Stimulation-Induced Muscle Deformation Measured with A-Mode Ultrasound Correlates with Muscle Fatigue

Muscle fatigue is a common physiological phenomenon whose onset can impair physical performance and increase the risk of injury. Traditional assessments of muscle fatigue are primarily constrained by their dependence on maximum voluntary contractions (MVCs), which not only rely heavily on participant motivation, reducing measurement accuracy, but also require large, stationary equipment such as isokinetic dynamometers, limiting their application to discrete assessments in lab-based environments. In this work, we introduce a wearable muscle fatigue tracking strategy that employs low-profile single-element ultrasound and electrical stimulation. This integrated approach demonstrates that muscle deformation from electrically-induced muscle contractions correlates with muscle fatigue, thus circumventing the need for bulky hardware and eliminating the variability associated with human volition. We define a deformation index, which fuses stimulation-induced changes in muscle thickness with baseline muscle swelling to track muscle fatigue. Our results demonstrate that the deformation index reliably tracks muscle fatigue (r = 0.85 ± 0.15), under specific conditions, namely extended joint angles and increased stimulation, as measured by changes in knee extension torque during a series of dynamic, volitional fatiguing contractions on 8 subjects on an isokinetic dynamometer. This approach has the potential to enable real-time, semi-continuous muscle fatigue monitoring in unconstrained environments.