic platforms can successfully reduce trunk and pelvis sway in spinal cord injury patients. The results of the pilot studies suggest that training with our robotic platforms and methods is beneficial in improving trunk control

A novel approach to apply gait synchronized external forces on the pelvis using A-TPAD to reduce walking effort

In this paper, we develop an intervention to apply external gait synchronized forces on the pelvis to reduce the user’s effort during walking. A cable-driven robot was used to apply the external forces and an adaptive frequency oscillator scheme was developed to adapt the timing of force actuation to the gait frequency during walking. The external forces were directed in the sagittal plane to assist the trailing leg during the forward propulsion and vertical deceleration of the pelvis during the gait cycle. A pilot experiment with five healthy subjects was conducted. The results showed that the subjects applied lower ground reaction forces in the vertical and anteriorposterior directions during the late stance phase. In summary, the current work provides a novel approach to study the role of external pelvic forces in altering the walking effort. These studies can provide better understanding for designing exoskeletons and prosthetic devices to reduce the overall walking effort.by Vineet Vashista, Moiz Khan and Sunil K. Agrawa