Capturing Upper Body Kinematics and Localization with Low-Cost Sensors for Rehabilitation Applications.

For upper extremity rehabilitation, quantitative measurements of a person's capabilities during activities of daily living could provide useful information for therapists, including in telemedicine scenarios. Specifically, measurements of a person's upper body kinematics could give information about which arm motions or movement features are in need of additional therapy, and their location within the home could give context to these motions. To that end, we present a new algorithm for identifying a person's location in a region of interest based on a Bluetooth received signal strength (RSS) and present an experimental evaluation of this and a different Bluetooth RSS-based localization algorithm via fingerprinting. We further present algorithms for and experimental results of inferring the complete upper body kinematics based on three standalone inertial measurement unit (IMU) sensors mounted on the wrists and pelvis. Our experimental results for localization find the target location with a mean square error of 1.78 m. Our kinematics reconstruction algorithms gave lower errors with the pelvis sensor mounted on the person's back and with individual calibrations for each test. With three standalone IMUs, the mean angular error for all of the upper body segment orientations was close to 21 degrees, and the estimated elbow and shoulder angles had mean errors of less than 4 degrees.

Modeling the metabolic reductions of a passive back-support exoskeleton.

Despite several attempts to quantify the metabolic savings resulting from the use of passive back-support exoskeletons (BSEs), no study has modeled the metabolic change while wearing an exoskeleton during lifting. The objectives of this study were to: 1) quantify the metabolic reductions due to the VT-Lowe's exoskeleton during lifting; and 2) provide a comprehensive model to estimate the metabolic reductions from using a passive BSE. In this study, 15 healthy adults (13 males, 2 females) of ages 20-34 yr (mean = 25.33, SD = 4.43) performed repeated freestyle lifting and lowering of an empty box and a box with 20% of their bodyweight. Oxygen consumption and metabolic expenditure data were collected. A model for metabolic expenditure was developed and fitted with the experimental data of two prior studies and the without-exoskeleton experimental results. The metabolic cost model was then modified to reflect the effect of the exoskeleton. The experimental results revealed that VT-Lowe's exoskeleton significantly lowered the oxygen consumption by ∼9% for an empty box and 8% for a 20% bodyweight box, which corresponds to a net metabolic cost reduction of ∼12% and ∼9%, respectively. The mean metabolic difference (i.e., without-exo minus with-exo) and the 95% confidence interval were 0.36 and (0.2-0.52) W/kg for 0% body weight and 0.43 and (0.18-0.69) W/kg for 20% body weight. Our modeling predictions for with-exoskeleton conditions were precise, with absolute freestyle prediction errors of <2.1%. The model developed in this study can be modified based on different study designs, and can assist researchers in enhancing designs of future lifting exoskeletons.NEW & NOTEWORTHY We present a new model of the metabolic cost of repetitive lifting, and how that is affected by wearing a passive back support exoskeleton. We compute the effective biomechanical efficiencies of moving the body and a carried load during lifting, and determine the effect of an exoskeleton's efficiency on its metabolic reduction. This model is useful for understanding the effects of exoskeletons on the body and for designing future exoskeletons.

Just noticeable differences for elbow joint torque feedback.

Joint torque feedback is a new and promising means of kinesthetic feedback imposed by a wearable device. The torque feedback provides the wearer temporal and spatial information during a motion task. Nevertheless, little research has been conducted on quantifying the psychophysical parameters of how well humans can perceive external torques under various joint conditions. This study aims to investigate the just noticeable difference (JND) perceptual ability of the elbow joint to joint torques. The paper focuses on the ability of two primary joint proprioceptors, the Golgi-tendon organ (GTO) and muscle spindle (MS), to detect elbow torques, since touch and pressure sensors were masked. We studied 14 subjects while the arm was isometrically contracted (static condition) and was moving at a constant speed (dynamic condition). In total there were 10 joint conditions investigated, which varied the direction of the arm's movement and the preload direction as well as torque direction. The JND torques under static conditions ranged from 0.097 Nm with no preload to 0.197 Nm with a preload of 1.28 Nm. The maximum dynamic JND torques were 0.799 Nm and 0.428 Nm, when the arm was flexing and extending at 213 degrees per second, respectively.

Kinematic effects of a passive lift assistive exoskeleton.

