Shear wave elastography of passive skeletal muscle stiffness: Influences of sex and age throughout adulthood

Background Numerous structural and compositional changes - related not only to age, but also activity level and sex - may affect skeletal muscle stiffness across the adult age-span. Measurement techniques available thus far have largely limited passive stiffness evaluations to those of entire joints and muscle-tendon units. Shear wave elastography is an increasingly popular ultrasound technique for evaluating the mechanical properties of skeletal muscle tissue. The purpose of this study was to quantify the passive stiffness, or shear modulus, of the biceps brachii throughout adulthood in flexed and extended elbow positions. We hypothesized that shear modulus would be higher in males relative to females, and with advanced age in both sexes. Methods Shear wave elastography quantified biceps brachii stiffness at 90° elbow flexion and full extension in a large sample of adults between 21 and 94 years old (n = 133; 47 males). Findings Regression analysis found sex and age were significant parameters for older adults (> 60 years) in full extension. As expected, shear modulus values increased with advancing age; however, shear modulus values for females tended to be higher than those for males. Interpretation This study begins to establish normative trends for skeletal muscle shear modulus throughout adulthood. Specifically, this work establishes for the first time that the higher passive joint torque often found in males relative to females likely relates to parameters other than muscle shear modulus. Indeed, perhaps increases in skeletal muscle passive stiffness, though potentially altering the length-tension curve, serve a protective role - maintaining the tendon-muscle-tendon length-tension curve within a functional range.