INDENTOR TESTS AND FINITE-ELEMENT MODELING OF BULK MUSCULAR TISSUE IN-VIVO

The quasi-static response of bulk muscular tissue to indentation was m easured on the posterior lower legs of living human subjects. No resid ual limbs were tested; all subjects had intact lower limbs. For loads up to 7.0 N on an 8.0 mm diameter flat-tipped indentor, the response w as repeatable without prior 'preconditioning.' The data at any test lo cation exhibited substantial random scatter, but did not trend up or d own with repeated cycles. At these limited loads (<7.0 N), hysteresis was always evident but was always less than or equal to 10% of the max imum reaction force generated. At these limited loads, stress relaxati on, in the time period between 5 and 1200 seconds after indentation, w as <10% (>90% confidence). At higher load levels (>12.0 N), greater hy steresis and prolonged stress relaxation were observed, accompanied by minor tissue damage. In order to estimate the composite material stif fness of the tissue, the indentations were modeled using a materially and geometrically nonlinear, large-strain finite element formulation. The resulting composite material stiffness was nonlinear, and could be approximated using the Jamus-Green-Simpson strain energy function; ty pical values for the coefficients were c(10)=0.0026 MPa, c(01)=0.00064 MPa, and c(11)=0.0057 MPa.