This child has had a common operation for equinus deformity, in which the Tendo-Achilles is lengthened. This fixes the equinus deformity, but unfortunately very often reduces push-off power becuase the muscles are now too long to work effectively. You can see that she has lost the normal plantarflexion during pre-swing . The knee should normally be close to full extension at initial contact. In this child, the knee is at an angle of nearly 30 degrees at contact, and in the video you can see that this makes his loading response very abnormal.
Common causes are a knee flexion contracture, or hamstrings tightness due to a muscle sprain.
When the quadriceps is not strong enough to eccentrically control the knee during loading, the plantarflexor-knee extensor couple is used instead. This requires body weight to be brought forward in front of the knee joint axis. Amount of swing phase flexion Normal swing-phase knee flexion should be around 65 degrees at normal cadence. In contrast, the man on the right has a fixed knee deformity from spastic contracture. The hip should normally extend about 20 degrees in terminal stance.