Patellar Ligament

The structure indicated is the patellar ligament.

The quadriceps muscles of the thigh attach to and encapsulate the patella (the kneecap), via the quadriceps tendon. The patella ligament is essentially the continuation of the quadriceps tendon inferior to the patella, attaching it to the tibial tuberosity.

Learn more about the muscles of the thigh in this tutorial.