Latent Factors Limiting the Performance of sEMG-Interfaces

Recent advances in recording and real-time analysis of surface electromyographic signals (sEMG) have fostered the use of sEMG human–machine interfaces for controlling personal computers, prostheses of upper limbs, and exoskeletons among others. Despite a relatively high mean performance, sEMG-interfaces still exhibit strong variance in the fidelity of gesture recognition among different users. Here, we systematically study the latent factors determining the performance of sEMG-interfaces in synthetic tests and in an arcade game. We show that the degree of muscle cooperation and the amount of the body fatty tissue are the decisive factors in synthetic tests. Our data suggest that these factors can only be adjusted by long-term training, which promotes fine-tuning of low-level neural circuits driving the muscles. Short-term training has no effect on synthetic tests, but significantly increases the game scoring. This implies that it works at a higher decision-making level, not relevant for synthetic gestures. We propose a procedure that enables quantification of the gestures’ fidelity in a dynamic gaming environment. For each individual subject, the approach allows identifying “problematic” gestures that decrease gaming performance. This information can be used for optimizing the training strategy and for adapting the signal processing algorithms to individual users, which could be a way for a qualitative leap in the development of future sEMG-interfaces

On the Utility of Representation Learning Algorithms for Myoelectric Interfacing

Electrical activity produced by muscles during voluntary movement is a reflection of the firing patterns of relevant motor neurons and, by extension, the latent motor intent driving the movement. Once transduced via electromyography (EMG) and converted into digital form, this activity can be processed to provide an estimate of the original motor intent and is as such a feasible basis for non-invasive efferent neural interfacing. EMG-based motor intent decoding has so far received the most attention in the field of upper-limb prosthetics, where alternative means of interfacing are scarce and the utility of better control apparent. Whereas myoelectric prostheses have been available since the 1960s, available EMG control interfaces still lag behind the mechanical capabilities of the artificial limbs they are intended to steer—a gap at least partially due to limitations in current methods for translating EMG into appropriate motion commands. As the relationship between EMG signals and concurrent effector kinematics is highly non-linear and apparently stochastic, finding ways to accurately extract and combine relevant information from across electrode sites is still an active area of inquiry.This dissertation comprises an introduction and eight papers that explore issues afflicting the status quo of myoelectric decoding and possible solutions, all related through their use of learning algorithms and deep Artificial Neural Network (ANN) models. Paper I presents a Convolutional Neural Network (CNN) for multi-label movement decoding of high-density surface EMG (HD-sEMG) signals. Inspired by the successful use of CNNs in Paper I and the work of others, Paper II presents a method for automatic design of CNN architectures for use in myocontrol. Paper III introduces an ANN architecture with an appertaining training framework from which simultaneous and proportional control emerges. Paper Iv introduce a dataset of HD-sEMG signals for use with learning algorithms. Paper v applies a Recurrent Neural Network (RNN) model to decode finger forces from intramuscular EMG. Paper vI introduces a Transformer model for myoelectric interfacing that do not need additional training data to function with previously unseen users. Paper vII compares the performance of a Long Short-Term Memory (LSTM) network to that of classical pattern recognition algorithms. Lastly, paper vIII describes a framework for synthesizing EMG from multi-articulate gestures intended to reduce training burden

Subject-independent modeling of sEMG signals for the motion of a single robot joint through GMM Modelization

This thesis evaluates the use of a probabilistic model, the Gaussian Mixture Model (GMM), trained through Electromyography (EMG) signals to estimate the bending angle of a single human joint. The GMM is created from the EMG signals collected by different people and the goal is to create a general model based on the data of different subjects. The model is then tested on new, unseen data. The goodness of the estimated data is evaluated by means of Normalized Mean Square Errorope

Force-Aware Interface via Electromyography for Natural VR/AR Interaction

While tremendous advances in visual and auditory realism have been made for virtual and augmented reality (VR/AR), introducing a plausible sense of physicality into the virtual world remains challenging. Closing the gap between real-world physicality and immersive virtual experience requires a closed interaction loop: applying user-exerted physical forces to the virtual environment and generating haptic sensations back to the users. However, existing VR/AR solutions either completely ignore the force inputs from the users or rely on obtrusive sensing devices that compromise user experience. By identifying users' muscle activation patterns while engaging in VR/AR, we design a learning-based neural interface for natural and intuitive force inputs. Specifically, we show that lightweight electromyography sensors, resting non-invasively on users' forearm skin, inform and establish a robust understanding of their complex hand activities. Fuelled by a neural-network-based model, our interface can decode finger-wise forces in real-time with 3.3% mean error, and generalize to new users with little calibration. Through an interactive psychophysical study, we show that human perception of virtual objects' physical properties, such as stiffness, can be significantly enhanced by our interface. We further demonstrate that our interface enables ubiquitous control via finger tapping. Ultimately, we envision our findings to push forward research towards more realistic physicality in future VR/AR.Comment: ACM Transactions on Graphics (SIGGRAPH Asia 2022