The VT-Lowe's exoskeleton was designed to help support the back during repetitive lifting tasks. This study focused on the kinematic differences between lifting with and without the exoskeleton (With-Exo and Without-Exo) over three different lifting styles (Freestyle, Squat, and Stoop) and two different box weights (0% and 20% of bodyweight). Twelve young and healthy males (Age 23.5 +/- 4.42 years; Height 179.33 +/- 6.37 cm; Weight 80.4 +/- 5.59 kg) participated in this study. Variables analyzed include the ankle and knee angles and angle between the Shoulder-Hip-Knee (SHK); the shoulder, elbow, and wrist heights; and the lifting speed and acceleration. The relationships between the torso angle, SHK angle, center of mass of the torso, torso torque, box height, as well as electromyography (EMG) data from a related study were also analyzed. On average, wearing the exoskeleton resulted in a 1.5 degree increase in ankle dorsiflexion, a 2.6 degree decrease in knee flexion, and a decrease of 2.3 degrees in SHK angle. Subjects' shoulder, elbow, and wrist heights were slightly higher while wearing the exoskeleton, and they lifted slightly more slowly while wearing the exoskeleton. Subjects moved more quickly while bending down as compared to standing up, and with the 0% bodyweight box as compared to the 20% bodyweight box. The values for Freestyle lifts generally fell in between Squat and Stoop lift styles or were not significantly different from Squat. EMG data from the leg muscles had relationships with torso torque while the back and stomach muscles showed no significant relationships.

Motion Inference Using Sparse Inertial Sensors, Self-Supervised Learning, and a New Dataset of Unscripted Human Motion.

In recent years, wearable sensors have become common, with possible applications in biomechanical monitoring, sports and fitness training, rehabilitation, assistive devices, or human-computer interaction. Our goal was to achieve accurate kinematics estimates using a small number of sensors. To accomplish this, we introduced a new dataset (the Virginia Tech Natural Motion Dataset) of full-body human motion capture using XSens MVN Link that contains more than 40 h of unscripted daily life motion in the open world. Using this dataset, we conducted self-supervised machine learning to do kinematics inference: we predicted the complete kinematics of the upper body or full body using a reduced set of sensors (3 or 4 for the upper body, 5 or 6 for the full body). We used several sequence-to-sequence (Seq2Seq) and Transformer models for motion inference. We compared the results using four different machine learning models and four different configurations of sensor placements. Our models produced mean angular errors of 10-15 degrees for both the upper body and full body, as well as worst-case errors of less than 30 degrees. The dataset and our machine learning code are freely available.

Quantification of Postures for Low-Height Object Manipulation Conducted by Manual Material Handlers in a Retail Environment.

Occupational Applications Manual material handlers performing stocking tasks spent substantial amounts of time in bent postures but used traditional stoops and squats infrequently. Instead, they often used split-legged stoops and squats, where one foot is further forward than the other, and one-legged ("golfer's") lifts. During object manipulation, the distance workers reached away from their body, and the height at which they manipulated objects, were correlated with the posture used by the worker. Workers also stayed in different postures for different lengths of time. It is likely that certain postures are more comfortable for the workers to remain in, provide additional mobility or operational radius, or require less energy to use. Understanding these factors in more detail could lead to improved worker training programs, where the postures taught not only have low injury risk but are comfortable so are actually adopted and used by the workers.

Technical Abstract Background Musculoskeletal disorders are relatively common among manual material handlers. This may be due in part to challenging postures used by workers. Purpose Studying the kinematics of manual material handlers in the workplace will provide quantitative data on how they move and what postures they adopt. With these data, some insights can be determined about why workers chose certain postures. Methods We conducted an on-site workplace study to capture the full-body kinematics of manual material handlers (stockers) using inertial measurement units. We organized the observed bends into six classes: stooping, fore-aft squatting, split-legged stooping with one-heel raised, split-legged stooping with no heels raised, one-legged lifting, and mixed lifting, which include multiple forms while remaining bent. These classes were based on a new general classification of bending and lifting postures that we developed, which enumerates all of the possible forms. We quantified how frequently and for what duration the workers bent and lifted, and determined how often they performed asymmetric motions while bending. We determined the range of motion of the hand positions during each bent posture, which provides a measure of the workspace afforded by the posture. Results Workers rarely used symmetric squats and infrequently used symmetric stoops typically studied in lab settings. Instead, they used a variety of different postures that have not been well-characterized. Of the 666 bending postures recorded during the experiment, 27.3% were stoops lifts, 22.1% were one-legged lifts, 20.3% were split-legged bends with both heels on the ground, and 12.3% were split-legged bends with a heel raised. Only 4.6% of the postures were squats and only one participant used this posture. Different bending postures were correlated with different ranges of hand position used in object manipulation. One-legged lifting corresponded to bends with the hands furthest away from the body along the sagittal axis. Conclusions While our study was exploratory, we observed many kinematic forms that have not been studied much in the past, such as split-legged stooping and one-legged lifting, suggesting that future work should be done to understand the biomechanics of these postures.

