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

BackgroundMuscle 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. MethodsTwenty-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 {micro}m (low) and 400 {micro}m (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. ResultsSWS increased significantly from 1.66{+/-}0.11{square}m/s at rest to 1.79{+/-}0.12{square}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.