Measurement of passive skeletal muscle mechanical properties in vivo: recent progress, clinical applications, and remaining challenges.

The ability to measure and quantify the properties of skeletal muscle in vivo as a method for understanding its complex physiological and pathophysiological behavior is important in numerous clinical settings, including rehabilitation. However, this remains a challenge to date due to the lack of a "gold standard" technique. Instead, there are a myriad of measuring techniques each with its own set of pros and cons. This review discusses the current state-of-the-art in elastography imaging techniques, i.e., ultrasound and magnetic resonance elastography, as applied to skeletal muscle, and briefly reviews other methods of measuring muscle mechanical behavior in vivo. While in vivo muscle viscoelastic properties can be measured, these techniques are largely limited to static or quasistatic measurements. Emerging elastography techniques are able to quantify muscle anisotropy and large deformation effects on stiffness, but, validation and optimization of these newer techniques is required. The development of reliable values for the mechanical properties of muscle across the population using these techniques are required to enable them to become more useful in rehabilitation and other clinical settings.