Pain in the hemiplegic knee in chronic stroke could be for a number of reasons. But the classic case is not because of spasticity or inability to flex the knee. It is true that many patients are unable to flex the knee in swing phase but this is usually not particularly painful. The inability to flex the knee is most commonly due to failure of the hip flexors to contract at the very start of swing phase. The hip flexors function in early swing to phase flex the hip and to create a flexor inertial moment at knee. In chronic stroke the quadriceps may have increased stiffness due to spasticity so this could als contribute to the lack of knee flexion. However neither weakness nor spasticity accounts for the pain.

The classic problem is that the knee hyperxtends in mid stance due failure to advance the tibia in early to mid stance phase. The abnormal hyperextension of the knee stresses the posterior structures of the knee joint that over time results in hypermobility and posterior knee pain. The most common reasons for the knee hyperextension and due to lack of tibial advancement is the failure of the tibialis anterior to contract at contact response to mid stance to pull the tibia forward, thereby reducing the extensor moment on the knee. Another reason that often goes hand-in-hand with weakenss of the dorsiflexors is stiffness of the triceps surae. This stiffness in chronic stroke is usually due to contracture, spasticity or a combination of both.