First, size the deformable envelope to affect the whole lower leg area and smoothly transition into the thigh and ankle areas. Next, you define a smaller rigid envelope to encompass the stiff unbendable section of the shin. Increase the weight of the rigid envelope to balance its effect against that of the deformable envelope.
Some users might use both envelopes as a way to have a bone affect two completely separate regions of a mesh. The envelopes are scaled so that the deformable envelope influences certain vertices on one side of the link and the rigid envelope affects the other side.