What muscles flex the knee?

Answer 1

The knee is a bone it doesn't flex it moves. The answer would be the Quadricep muscles.

Answer 2

Although the knee joint may look like a simple joint, it is one of the most complex. Moreover, the knee is more likely to be injured than is any other joint in the body. We tend to ignore our knees until something happens to them that causes pain. As the saying goes, however, "an ounce of prevention is worth a pound of cure."
If we take good care of our knees now, before there is a problem, we can really help ourselves. In addition, if some problems with the knees develop, an exercise program can be extremely beneficial.
The knee is essentially made up of four bones. The femur, which is the large bone in your thigh, attaches by ligaments and a capsule to your tibia. Just below and next to the tibia is the fibula, which runs parallel to the tibia. The patella, or what we call the knee cap, rides on the knee joint as the knee bends.
When the knee moves, it does not just bend and straighten, or, as it is medically termed, flex and extend. There is also a slight rotational component in this motion. This component was recognized only within the last 50 years, which may be part of the reason people have so many unknown injuries. The knee muscles which go across the knee joint are the quadriceps and the hamstrings. The quadriceps muscles are on the front of the knee, and the hamstrings are on the back of the knee. The ligaments are equally important in the knee joint because they hold the joint together. Problems with ligaments are common. In review, the bones support the knee and provide the rigid structure of the joint, the muscles move the joint, and the ligaments stabilize the joint
The knee joint also has a structure made of cartilage, which is called the meniscus or meniscal cartilage. The meniscus is a C-shaped piece of tissue which fits into the joint between the tibia and the femur. It helps to protect the joint and allows the bones to slide freely on each other. There is also a bursa around the knee joint. A bursa is a little fluid sac that helps the muscles and tendons slide freely as the knee moves.
To function well, a person needs to have strong and flexible muscles. In addition, the meniscal cartilage, articular cartilage and ligaments must be smooth and strong. Problems occur when any of these parts of the knee joint are damaged or irritated.

http://www.arthroscopy.com/sp05001.htm

Answer 3

The hamstrings flex the leg at the knee, the quadriceps extends it.

Answer 4

The Biceps Femoris.