Muscle fatigue represents a complex physiological and psychological phenomenon that impairs physical performance and increases the risks of injury. It is important to continuously monitor fatigue levels for early detection and management of fatigue. The detection and classification of muscle fatigue also provide important information in human-computer interactions (HMI), sports injuries and performance, ergonomics, and prosthetic control. With this purpose in mind, this review first provides an overview of the mechanisms of muscle fatigue and its biomarkers and further enumerates various non-invasive techniques commonly used for muscle fatigue monitoring and detection in the literature, including electromyogram (EMG), which records the muscle electrical activity during muscle contractions, mechanomyogram (MMG), which records vibration signals of muscle fibers, near-infrared spectroscopy (NIRS), which measures the amount of oxygen in the muscle, ultrasound (US), which records signals of muscle deformation during muscle contractions. This review also introduces the principle and mechanism, parameters used for fatigue detection, application in fatigue detection, and advantages and disadvantages of each technology in detail. To conclude, the limitations/challenges that need to be addressed for future research in this area are presented.

Recently, surface electromyography (sEMG) emerged as a novel biometric authentication method. Since EMG system parameters, such as the feature extraction methods and the number of channels, have been known to affect system performances, it is important to investigate these effects on the performance of the sEMG-based biometric system to determine optimal system parameters. In this study, three robust feature extraction methods, Time-domain (TD) feature, Frequency Division Technique (FDT), and Autoregressive (AR) feature, and their combinations were investigated while the number of channels varying from one to eight. For these system parameters, the performance of sixteen static wrist and hand gestures was systematically investigated in two authentication modes: verification and identification. The results from 24 participants showed that the TD features significantly ( ${p} < 0.05$ ) and consistently outperformed FDT and AR features for all channel numbers. The results also showed that the performance of a four-channel setup was not significantly different from those with higher number of channels. The average equal error rate (EER) for a four-channel sEMG verification system was 4% for TD features, 5.3% for FDT features, and 10% for AR features. For an identification system, the average Rank-1 error (R1E) for a four-channel configuration was 3% for TD features, 12.4% for FDT features, and 36.3% for AR features. The electrode position on the flexor carpi ulnaris (FCU) muscle had a critical contribution to the authentication performance. Thus, the combination of the TD feature set and a four-channel sEMG system with one of the electrodes positioned on the FCU are recommended for optimal authentication performance.

Recent advances in biometric research have established surface electromyogram (sEMG) as a potential spoof-free solution to address some key limitations in current biometric traits. The nature of sEMG signals provide a unique dual-mode security: sEMGs have individual-specific characteristics (biometrics), and users can customize and change gestures just like passcodes. Such security also facilitates the use of code sequences (multicode) to further enhance the security. In this study, three levels of fusion, score, rank, and decision were investigated for two biometric applications, verification and identification. This study involved 24 subjects performing 16 hand/finger gestures, and code sequences with varying codelengths were generated. The performance of the verification and identification system was analyzed for varying codelength ( M : 1–6) and rank ( K : 1–4) to determine the best fusion scheme and desirable parameter values for a multicode sEMG biometric system. The results showed that the decision-level fusion scheme using a weighted majority voting resulted in an average equal error rate of 0.6% for the verification system when M = 4. For the identification system, the score-level fusion scheme with score normalization based on fitting a Weibull distribution resulted in a minimum false rejection rate of 0.01% and false acceptance rate of 4.7% using a combination of K = 2 and M = 4. The results also suggested that the parameters M and K could be adjusted based on the number of users in the database to facilitate optimal performance. In summary, a multicode sEMG biometric system was developed to provide improved dual-mode security based on the personalized codes and biometric traits of individuals, with the combination of enhanced security and flexibility.

Previous research has demonstrated that children with autism walk with atypical ankle kinematics and kinetics. Although these studies have utilized single-segment foot (SSF) data, multisegment foot (MSF) kinematics can provide further information on foot mechanics. Machine learning (ML) tools allow the combination of MSF kinematic features for classifying autism gait patterns. In this study, multiple ML models are investigated, and the most contributing features are determined. This study involved 19 children with autism and 21 age-matched controls performing walking trials. A 34-marker system and a 12-camera motion capture system were used to compute SSF and MSF angles during walking. Features extracted from these foot angles and their combinations were used to develop support vector machine (SVM) models. Additional techniques-S Hapley Additive exPlanations (SHAP) and the Shapley Additive Global importancE (SAGE) are used for local and global importance of the black-box ML models. The results suggest that models based on combinations of MSF kinematic features classify autism patterns with an accuracy of 96.3%, which is higher than using SSF kinematic features (83.8%). The relative angle between the metatarsal and midfoot segments had the highest contribution to the classification of autism gait patterns. The study demonstrated that kinematic features from MSF angles, supported by ML models, can provide an accurate and interpretable classification of autism and control patterns in children.

Recently, electromyogram (EMG) has been proposed for addressing some key limitations of current biometrics. Wrist-worn wearable sensors can provide a non-invasive method for acquiring EMG signals for gesture recognition or biometric applications. EMG signals contain individuals’ information and can facilitate multi-length codes or passwords (for example, by performing a combination of hand gestures). However, current EMG-based biometric research has two critical limitations: small subject-pool for analysis and limited to single-session datasets. In this study, wrist EMG data were collected from 43 participants over three different days (Days 1, 8, and 29) while performing static hand/wrist gestures. Multi-day analysis involving training data and testing data from different days was employed to test the robustness of the EMG-based biometrics. The multi-day authentication resulted in a median equal error rate (EER) of 0.039 when the code is unknown, and an EER of 0.068 when the code is known to intruders. The multi-day identification achieved a median rank-5 accuracy of 93.0%. With intruders, a threshold-based identification resulted in a median rank-5 accuracy of 91.7% while intruders were denied access at a median rejection rate of 71.7%. These results demonstrated the potential of EMG-based biometrics in practical applications and bolster further research on EMG-based biometrics.

Accidental falls are a major health concern among older adults. Currently, fall prevention programs employ clinical assessment scores for identifying elderly fallers based on cut-off values. Biomechanical parameters provide crucial information differentiating pathological gait and posture and can be used to classify elderly fallers and non-fallers. Pattern recognition models based on biomechanical parameters may provide greater insight for such classification. The purpose of this study was to compare the classification accuracy of different pattern recognition models for identifying elderly fallers using biomechanical parameters measured during balance and gait tasks. Pattern recognition models were also developed using clinical assessment scores and compared to the models based on biomechanical parameters for accurately identifying elderly fallers. Participants included 58 non-fallers (age = 72.3 ± 5.7) and 41 fallers (age = 74.0 ± 12.3) who performed balance and gait tasks on a walkway with embedded force plates and pressure mats. The parameters included 2D ground reaction force (GRF), center of pressure (COP), and the plantar pressure (PP). Using this data as input, different classification algorithms were used to build models. Maximum accuracy of 86.02% for classifying faller/non-faller categories was obtained using a classifier based on biomechanical parameters from combined gait and balance tasks. The GRF parameters ranked higher than COP and PP parameters based on F-score ranking suggesting predictor importance of GRF parameters. The classification performance was further improved by adding GRF parameters to the more commonly used COP parameters. However, the classifiers based on clinical assessment scores resulted in a maximum accuracy of 92.93% suggesting that elderly fallers can be accurately classified using pattern recognition models based on clinical assessment scores.