Photoacoustic Imaging and High-Density Surface Electromyography: A Feasibility Study on Muscle Fatigue Monitoring

Muscle fatigue, defined as the decline in skeletal muscle force or power during sustained activity, is a complex, multifactorial phenomenon critical to physical performance. While myoelectric manifestations are well studied, simultaneous hemodynamic assessment is emerging. Unlike near-infrared spectroscopy (NIRS), this study employs photoacoustic imaging (PAI), combining laser illumination and ultrasound detection for enhanced depth sensitivity. Simultaneous high-density surface electromyography (HD-sEMG) and multispectral PAI were performed using a Verasonics Vantage 256 system with an Opotek Phocus Mobile SE laser and a 128-element linear ultrasound transducer, acquiring 10 wavelengths per acquisition. HD-sEMG was recorded via a 32-channel electrode matrix over the forearm. Two healthy subjects completed two to four trials of rest and sustained isometric contraction (little finger, right hand, until exhaustion), yielding a preliminary multimodal dataset. HD-sEMG signals were analyzed via root mean square (RMS) mapping at contraction start, middle, and end, and RMS and mean frequency (MNF) slopes in the most active muscle region. PAI data were reconstructed and unmixed into oxygenated hemoglobin over total hemoglobin (HbO₂/HbT) distributions. HD-sEMG enabled localized muscle activation assessment, showing trends consistent with fatigue. PAI provided localized monitoring of muscle oxygenation, although quantitative interpretation was constrained by the linear transducer geometry and the linear spectral unmixing algorithm. Despite these limitations, the results demonstrate the feasibility of simultaneous multimodal acquisition, enabling localized monitoring of muscle activity and preliminary observation of fatigue-related changes. This study lays the groundwork for future investigations aimed at refining multimodal assessment of muscle fatigue.