The patellar extensor apparatus is composed of the quadriceps muscles that converge to a central tendon that inserts on and invests the patella. It continues by the patellar tendon to act on the tibial tuberosity and thereby extends the leg at the knee. The structure can be thought of as a chain with pathology able to occur at each level. Pathological processes are generally caused by the great force experienced at each level both chronically and acutely. The forces are, however, greatly modified by the particular geometries present at each level. The various pathological processes and factors that modify them are reviewed at each level, beginning with the quadriceps musculature and ending at the terminal point of action, the tibial tuberosity.