Muscle activation assessment using ultrasound time-harmonic elastography and tonic vibration reflex

Muscle activation increases tissue stiffness as measured by elastography. We applied ultrasound time‑harmonic elastography (THE) to the vastus lateralis muscle during passive tonic vibration reflex (TVR) and active voluntary contraction (VC) to test whether TVR can serve as a stimulation method for functional assessment of skeletal muscle stiffness. Twenty‑five asymptomatic volunteers (8 females, mean age 34 ± 8 years) underwent five consecutive THE examinations at three VC loads (15, 22, and 37 N) and during TVR stimulation with 100 Hz frequency and 200 µm (low) and 400 µm (moderate) amplitude. Using standard line‑by‑line ultrasound, THE acquired induced shear waves of 60, 70, and 80 Hz frequency with a frame rate of 100 Hz. Shear wave speed (SWS) was reconstructed as a proxy for muscle stiffness and statistically analyzed. SWS increased significantly from 1.66 ± 0.11 m/s at rest to 1.79 ± 0.12 m/s, 1.93 ± 0.13 m/s, and 2.16 ± 0.12 m/s with 15, 22, and 37 N VC load (p < 0.001). Similar effects were observed during TVR activation with increases to 1.93 ± 0.13 m/s and 2.18 ± 0.14 m/s for low and moderate TVR amplitudes (p < 0.001). Increase of SWS at moderate TVR amplitudes correlated with that of 37 N VC load (r = 0.67, p < 0.001). TVR-induced stiffness changes at a 100 Hz vibration frequency and moderate amplitude may serve as a more objective alternative to VC-based loading for assessing functional muscle stiffness in asymptomatic individuals. However, clinical translation of TVR-THE as a standardized tool for assessing activation-related stiffness requires further evaluation in patient populations.