Indentor tests and finite element modeling of bulk muscular tissue in vivo

The quasi-static response of bulk muscular tissue to indentation was measured on the posterior lower legs of living human subjects. No residual 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 was repeatable without prior 'preconditioning'. The data at any test location exhibited substantial random scatter, but did not trend up or down with repeated cycles. At these limited loads (< 7.0 N), hysteresis was always evident but was always < or = 10% of the maximum reaction force generated. At these limited loads, stress relaxation, in the time period between 5 and 1200 seconds after indentation, was < 10% (> 90% confidence). At higher load levels (> 12.0 N), greater hysteresis and prolonged stress relaxation were observed, accompanied by minor tissue damage. In order to estimate the composite material stiffness 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; typical values for the coefficients were c10 = 0.0026 MPa, c01 = 0.00064 MPa, and c11 = 0.0057 MPa.