Conclusions and outlook
pronation, supination, gestures or implicit learning
REGRESSION
In this work, a detailed examination of kNN-based learning techniques in the context of electromyographically controlled prostheses was conducted.Summing up, with the proposed and implemented algorithms, all requirements stated in Table First, the influence of several parameters on the block-wise cross-validation was examined for kNN classification. This showed that setting The analysis of numbers With the choice of In contrast to the Mahalanobis distance, the distance metrics based on the Minkowski norm proved well. In some cases, a higher order of norm yielded better results. This was the case when not considering the wrist rotation gestures where the Chebyshev distance performed the best. With pronation and supination included, there was the reverse effect, i.e. the Manhattan norm performing best. The Euclidean distance seems to be a good compromise to equalize both effects, although its calculation (multiplications) is slightly more computationally expensive than those of the Manhattan and Chebyshev norms.The chosen factor of distance weighting seemed to not heavily influence the classification accuracy if Regarding the pilot experiments, the offset scaling showed the best effect in optimizing the trade-off between a high value range of exertion levels and low-intensity gestures still being reachable. A scale offset divisor set to 5 could increase the success rate up to over 90% both for the gesture set (rs, pw, pn, fl, ex) as well as the specifically problematic set of additionally including another degree of freedom in the form of pronation and supination. Nevertheless, possibilities for a usage of more sophisticated proportionality schemes have to be evaluated. Instead of using linear dependencies, other functions to realize interpolation might be tested.The approach of rest magnitude thresholding could overcome the problem related to gestures exerted with less than half the full intensity being detected as rest state. A value of about 2.5 times the average magnitude across all rest training samples showed the best results.The adaptions made to kNN have no influence on kNN’s original incrementality so that requirement R4 is inherently guaranteed.The motivation behind investigating prototype reduction mechanisms was to cope with the inherent issue of instance-based learning manifesting in very high computational demands during prediction. The concept of prototype reduction promised to reduce these demands by preponing calculations to the training phase where the amount of data to be processed in prediction is reduced, thus accomplishing both requirement R5.1 and requirement R5.2.From the multitude of algorithms proposed in literature, the DSM algorithm singled out as highly appropriate. It is deterministic with regard to the size of the final reduced prototype set to be generated (memory determinism, requirement R5.2), yielded high cross-validation accuracies using EMG datasets captured with the Myo armband (requirement R1) at a low amount of time needed for reduction (requirement R5.3), and is considered to be incremental (requirement R4).So far, the best results for DSM were shown when using the centres of the classes as initial prototypes. This was implemented by means of calculating the class-wise means. Nevertheless, this can lead to misclassification in the case of overlapping classes, specifically if they are concentric [Furthermore, requirement R5.4 is also met due to DSM’s elementary composition of an initialization of prototypes in the class centres and a correction phase shifting them by either penalizing or rewarding them depending on different basic criteria. As it is meant to be used with the proposed kNN implementation which guarantees this requirement, R2 is also fulfilled.In the final user study, requirement R3 was evaluated for both the standard kNN approach (extended by the introduced adaptions) and kNN applied on the dataset reduced by DSM. It could be shown that the kNN methods performed significantly better than the ridge regression methods. Within the groups themselves, there was no statistical significance determinable.Interestingly, DSM-kNN and kNN performed equally well; DSM-kNN sometimes even better, even though a reduction of over 99% was achieved by relying on only seven prototypes in total. By this, requirement R3 as to user satisfaction in real scenarios is fulfilled. The extended user study on additionally including the wrist rotation gestures, achieving very good success rates, might give further motivation to deeper analyze this influence in the context of representative studies.Regarding the measured online timing behaviour in the proposed setup, it could be shown that DSM reduces kNN’s classification times by three orders of magnitude. With that, it achieves the same order as standard RR and is one order of magnitude faster than RR-RFF. This means, DSM-kNN is excellently suited for real-time control [It could be shown that DSM-kNN is an appropriate method to be integrated into wearable prosthetic devices. Its properties lead to fulfilling non-functional requirements with respect to dependability and energy consumption, among other properties, favouring battery-powered portable myocontrol implementations.A limitation of the conducted user study is the combined comparison of simultaneous and non-simultaneous control. The advantage of higher stability and robustness in the kNN-based methods comes at the disadvantage of not allowing to predict multiple degrees of freedom in parallel. On the contrary, the RR-based methods are subject to instabilities because of their tendency towards simultaneous predicting multiple degrees of freedoms. Approaches of how to handle mixed states in the case of kNN could comprise explicit learning on mixed gestures or implicit learning by automatically creating mixtures of gestures. Another approach is suggested in [For a further evaluation of the kNN-based methods, a user study with handicapped subjects is of high importance. Additionally, besides using the visual feedback of the hand model, experiments with prosthetic devices have to conducted to identify the potential of the methods in terms of helpfulness for amputees. Longer-term studies may provide information about the influence of potential electrode shift as well as how to counteract this effect (as in [Time complexity of DSM, training phase consists of initialization and reduction, see Sect. Such experiments could also reveal further insights with regard to preprocessing, choice of the features (potentially combined with feature selection for horizontal data reduction), additional modalities or coping with the limb position effect (where the armband’s integrated IMU might be useful). Since we solely rely on the linear envelope, a combination of our work with an embedded feature analysis [All in all, this paper confirmed the suitability of nearest neighbour learning techniques in the context of proportional myocontrol. Specifically, the results of using decision surface mapping at very high reduction rates (>99%) motivate to further look into this promising method.