Paul Stegall

Effects of Complementary Auditory Feedback in Robot-Assisted Lower Extremity Motor Adaptation

This study investigates how complementary auditory feedback may affect short-term gait modifications induced by four training sessions with a robotic exoskeleton. Healthy subjects walked on a treadmill and were instructed to match a modified gait pattern derived from their natural one, while receiving assistance by the robot (kinetic guidance). The main question we wanted to answer is whether the most commonly used combination of feedback (i.e., haptic and visual) could be either enhanced by adding auditory feedback or successfully substituted with a combination of kinetic guidance and auditory feedback. Participants were randomly assigned to one of four groups, all of which received kinetic guidance. The control group received additional visual feedback, while the three experimental groups were each provided with a different modality of auditory feedback. The third experimental group also received the same visual feedback as the control group. Differences among the training modalities in gait kinematics, timing and symmetry were assessed in three post-training sessions.

Knee Joint Misalignment in Exoskeletons for the Lower Extremities: Effects on User's Gait

Due to the complexity of the human musculoskeletal system and intra/intersubjects variability, powered exoskeletons are prone to human-robot misalignments. These induce undesired interaction forces that may jeopardize safe operation. Uncompensated inertia of the robotic links also generates spurious interaction forces. Current design approaches to compensate for misalignments rely on the use of auxiliary passive degrees of freedom that unavoidably increase robot inertia, which potentially affects their effectiveness in reducing undesired interaction forces. Assessing the relative impact of misalignment and robot inertia on the wearer can, therefore, provide useful insights on how to improve the effectiveness of such approaches, especially in those situations where the dynamics of the movement are quasi-periodic and, therefore, predictable such as in gait. In this paper, we studied the effects of knee joint misalignments on the wearers gait, by using a treadmill-based exoskeleton developed by our group, the ALEX II. Knee joint misalignments were purposely introduced by adjusting the mismatch between the length of the robot thigh and that of the human thigh. The amount of robot inertia reflected to the user was adjusted through control. Results evidenced that knee misalignment significantly changes human-robot interaction forces, especially at the thigh interface, and this effect can be attenuated by actively compensating for robot inertia. Misalignments caused by an excessively long robot thigh are less critical than misalignments of equal magnitude deriving from an excessively short robot thigh.

Adaptive assist-as-needed controller to improve gait symmetry in robot-assisted gait training

This paper introduces the overall design of ALEX III, the third generation of Active Leg Exoskeletons developed by our group. ALEX III is the first treadmill-based rehabilitation robot featuring 12 actively controlled degrees of freedom (DOF): 4 at the pelvis and 4 at each leg. As a first application of the device, we present an adaptive controller aimed to improve gait symmetry in hemiparetic subjects. The controller continuously modulates the assistive force applied to the impaired leg, based on the outputs of kernel-based non-linear filters, which learn the movements of the healthy leg. To test the effectiveness of the controller, we induced asymmetry in the gait of three young healthy subjects adding ankle weights (2.3kg). Results on kinematic data showed that gait symmetry was recovered when the controller was active.

Directed neural connectivity changes in robot-assisted gait training: A partial Granger causality analysis

Now-a-days robotic exoskeletons are often used to help in gait training of stroke patients. However, such robotic systems have so far yielded only mixed results in benefiting the clinical population. Therefore, there is a need to investigate how gait learning and de-learning get characterised in brain signals and thus determine neural substrate to focus attention on, possibly, through an appropriate brain-computer interface (BCI). To this end, this paper reports the analysis of EEG data acquired from six healthy individuals undergoing robot-assisted gait training of a new gait pattern. Time-domain partial Granger causality (PGC) method was applied to estimate directed neural connectivity among relevant brain regions. To validate the results, a power spectral density (PSD) analysis was also performed. Results showed a strong causal interaction between lateral motor cortical areas. A frontoparietal connection was found in all robot-assisted training sessions. Following training, a causal “top-down” cognitive control was evidenced, which may indicate plasticity in the connectivity in the respective brain regions.

Dynamic brace for correction of abnormal postures of the human spine

This paper describes the design and control architectures for a novel active thoracolumbosacral orthosis targeted at correction of abnormal postures and treatment of the human spine, often seen in adolescent idiopathic or neuromuscular scoliosis. Our novel device is motivated by the current limitations of the rigid braces used for this purpose which do not adapt to changes in the skeletal system in response to treatment. In addition, the dynamic brace can open possibilities for new treatment methods which currently do not exist. Previous brace designs were not capable of providing dynamic controlled forces. Our design utilizes two Stewart-Gough platforms in series, each controlled independently, either in position or force modes. The design can provide controlled forces/torques on different regions of spine to modify the posture. Additionally, it can control the motion of different regions of the spine through independent position control of each platform using six parallel actuators. Both control methods were validated in benchtop tests. A range of motion study was also performed with a healthy subject wearing the device while the system was controlled in transparent mode.