Myoelectric Control Architectures to Drive Upper Limb Exoskeletons

Myoelectric interfaces are sensing devices based on electromyography (EMG) able to read the electrical activity of motoneurons and muscles. These interfaces can be used to infer movement volition and to control assistive devices. Currently, these interfaces are widely used to control robotic prostheses for amputees, but their use could be beneficial even for people suffering from motor disabilities where the peripheral nervous system is intact and the impairment is only due to the muscles, e.g. muscular dystrophy, myopathies, or ageing. In combination with recent robotic orthoses and exoskeletons, myoelectric interfaces could dramatically improve these patients’ quality of life. Unfortunately, despite a wide plethora of methodologies has been proposed so far, a natural, intuitive, and reliable interface able to follow impaired subjects’ volition is still missing. The first contribution of this work is to provide a review of existing approaches. In this work we found that existing EMG-based control interfaces can be viewed as specific cases of a generic myoelectric control architecture composed by three distinct functional modules: a decoder to extract the movement intention from EMG signals, a controller to accomplish the desired motion through an actual command given to the actuators, and an adapter to connect them. The latter is responsible for translating the signal from decoder’s output to controller’s input domain and for modulating the level of provided assistance. We used this concept to analyse the case of study of linear regression decoders and an elbow exoskeleton. This thesis has the scientific objective to determine how these modules affect performance of EMG-driven exoskeletons and wearer’s fatigue. To experimentally test and compare myoelectric interfaces this work proposes: (1) a procedure to automatically tune the decoder module in order to equally compare or to normalize the decoder output among different sessions and subjects; (2) a procedure to automatically tune gravity compensation even for subjects suffering from severe disabilities, allowing them to perform the experimental tests; (3) a methodology to guide the impaired patients through the experimental session; (4) an evaluation procedure and metrics allowing statistically significant and unbiased comparison of different myoelectric interfaces. A further contribution of this work is the design of an experimental test bed composed by an elbow exoskeleton and by a software framework able to collect EMG signals and make them available to the exoskeleton’s actuators with minimal latency. Using this test bed, we were able to test different myoelectric interfaces based on our architecture, with different modules choices and tunings. We used linear regression decoders calibrated to predict the muscular torque, low-level controllers having torque or velocity as reference, and adapters consisting of a properly dimensioned gain or simple dynamic systems, such as an integrator or a mass-damping system. The results we obtained allow to conclude that EMG-based control is a viable technology to assist muscular weakness patients. Moreover, all the components of the myoelectric control architecture – decoder, adapter, controller, and their tuning – significantly affect the task-based performance measures we collect. Further investigations should be devoted to a methodology to automatically tune all the components, not the decoders only, and to the quantitative study of the effect the adapter has on the regulation of the assistance level and of the tradeoff between speed and accuracy

Applications of EMG in Clinical and Sports Medicine

This second of two volumes on EMG (Electromyography) covers a wide range of clinical applications, as a complement to the methods discussed in volume 1. Topics range from gait and vibration analysis, through posture and falls prevention, to biofeedback in the treatment of neurologic swallowing impairment. The volume includes sections on back care, sports and performance medicine, gynecology/urology and orofacial function. Authors describe the procedures for their experimental studies with detailed and clear illustrations and references to the literature. The limitations of SEMG measures and methods for careful analysis are discussed. This broad compilation of articles discussing the use of EMG in both clinical and research applications demonstrates the utility of the method as a tool in a wide variety of disciplines and clinical fields

Machine Learning-Based Hand Gesture Recognition via EMG Data

Electromyography (EMG) data gives information about the electrical activity related to muscles. EMG data obtained from arm through sensors helps to understand hand gestures. For this work, hand gesture data were taken from UCI2019 EMG dataset obtained from MYO thalmic armband were classied with six dierent machine learning algorithms. Articial Neural Network (ANN), Support Vector Machine (SVM), k-Nearest Neighbor (k-NN), Naive Bayes (NB), Decision Tree (DT) and Random Forest (RF) methods were preferred for comparison based on several performance metrics which are accuracy, precision, sensitivity, specicity, classication error, kappa, root mean squared error (RMSE) and correlation. The data belongs to seven hand gestures. 700 samples from 7 classes (100 samples per group) were used in the experiments. The splitting ratio in the classication was 0.8-0.2, i.e. 80% of the samples were used in training and 20% of data were used in testing phase of the classier. NB was found to be the best among other methods because of high accuracy (96.43%) and sensitivity (96.43%) and the lowest RMSE (0.189). Considering the results of the performance parameters, it can be said that this study recognizes and classies seven hand gestures successfully in comparison with the literature

Machine Learning for Hand Gesture Classification from Surface Electromyography Signals

Classifying hand gestures from Surface Electromyography (sEMG) is a process which has applications in human-machine interaction, rehabilitation and prosthetic control. Reduction in the cost and increase in the availability of necessary hardware over recent years has made sEMG a more viable solution for hand gesture classification. The research challenge is the development of processes to robustly and accurately predict the current gesture based on incoming sEMG data. This thesis presents a set of methods, techniques and designs that improve upon evaluation of, and performance on, the classification problem as a whole. These are brought together to set a new baseline for the potential classification. Evaluation is improved by careful choice of metrics and design of cross-validation techniques that account for data bias caused by common experimental techniques. A landmark study is re-evaluated with these improved techniques, and it is shown that data augmentation can be used to significantly improve upon the performance using conventional classification methods. A novel neural network architecture and supporting improvements are presented that further improve performance and is refined such that the network can achieve similar performance with many fewer parameters than competing designs. Supporting techniques such as subject adaptation and smoothing algorithms are then explored to improve overall performance and also provide more nuanced trade-offs with various aspects of performance, such as incurred latency and prediction smoothness. A new study is presented which compares the performance potential of medical grade electrodes and a low-cost commercial alternative showing that for a modest-sized gesture set, they can compete. The data is also used to explore data labelling in experimental design and to evaluate the numerous aspects of performance that must be traded off