Answer:
Humans may have evolved by being able to augment their diet with the concentrated form of protein derived from eating other animals. To obtain protein from this source was difficult as animals typically run faster, jump higher, and some are meat-eating themselves and thusly better approached from a distance. To successfully accomplish the hunt for food, the human species developed two significant physical traits, the opposable thumb for grasping and the LOCKABLE knee joint for supporting powerful and aggressive throwing.
The knee locks in extension when the lower leg is internally rotated by the contraction of the SARTORIUS muscle. Further contraction of the surrounding thigh muscles, RECTUS FEMORIS, ILIOTIBIAL BAND, along with the BICEPS FEMORIS, all serve to firmly pull the lower leg proximally into the closed and locked position, ready now to support the entire body needed for standing, running, or throwing (spears, baseballs, or whatever).
To reverse the above process of unlocking, the knee must be first unweighted by shifting the body weight to the opposite leg - then the lower leg is externally rotated by the contraction of the POPLITEUS muscle followed by full knee flexion by contraction of the BICEPS FEMORIS muscle pair. All of this activity happens automatically even though these muscles are under voluntary control.
The overall twist (internal/external rotation) of the lower leg (tibia) on the upper leg (femur) is approximately 5 degrees.
The medial and lateral menisci serve to guide and center the femoral and tibial articular surfaces and often contribute to inadvertant LOCKING when they are defective (torn or folded) or misplaced due to swelling. An uncomplicated (no fracture, degenerative arthritis, ripped or torn ACL/PCL/collateral ligament, or joint mice) and locked knee joint is a frightening experience, to be sure, however, the knee is easily unlocked (with assistance, typically) by pressing deeply with both thumbs just below the knee cap and slightly to the outside of the patellar ligament. Pressing firmly there as the knee is slowly and gently extended will release the meniscus. The lower leg will actually be felt to 'thunk' free and the individual will be able to immediately stand and use the leg normally although locking may occur again.