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

Background Muscle activation is associated with increased tissue stiffness as measured by elastography in diagnostic applications. For this reason, we present ultrasound time-harmonic elastography (THE) applied to the vastus lateralis (VL) muscle during passive tonic vibration reflex (TVR) and active voluntary contraction (VC) stimulation to test whether TVR can serve as a stimulation method for functional assessment of skeletal muscle stiffness. Methods Twenty-five asymptomatic volunteers (8 females, mean age 34 ± 8 years) underwent five consecutive THE examinations of the VL 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 the 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 with repeated-measures ANOVA and nonparametric Friedman tests. Results 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 < 10 -3 ). 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 < 10 -3 ). Increase of SWS at moderate TVR amplitudes correlated with that of 37 N VC load ( r = 0.67, p < 10 -3 ). TVR-induced stiffness changes at 100 Hz vibration frequency and moderate amplitude can substitute the more subjective VC forces for muscle function testing. TVR stimulation combined with skeletal-muscle THE may be a useful tool for the routine clinical assessment of stiffness during muscle activation.