Towards objective identification of myofascial trigger points: A high-density surface electromyography method

Myofascial trigger points (MTrPs), which arise from muscle overload and subsequent ischemia, contribute to myofascial pain syndrome (MPS). Despite the high prevalence of MPS, diagnosis remains challenging. There is a lack of objective methods for identifying MTrPs using high-density surface electromyography (HD-sEMG), particularly through analysis of EMG activity and decomposed motor unit (MU) characteristics between active and non-MTrPs.

EMG and MU activities were compared between active MTrPs (n = 15) and non-MTrPs (n = 22) in the upper and middle trapezius. MU activities were extracted during three 2-s maximal contractions using gradient Convolution Kernel Compensation (gCKC)-based decomposition. Features were classified using five machine-learning models, and global explanation plots were generated to identify important parameters that predicted MTrP classification and to illustrate trends in EMG and MU activity values.

Participants with active MTrP reported severe pain (7 ± 1.25) and contributed 27 EMG samples, while non-MTrPs provided 41 samples. Active MTrPs exhibited lower mean ranks for EMG activity (RMS and MDF) and MU discharge rate (MU_DR) but higher mean ranks for MU number and MU action potential (MUAP) amplitude compared to non-MTrPs (p > 0.05). The artificial neural network (ANN) classifier achieved the highest performance for sample-wise cross-validation using the scaled dataset (82.35%). Although global explanations revealed MU_number as the lowest predictor, active MTrPs tend to show low RMS and MDF but elevated MUAP-amplitude features.

HD-sEMG is a promising method for identifying MTrPs. Machine learning methods, such as ANN-based classification, have demonstrated strong performance. However, further investigation is needed to improve classification accuracy for MTrP identification based on EMG decomposition.