Abstract
BackgroundMyoelectric signals offer significant insights in interpreting the motion intention and extent of effort involved in performing a movement, with application in prostheses, orthosis and exoskeletons. Feature extraction plays a vital role, and follows two approaches: EMG and synergy features. More recently, muscle synergy based features are being increasingly explored, since it simplifies dimensionality of control, and are considered to be more robust to signal variations. Another important aspect in a myoelectrically controlled devices is the learning capability and speed of performance for online decoding. Extreme learning machine (ELM) is a relatively new neural-network based learning algorithm: its performance hasn’t been explored in the context of online control, which is a more reliable measure compared to offline analysis. To this purpose we aim at focusing our investigation on a myoelectric-based interface which is able to identify and online classify, upper limb motions involving shoulder and elbow. The main objective is to compare the performance of the decoder trained using ELM, for two different features: EMG and synergy features.MethodsThe experiments are broadly divided in two phases training/calibration and testing respectively. ELM is used to train the decoder using data acquired during the calibration phase. The performance of the decoder is then tested in online motion control by using a simulated graphical user interface replicating the human limb: subjects are requested to control a virtual arm by using their muscular activity. The decoder performance is quantified using ad-hoc metrics based on the following indicators: motion selection time, motion completion time, and classification accuracy.ResultsPerformance has been evaluated for both offline and online contexts. The offline classification results indicated better performance in the case of EMG features, whereas a better classification accuracy for synergy feature was observed for online decoding. Also the other indicators as motion selection time and motion completion time, showed better trend in the case of synergy than time-domain features.ConclusionThis work demonstrates better robustness of online decoding of upper-limb motions and motor intentions when using synergy feature. Furthermore, we have quantified the performance of the decoder trained using ELM for online control, providing a potential and viable option for real-time myoelectric control in assistive technology.Electronic supplementary materialThe online version of this article (doi:10.1186/s12984-016-0183-0) contains supplementary material, which is available to authorized users.
Highlights
Myoelectric signals offer significant insights in interpreting the motion intention and extent of effort involved in performing a movement, with application in prostheses, orthosis and exoskeletons
Shi et al [26] shows that the running time of Extreme learning machine (ELM) is much faster than linear discriminant analysis (LDA) and support vector machines (SVM); the results indicate the classification accuracy of ELM is overall higher than LDA, and almost comparable with that of SVM, showing the potential of ELM for real-time myoelectric control of assistive devices
Performance metrics In order to compare the accuracy of the ELM algorithm for EMG features and synergy feature, we evaluate the offline performance and online performance
Summary
Myoelectric signals offer significant insights in interpreting the motion intention and extent of effort involved in performing a movement, with application in prostheses, orthosis and exoskeletons. Muscle synergy based features are being increasingly explored, since it simplifies dimensionality of control, and are considered to be more robust to signal variations Another important aspect in a myoelectrically controlled devices is the learning capability and speed of performance for online decoding. Electromyogram (EMG) signals are nowadays the most widely used biometric information to translate human motion intention into action Their main use ranges from interfaces in human-machine interaction based applications like prosthesis [1,2,3], orthosis [4,5,6] and telemanipulation [7,8,9,10], to functional electrical stimulation as well [11, 12]. Synergy features have shown to be consistent and robust to slight shift in electrode position [22]
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