A Layered Manufacturing Approach for Soft and Soft-Rigid Hybrid Robots.

We present a manufacturing process for creating centimeter-scale multichambered inflatable robots and structures that can include both soft and rigid components. Our process uses a thermoplastic polyurethane (TPU) adhesive film to bond together layers of textiles, plastics, or other materials. The structures are heated and compressed a few layers at a time with a heat press machine or bonded in an oven all at once. We present two methods for arranging textiles and thermal adhesive film to achieve airtight structures and perform modeling and measurements on the resulting inflatable chambers. We characterize the set of textiles and rigid materials that will work with this process, measuring how strongly the TPU film bonds with them. We also describe how to include corners, where several pieces of material come together at a point, and determine which corner constructions are airtight. We characterize how different seam widths behave, determine the maximum pressure chambers fabricated with this process can support, and determine the cycle life of actuators built with this process. Finally, we present an actuator with an embedded sensor and three examples of robots constructed with textiles and TPU film, including a hybrid soft/rigid robotic arm, a soft robot that can roll along the ground, and a robot that can climb inside tubes or other confined spaces.

An elbow exoskeleton for haptic feedback made with a direct drive hobby motor.

A direct drive motor is one of the simplest mechanisms that can be used to move a mechanical joint. In particular, a brushless direct current (BLDC) motor with no gearing produces a low parasitic torque due to its backdrivability and low inertia, which is ideal for some applications such as wearable systems. While capable of operating with a higher power density than brushed motors, BLDC motors require accurate position feedback to be controlled via vector control at slow speeds. The MotorWare ™ library from Texas Instruments (TI), which is designed to run with a C2000 microcontroller, is written to run BLDCs. However, the code was written to run the motor continuously with an incremental encoder and requires further engineering to be used at low speeds such as in an exoskeleton. In this paper, we present the design of an elbow exoskeleton that can be used for haptic feedback. We provide instructions to build the exoskeleton hardware, custom code to modify software provided by TI so that a motor can provide a controlled torque at low speeds, code to enable the microcontroller to communicate with a computer for high-level commands and data storage, and also provide an overview of how alternate motors could be used with this software setup.

A passive exoskeleton reduces peak and mean EMG during symmetric and asymmetric lifting.

The VT-Lowe's exoskeleton is a novel passive lift-assistive device designed to offload the back muscles during repetitive lifting. In this study, the effect of the exoskeleton on electromyographic (EMG) signals was investigated in four different lifting types (stoop, squat, freestyle and asymmetric) and two box weights (0% and 20% of body weight). Twelve young healthy adults ages 18-31 years (mean = 22.75, SD = 4.35) were participants. The EMG signals for twelve muscles (iliocostalis erector spinae (IL), longissimus erector spinae (LT), multifidus (MF), bicep femoris (BF), vastus lateralis (VL) and abdominal external oblique (AEO) muscles) were measured. The exoskeleton significantly decreased the peak and mean activity of back muscles (IL and LT) by 31.5% and 29.3%, respectively, for symmetric lifts and by 28.2% and 29.5%, respectively, for asymmetric lifts. The peak and mean EMG of leg muscles were significantly reduced by 19.1% and 14.1% during symmetric lifts, and 17.4% and 14.6% during asymmetric lifts. Although the exoskeleton reduced the activation of back and leg muscles, it slightly increased the activity of external oblique muscles, although this was not statistically significant. In conclusion, the exoskeleton is promising as a lift-assist device for manual material handlers and workers performing repetitive lifting.

Modeling and Analysis of a High-Displacement Pneumatic Artificial Muscle With Integrated Sensing.

We present a high-displacement pneumatic artificial muscle made of textiles or plastics that can include integrated electronics to sense its pressure and displacement. Compared to traditional pneumatic muscle actuators such as the McKibben actuator and other more recent soft actuators, the actuator described in this paper can produce a much higher (40~65%) contraction ratio. In this paper, we describe the design, fabrication, and evaluation of the actuator, as well as the manufacturing process used to create it. We demonstrate the actuator design with several examples that produce 120 and 300 N at pressures of 35 and 105 kPa, respectively, and have contraction ratios of 40-65%.