Walking With aBackpack Using Load Distribution and Dynamic Load Compensation Reduces Metabolic Cost and Adaptations to Loads

In this study, we showed a way of reducing the metabolic cost of walking with a backpack using load distribution and dynamic load compensation, provided by a wearable upper body device. This device distributes the backpackload between the shouldersand the pelvis, senses the vertical motion of the pelvis, and provides gait synchronized compensatory forces to reduce the dynamic loads from a backpack. It was hypothesized that by reducing dynamic loads from a backpack during load carriage, the user’s gait and postural adaptation, muscular effort and metabolic cost would be reduced. This hypothesis was supported by biomechanical and physiological measurements on a group of young healthy subjects, as they walked on a treadmill under four different conditions: unloaded; with a backpack, loaded with 25% of their body weight, supported on the shoulders; with the same load distributed between the shoulders and the pelvis; and with dynamic load compensation in addition to load distribution. The results showed reductions in gait and postural adaptations, muscle activity, vertical and braking ground reaction forces, and metabolic cost while carrying the same backpack load with the device. We conclude that the device can potentially reduce the risk of musculoskeletal injuries and muscle fatigue associated with carrying heavy backpack loads while reducing the metabolic cost of loaded walking.

Analytic Determination of Wrench Closure Workspace of Spatial Cable Driven Parallel Mechanisms

This paper proposes an efficient way of determining analytically the Wrench Closure Workspace (WCW) of spatial redundant cable-driven parallel mechanisms (CDPM). The method builds upon the boundary surface equations obtained from the null space of the structure matrix of CDPM. The set of feasible solutions is obtained that satisfies positive tension in the cables. This method was applied to characterize the WCW of spatial CDPM which has redundancy of 1 or 2. A simulation study was carried out to validate the accuracy and efficiency of the method. Several advantages over conventional approaches for determining the WCW were identified through simulation.Copyright

Wearable Upper Body Suit for Assisting Human Load Carriage

This paper presents a wearable upper body suit designed to assist in human load carriage. The two functions of the suit are: (i) load distribution between the shoulders and the waist, and (ii) reduction of the dynamic load on the waist during walking. These are achieved through two cable driven modules — passive and active — within a custom fitted shirt integrated with motion/force sensors, actuators, and a real time controller. The load distribution between the shoulders and the waist is achieved through the load bearing columns connecting the shoulder pads and the waist belt whose load bearing capacity is modulated by a nominal cable tension in the passive module via a ratchet mechanism. The dynamic load is reduced in addition in the active module by modulating the cable tension via external actuator. Mathematical model of the system is presented and a state feedback controller is designed. Simulation study was performed to investigate the system response under different disturbance conditions as a result of vertical motion of the waist during human walking. Experiment evaluation of the suit was performed with a subject walking on a treadmill while carrying a backpack load. The results show that the developed suit can transfer the load from the shoulders to the waist as well as reduce the dynamic load induced during human walking. This can potentially reduce the energy expenditure and the risk of musculoskeletal injuries associated with human load carriage.Copyright

Second Spine: A device to relieve stresses on the upper body during loaded walking

The target of this work is to experimentally validate the Second Spine, a wearable device recently developed by our group to transfer forces from shoulder to pelvis during loaded walking. A key-feature of the Second Spine compared to traditional framed backpacks is the adjustable stiffness of its structure, which allows the wearer to change the load-bearing behavior of the device. In line with previous studies on loaded walking, we investigate biomechanical and physiological variables on a small group of young healthy subjects, as they walked on a treadmill under 3 different conditions: free walking, walking with a backpack of 25% of subjects Body Weight (BW), and walking with the same backpack while wearing the device. Results indicate that wearing the Second Spine significantly reduces the pressure on shoulders and induces smaller deviations from unloaded walking in terms of gait timing and stride length. The activations of the rectus femoris and the gastrocnemius muscles, along with the kinematics of the knee joint, provide indirect evidence that dynamic loads were rigidly transmitted from the shoulder to the waist. We discuss how these preliminary findings might be relevant for the prevention of injuries related to load carriage, and how they set important guidelines for the next generation of the Second Spine.