Table 1.

The material properties of the anterior cruciate ligament found by fitting the constitutive model (Eq. 24 ) to tensile experimental data

Elastic properties	Short-term viscous properties	Long-term viscous properties
a 2 = 2.213 × 106 Pa a 3 = 3.879 (dimensionless)	a 4 = 0.3653 × 106 Pa.s, a 5 = 0.652 (dimensionless)	τ i = 3.77, 148.24, 10987.98 s
		w i = 0.154, 0.161, 0.682 s−1
		(i = 1, 2, 3)

The elastic properties, a ₂ and a ₃ , were found from the near equilibrium loading (1.2%/s); and the short-term viscous properties, a ₄ and a ₅ , were determined using the ramp loading under 25%/s strain-rate (Figure 1). Finally, the long term viscous parameters, τ _i and w _i, were determined from the stress relaxation data (Figure 2). The compressive property of the ligament, a ₁, was neglected in the fit. All parameters were enforced to be positive to satisfy the second law of thermodynamics. Additionally, the constraint of Σw _i = 1 was also applied (Eq. 7) during the optimization.