Quantitative analysis of ultrasound echo intensity dynamics in pennate skeletal muscle during passive stretching,...

Quantitative ultrasound imaging enables real-time assessment of skeletal muscle contraction. However, traditional muscle structural parameters can be compromised by unstable imaging during intense contractions.

We introduce an echo intensity (EI)-based method to quantify contraction activity in pennate skeletal muscle. The approach was evaluated in three representative scenarios using longitudinal ultrasound images: passive stretching (gastrocnemius, n = 20), active plantar flexion (gastrocnemius, n = 8), and treadmill sprinting (biceps femoris long head, n = 5).

In passive stretching, inverted normalized EI correlated strongly with algorithm-derived pennation angle (Pearson r = 0.81±0.17, p<0.001). During active plantar flexion, inverted EI signals were temporally aligned with muscle torque via cross-correlation, yielding a high correlation (Pearson r = 0.91±0.06, p<0.001). In comparison, algorithm-derived traditional structural parameters showed moderate correlations with torque, including pennation angle (r = 0.68±0.19), fascicle length (r = -0.69±0.31), and muscle thickness (r = -0.59±0.37). For rapid sprinting, inverted EI exhibited clear periodicity consistent with subjects' stride cycles and correlated with the root mean square (RMS) of the surface electromyography signal after alignment (Pearson r = 0.59±0.16, p<0.001). Notably, EI provides accurate temporal tracking of contraction events observed in conventional reference measures, while amplitude correspondence remains variable.

These findings demonstrate that EI can serve as a quantitative, real-time ultrasonic biomarker for dynamic muscle contraction, highlighting its potential for further methodological development and imaging-based assessments.