title: Skeletal muscle tensile strain dependence: Hyperviscoelastic nonlinearity.
abstract: BACKGROUND
  Computational modeling of skeletal muscle requires characterization at the tissue level. While most skeletal muscle studies focus on hyperelasticity, the goal of this study was to examine and model the nonlinear behavior of both time-independent and time-dependent properties of skeletal muscle as a function of strain.
  
  
  METHODS
  Nine tibialis anterior muscles from New Zealand White rabbits were subject to five consecutive stress relaxation cycles of roughly 3% strain. Individual relaxation steps were fit with a three-term linear Prony series. Prony series coefficients and relaxation ratio were assessed for strain dependence using a general linear statistical model. A fully nonlinear constitutive model was employed to capture the strain dependence of both the viscoelastic and instantaneous components.
  
  
  RESULTS
  Instantaneous modulus (p<0.0005) and mid-range relaxation (p<0.0005) increased significantly with strain level, while relaxation at longer time periods decreased with strain (p<0.0005). Time constants and overall relaxation ratio did not change with strain level (p>0.1). Additionally, the fully nonlinear hyperviscoelastic constitutive model provided an excellent fit to experimental data, while other models which included linear components failed to capture muscle function as accurately.
  
  
  CONCLUSIONS
  Material properties of skeletal muscle are strain-dependent at the tissue level. This strain dependence can be included in computational models of skeletal muscle performance with a fully nonlinear hyperviscoelastic model.
authors.affiliation: Soft Tissue Mechanics Laboratory, Department of Mechanical Engineering, Colorado State University, 1374 Campus Delivery, Fort Collins, CO 80523, United States.
authors.forename: Benjamin B
authors.initials: BB
authors.lastname: Wheatley
authors.affiliation: Motion Analysis Laboratory, Department of Orthopedic Surgery, Mayo Clinic, 200 First Street SW, Rochester, MN 55906, United States.
authors.forename: Duane A
authors.initials: DA
authors.lastname: Morrow
authors.affiliation: Department of Mechanical Engineering - Engineering Mechanics, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, United States.
authors.forename: Gregory M
authors.initials: GM
authors.lastname: Odegard
authors.affiliation: Motion Analysis Laboratory, Department of Orthopedic Surgery, Mayo Clinic, 200 First Street SW, Rochester, MN 55906, United States.
authors.forename: Kenton R
authors.initials: KR
authors.lastname: Kaufman
authors.affiliation: Soft Tissue Mechanics Laboratory, Department of Mechanical Engineering, Colorado State University, 1374 Campus Delivery, Fort Collins, CO 80523, United States; School of Biomedical Engineering, Colorado State University, Fort Collins, CO 80523, United States. Electronic address: Tammy.Donahue@colostate.edu.
authors.forename: Tammy L
authors.initials: TL
authors.lastname: Haut Donahue
mesh_terms: D000818:Animals; D004548:Elasticity; D008954:Models, Biological; D018482:Muscle, Skeletal; D017711:Nonlinear Dynamics; D011817:Rabbits; D013314:Stress, Mechanical; D013718:Tensile Strength; D013997:Time Factors; D014783:Viscosity