Using the frequency signature to detect muscular activity in weak and noisy myoelectric signals
Introduction
The detection of skeletal muscle activation is a critical issue in myoelectric signal (EMG) processing for clinical [1], motor control [2], ergonomics [3] and sports applications [4]. Different approaches were developed to estimate EMG onset–offset based on several automatic computer-based algorithms, including: single-threshold [5] and double-threshold [6] detection, adaptive threshold method [7], advanced statistical procedures [8], artificial neural network [9] and fuzzy logic [10] techniques. Unfortunately, these methods often fail when the muscle activity is recorded by damaged (i.e amputation) [12], pathological muscles (i.e myopathology) [13,14], or by complex muscles (i.e upper trapezius) during sustained and prolonged activity [15]. In these situations, the signals are characterized by a low activity level due to low firing rate, low number of motor units recruited, low activation threshold and very low signal-noise ratio (SNR) [11].
For these weak and noisy [19] EMG signals, several approaches have been recently proposed to improve SNR and to minimize erroneous onset detection [16]: Xu et al. [17] developed an adaptive algorithm for the determination of the onset and offset of the muscle contractions based on the generalized likelihood ratio test (GLR); Merlo et al. [11] proposed a time-frequency analysis to identify the single motor unit activation potentials in noisy EMG, as they used the continuous wavelet transform (CWT), permitting to detect the intervals of activation through an optimal threshold definition when the SNR is low (2 dB); Zhang and Zhou [18], instead, proposed an analysis based on Sample Entropy to distinguish the EMG signal from spurious background spikes. A method based on the non-linear properties of the Teager- Kaiser energy (TKE) operator, applied to surface EMG signal, was proposed for the first time by Li et al [16] considering simultaneously the amplitude and the instantaneous frequency of the signal: it improves SNR and favours a more robust detection of the onset muscle activity. This method was perfected by Yang et al. [19] applying a filtering method that is borrowed from an advanced image enhancement technique in the TKE domain. Solnik and colleagues [20] compared the results obtained by the application of the TKE with the three more-used classical onset methods for detection (visual detection, threshold-based method and approximated generalized likelihood-ratio), showing that the application of TKE operator in signal conditioning significantly reduces the mean detection error with respect to the classical methods.
However, all the mentioned methods depend, either fully (i.e GLR) or partially (i.e TKE or CWT), on the amplitude characteristics of the weak EMG signal, and their performance often depends on the SNR level; this detrimentally affects the detection performance when SNR is less than 10 dB.
To date, despite the frequency analysis of the EMG is widely used to track muscular changes and to evaluate muscle fatigue [21,22], the possibility to extract the intervals of muscular activity only from the changes in frequency domain was only partially investigated: to our knowledge only one preliminary study was proposed in the literature, which uses frequency characteristics such as zero-crossings and mean instantaneous frequency to detect the muscular activity of a real EMG signal, by an empirical threshold approach [23].
In our work the aim is to evaluate the possibility to extract the intervals of muscular activation in weak and noisy signals, where amplitude-based parameters display a reduced efficacy driven by the low SNR characteristics, leveraging on frequency features only. In particular, we developed a cluster-based algorithm which is based on the extraction of the number of zero-crossings and the mean instantaneous frequency [23], which we can comprehensively denote as frequency signature. This kind of approach to muscle activity detection allows to overcome the problems related to the definition of empirical thresholding, which in turns makes comparisons of results among different operators and laboratories harder.
The algorithm was validated on a set of simulated EMG signals and the performance was assessed showing good results in terms of detection and robustness with respect to both noise level and implementation settings.
Section snippets
EMG simulated signals
The EMG signals were simulated using an EMG simulation software based on the model developed by Hamilton-Wright and Stashuk [24,25]. The details of the theoretical bases of the EMG modelling method can be found in [24,25].
The EMG signals were supposed to be generated from a small muscle (100 motor units) with low firing rate (10 pps). The muscular contraction was generated in terms of percentage of the maximum voluntary contraction (5% MVC). The maximum recruitment threshold was set at 50% of
Statistical analysis
For each parameter, descriptive statistics was calculated (mean ± standard deviation) and statistical analysis was done: to verify the normal distribution of data, Kolmogorov-Smirnov test was done. Bias and error time (τ, terror) parameters were then considered separately as dependent variables for a 3-way ANOVA test considering window length (win200, win400, win600,win800, win1000, win1200, win1400, win1600, win1800, win2000, win2200, win 2400, win2600, win3000), overlap (0%, 25%, 50%,75%) and
Results
For both τ and terror the statistical analysis has shown a global effect for window length, overlap, SNR and the interaction between window length and overlap, between window length and SNR and between overlap and SNR. The statistical values are reported in the following Table 2.
Discussion and conclusions
The detection of the muscular activity in signals characterized by low amplitude and low signal-to noise ratio is a challenge in biomedical data processing for application in different fields such as prosthesis control and ergonomics.
To date the main approaches proposed to detect muscular activity are essentially based either on the amplitude or on the combination of both frequency and amplitude characteristics of the signals, and their performance are strictly related to the SNR level.
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