Quantitative Estimation of Passive Elastic Properties of Individual Skeletal Muscle in Vivo Using Normalized Elastic Modulus-Length Curve.

Abstract

Precise characterization of the passive elastic properties of human skeletal muscle could provide valuable information for functional assessment and medical diagnosis. Using normalized elastic modulus-length curve based on a piecewise exponential model, a non-invasive ultrasonography (US) method was developed for estimating three physiologically meaningful parameters, termed as passive-elastic coefficient k, slack length l₀ and slack elastic modulus G₀, to quantify the passive elastic properties of human gastrocnemius (GM) muscle, and correlate different findings with healthy and atrophic subjects. Dynamic B-mode US and shear wave elastography (SWE) videos of right GM muscles were performed in 32 healthy subjects and 18 atrophic patients with their ankle angles from 40° plantarflexion to 30° dorsiflexion by an isokinetic dynamometer. During muscle passive stretching induced by ankle rotation, consecutive muscle lengths were measured by automatically tracking the distal muscle-tendon junction positions in B-mode imaging frames using Hough transform and an active contour method. The corresponding elastic moduli were obtained using SWE imaging frames. The Gauss-Newton algorithm was used to estimate the parameters k, l₀ and G₀ from normalized elastic modulus-length curve. In-vivo experimental results showed that the measured muscle elastic modulus-length data fitted well to the proposed model with three estimated parameters for all subjects (0.942 < R² < 0.997). The passive-elastic coefficient k was significantly higher for atrophic subjects compared with normal subjects (105.5 ± 45.3 versus 48.4 ± 16.0, p < 0.001). The proposed parameters allow further characterization of muscle essential mechanical properties and have a potential to become effective indexes for muscular disease diagnosis.