Assessment of anisotropy using viscoelastic response (VisR) ultrasound in the biceps brachii of healthy older adults and stroke patients

While skeletal muscle is often assumed to be structurally transversely isotropic (TI), its mechanical anisotropy is not well characterized in vivo. VisR ultrasound, a technique that provides information about the viscoelastic mechanical response of tissue, can be used for such in vivo characterization. Specifically, VisR-derived Relative Elasticity (RE) provides directionally dependent information about the shear modulus of materials. We hypothesize that VisR RE can be used in healthy older adults and in stroke patients to 1) assess the degree of anisotropy (DoA) of the biceps brachii (BB) and 2) characterize the effect of passive loading on the BB. VisR imaging was performed in the BB long head of 2 healthy controls and 1 stroke patient. The subjects' arms were imaged in full extension and approximately 90° flexion at rest. Data were acquired in the transverse and longitudinal imaging planes at the medial location of the BB. The ratio of RE obtained in the transverse to the longitudinal muscle views was found to reflect the DoA in shear modulus, where DoA = 1 indicates perfect isotropy. In control subjects, RE values for flexion and extension in the transverse imaging plane (previously shown to reflect the longitudinal shear modulus, μL) were higher than the RE values in the longitudinal imaging plane (which, primarily reflects the transverse shear modulus, μT). Additionally, both the longitudinal and transverse shear moduli increased in extension vs. flexion. Both the magnitude and direction of anisotropy appeared to change in the stroke patient compared to the healthy controls. This work indicates VisR can be used to assess DoA in skeletal muscle of older adults and examine effects of passive loading on mechanical properties, in vivo, which may be clinically relevant to studying impairment post stroke.