Full-Field Ultrasound Time-Harmonic Elastography Quantifies Task-Dependent Thoracolumbar Fascia and Multifidus Mechanics During Back Extension

Purpose: Ultrasound time-harmonic elastography (THE) uses continuous multifrequency vibration and standard-frame-rate ultrasound to generate full-field mechanical maps. We tested whether THE concurrently resolves task-dependent responses in the thin thoracolumbar fascia (TLF) and deeper multifidus muscle (MFM) during back extension. Methods: Eighteen healthy adults (9 female and 9 male; age 19.4 +/- 1.9 years) underwent multifrequency THE (60-80 Hz; 100 frames/s) at rest and during active back extension. Shear wave speed (SWS) and penetration rate (PR), an attenuation-sensitive measure, were quantified in manually defined TLF and MFM regions. Four within-tissue contrasts were tested using paired t tests with Holm adjustment; between-tissue, superficial-tissue-thickness, and sex analyses were exploratory. Results: Back extension increased SWS in the TLF (0.43 +/- 0.43 m/s; dz = 0.99) and MFM (0.62 +/- 0.23 m/s; dz = 2.73) and decreased PR in the TLF (-1.01 +/- 0.54 m/s; dz = -1.88) and MFM (-1.20 +/- 0.51 m/s; dz = -2.34); all Holm-adjusted p < 0.001. The SWS increase was 0.19 m/s greater in MFM than TLF (95% confidence interval, 0.01-0.38 m/s; unadjusted p = 0.042), whereas PR changes did not differ between tissues (p = 0.233). Conclusion: THE simultaneously captured spatially resolved, task-dependent responses in adjacent fascial and muscle tissues using standard-frame-rate acquisition. The findings establish biomechanical sensitivity in healthy adults and support further validation with standardized loading, repeatability testing, and comparison with conventional shear wave elastography.