title: Length-force characteristics of in vivo human muscle reflected by supersonic shear imaging.
abstract: Recently, an ultrasound-based elastography technique has been used to measure stiffness (shear modulus) of an active human muscle along the axis of contraction. Using this technique, we explored 1) whether muscle shear modulus, like muscle force, is length dependent; and 2) whether the length dependence of muscle shear modulus is consistent between electrically elicited and voluntary contractions. From nine healthy participants, ankle joint torque and shear modulus of the tibialis anterior muscle were measured at five different ankle joint angles during tetanic contractions and during maximal voluntary contractions. Fascicle length, pennation angle, and tendon moment arm length of the tetanized tibialis anterior calculated from ultrasound images were used to reveal the length-dependent changes in muscle force and shear modulus. Over the range of joint angles examined, both force and shear modulus of the tetanized muscle increased with increasing fascicle length. Regression analysis of normalized data revealed a significant linear relationship between force and shear modulus (R(2) = 0.52, n = 45, P < 0.001). Although the length dependence of shear modulus was consistent, irrespective of contraction mode, the slope of length-shear modulus relationship was steeper during maximal voluntary contractions than during tetanic contractions. These results provide novel evidence that length-force relationship, one of the most fundamental characteristics of muscle, can be inferred from in vivo imaging of shear modulus in the tibialis anterior muscle. Furthermore, the estimation of length-force relationship may be applicable to voluntary contractions in which neural and mechanical interactions of multiple muscles are involved.
authors.affiliation: Faculty of Human Sciences and Design, Japan Women's University, Tokyo, Japan; and Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, Tokyo, Japan sasakik@fc.jwu.ac.jp.
authors.forename: Kazushige
authors.initials: K
authors.lastname: Sasaki
authors.affiliation: Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, Tokyo, Japan.
authors.forename: Sho
authors.initials: S
authors.lastname: Toyama
authors.affiliation: Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, Tokyo, Japan.
authors.forename: Naokata
authors.initials: N
authors.lastname: Ishii
mesh_terms: D000328:Adult; D000843:Ankle Joint / Q000502:physiology; D003952:Diagnostic Imaging / Q000379:methods; D055119:Elastic Modulus / Q000502:physiology; D054459:Elasticity Imaging Techniques / Q000379:methods; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology; D008297:Male; D018482:Muscle, Skeletal / Q000502:physiology*; D033081:Shear Strength / Q000502:physiology*; D013710:Tendons / Q000502:physiology; D013977:Tibia / Q000502:physiology; D019415:Torque
keywords: electrical stimulation; muscle mechanics; muscle stiffness; tibialis anterior; ultrasonography