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

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 l(0) and slack elastic modulus G(0), 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 degrees plantarflexion to 30 degrees 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, l(0) and G(0) 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 < R-2 < 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.