Ligaments are viscoelastic material which means their stress-strain curve patterns are time/rate dependent (as a result of the internal friction).
The inital portion of the stress-strain curve, called the toe region, exhibits a high deformation/low force characteristic due to the uncrimping of collagen fibers and the elasticity of elastin. Next is the linear region where slippage within and then between collagen fibrils occurs. In this stage, ligaments gets stiffer (increased tensile strength) at higher strain rates.
Illustration A shows the different regions of the stress-strain curve.
Buckwalter JA, Einhorn RA, Simon SR (eds): Orthopaedic Basic Science: Biology and Biomechanics of the Musculoskeletal System, Rosemont IL, American Academy of Orthopaedic Surgeons, 2000.