Extending the knee involves several structures, which can be injured when people try to straighten their knee and are blocked from doing so. These injuries include

A tear (rupture) in the quadriceps tendon, which attaches the main muscle in the thigh (quadriceps) to the kneecap (patella)

A tear in the patellar tendon, which attaches the kneecap to the shinbone (tibia)

Fracture of the kneecap or top of the shinbone