Applying a stress to a long muscle fiber will result in a larger relative length change than the same stress applied to a shorter muscle fiber, all other things being equal. (strengthandconditioningresearch.com)
This rapid deceleration causes a large and fast strain (change in relative length) to occur in the adductor muscles as well as a very high stress (force per unit area). (strengthandconditioningresearch.com)
Stiffness is the gradient of the stress - strain curve, which can be calculated as stress (force per unit area) divided by strain (relative length change). (strengthandconditioningresearch.com)