The tibial nerve is a branch of the sciatic nerve. The tibial nerve passes through the popliteal fossa to pass below the arch of soleus. In the popliteal fossa the nerve gives off branches to gastrocnemius, popliteus, soleus and plantaris muscles, an articular branch to the knee joint, and a cutaneous branch that will become the sural nerve. The sural nerve is joined by fibers from the common peroneal nerve...
Tibial nerve distribution includes (as you thought) extensors (plantar flexors) of the ankle and flexors of knee (here's the muscle group if you really want specifics): popliteus, gastrocnemius, soleus, and...
The tibial nerve is an off shoot of the sciatic nerve.
I would recommend you see a chiropractor who will determine the cause. The knee may not be tracking right...