i would think that for deformations such as these, creating blendshapes would work well, but in order to be able to utilize them, set driven keys would help... you create a blendshape for your leg in the 'entirely flexed knee' mode and then create a set driven key to allow the original mesh to deform to your 'entirely flexed knee' when the bone flexes to a certain degree. sadly i haven't worked with SDK (set driven key)s in a while, but (i have seen) people either use SDKs or flexors to control skeleton deformations when dealing with improper movement or reaction.