Biomechanical and Physiological Evaluation of Multi-Joint Assistance With Soft Exosuits.

To understand the effects of soft exosuits on human loaded walking, we developed a reconfigurable multi-joint actuation platform that can provide synchronized forces to the ankle and hip joints. Two different assistive strategies were evaluated on eight subjects walking on a treadmill at a speed of 1.25 m/s with a 23.8 kg backpack: 1) hip extension assistance and 2) multi-joint assistance (hip extension, ankle plantarflexion and hip flexion). Results show that the exosuit introduces minimum changes to kinematics and reduces biological joint moments. A reduction trend in muscular activity was observed for both conditions. On average, the exosuit reduced the metabolic cost of walking by 0.21 ±0.04 and 0.67 ±0.09 W/kg for hip extension assistance and multi-joint assistance respectively, which is equivalent to an average metabolic reduction of 4.6% and 14.6%, demonstrating that soft exosuits can effectively improve human walking efficiency during load carriage without affecting natural walking gait. Moreover, it indicates that actuating multiple joints with soft exosuits provides a significant benefit to muscular activity and metabolic cost compared to actuating single joint.

A biologically-inspired multi-joint soft exosuit that can reduce the energy cost of loaded walking.

BACKGROUND: Carrying load alters normal walking, imposes additional stress to the musculoskeletal system, and results in an increase in energy consumption and a consequent earlier onset of fatigue. This phenomenon is largely due to increased work requirements in lower extremity joints, in turn requiring higher muscle activation. The aim of this work was to assess the biomechanical and physiological effects of a multi-joint soft exosuit that applies assistive torques to the biological hip and ankle joints during loaded walking. METHODS: The exosuit was evaluated under three conditions: powered (EXO_ON), unpowered (EXO_OFF) and unpowered removing the equivalent mass of the device (EXO_OFF_EMR). Seven participants walked on an instrumented split-belt treadmill and carried a load equivalent to 30 % their body mass. We assessed their metabolic cost of walking, kinetics, kinematics, and lower limb muscle activation using a portable gas analysis system, motion capture system, and surface electromyography. RESULTS: Our results showed that the exosuit could deliver controlled forces to a wearer. Net metabolic power in the EXO_ON condition (7.5 ± 0.6 W kg(-1)) was 7.3 ± 5.0 % and 14.2 ± 6.1 % lower than in the EXO_OFF_EMR condition (7.9 ± 0.8 W kg(-1); p = 0.027) and in the EXO_OFF condition (8.5 ± 0.9 W kg(-1); p = 0.005), respectively. The exosuit also reduced the total joint positive biological work (sum of hip, knee and ankle) when comparing the EXO_ON condition (1.06 ± 0.16 J kg(-1)) with respect to the EXO_OFF condition (1.28 ± 0.26 J kg(-1); p = 0.020) and to the EXO_OFF_EMR condition (1.22 ± 0.21 J kg(-1); p = 0.007). CONCLUSIONS: The results of the present work demonstrate for the first time that a soft wearable robot can improve walking economy. These findings pave the way for future assistive devices that may enhance or restore gait in other applications.

Biologically-inspired soft exosuit.

In this paper, we present the design and evaluation of a novel soft cable-driven exosuit that can apply forces to the body to assist walking. Unlike traditional exoskeletons which contain rigid framing elements, the soft exosuit is worn like clothing, yet can generate moments at the ankle and hip with magnitudes of 18% and 30% of those naturally generated by the body during walking, respectively. Our design uses geared motors to pull on Bowden cables connected to the suit near the ankle. The suit has the advantages over a traditional exoskeleton in that the wearer's joints are unconstrained by external rigid structures, and the worn part of the suit is extremely light, which minimizes the suit's unintentional interference with the body's natural biomechanics. However, a soft suit presents challenges related to actuation force transfer and control, since the body is compliant and cannot support large pressures comfortably. We discuss the design of the suit and actuation system, including principles by which soft suits can transfer force to the body effectively and the biological inspiration for the design. For a soft exosuit, an important design parameter is the combined effective stiffness of the suit and its interface to the wearer. We characterize the exosuit's effective stiffness, and present preliminary results from it generating assistive torques to a subject during walking. We envision such an exosuit having broad applicability for assisting healthy individuals as well as those with muscle weakness.