Answer
What happened is that the patient severed the Achilles tendon (calcaneum tendon) that attaches the gastrocnemius and the soleus muscle to the calcaneum. The upper leg looked swollen because both large muscles were contracted. Also there was probably no way for the muscles to pump out fluid that accumulated in the veins and interstitial space.
Work Step by Step
This is a case where the calcaneum tendon was severed. The bulge is caused by the contraction of the gastrocnemius and the soleus muscles which normally are attached to the calcaneum phalanx by the Achilles tendon. The injury would result in increased blood flow to the area, but venous and lymphatic system fluid return would be impaired because of the paralysis of the skeletal muscle pump in the area.
