The knee is a hinge joint that has two large bones above and below it--the femur and tibia. The knee makes it possible to bend and straighten the lower leg. When the leg bends, it is called flexion and when it straightens out it is called extension. These actions are due to muscles found above and below the knee that cross over the joint.
Vastus Muscles
The vastus medialis, vastus intermedius and vastus lateralis are three muscles of the quadriceps that all cross the knee joint. These are found on the front of the thigh, right above the knee. The medialis is on the lower inner thigh, the intermedius is in the center of the thigh and the lateralis is on the outside.
Rectus Femoris
The rectus femoris is the fourth part of the quadriceps muscle and it is found right in the center of the thigh. Not only does it cross the knee joint, but it is also responsible for two movements--knee extension and hip flexion. Hip flexion takes place when the thigh moves up toward the stomach. You would experience this if you did a lying leg raise.
Sartorius
The sartorius is a small strip of muscle that crosses over the quadriceps. It runs from the outer pelvic bone all the way down to the inside of the tibia across the knee. It is responsible for knee flexion.
Gracilis
The gracilis is one of the adductor muscles found on the inside of the thighs. This muscle crosses the knee and it is also responsible for medial rotation when the knee is flexed. To experience this, sit in a chair, lift your foot off the floor and turn your lower leg inward.
Biceps Femoris
The biceps femoris is the largest of the three hamstring muscles which are found on the back of the thighs. This segment of the hamstrings is furthest outward and it runs from the pelvic bone to the outer, or lateral, side of the tibia below the knee. Not only do the hamstrings flex the knee, but they also extend the hip. You would experience this if you moved your upper leg backward.
Semitendinosus
The semitendinosus is the middle portion of the hamstrings. It starts at the same location as the biceps femoris and it runs down to the middle part of the tibia, across the back of the knee.
Semimembranosus
The semimembranosus is the third part of the hamstrings and it starts at the same area as the biceps femoris and semitendinosus. It runs across the back of the knee and ends on the innermost part of the tibia called the medial condyle. A condyle is a protrusion on a bone where joints come together.
Gastrocnemius
The gastrocnemius is the upper calf muscle, which has a lateral and medial head. The lateral head is on the outside of the lower leg and the medial is on the inside. Both of these cross the back of the knee and end at the medial condyle of the femur.
Popliteus
The popliteus is a small muscle that is found right at the back of the knee joint. It starts on the lateral condyle of the femur and runs down to the back of the tibia.
Plantaris
The plantaris is a small strip of muscle that runs from the bottom of the femur all the way down to the calcaneus, which is in the ankle. Not only does it flex the knee, but it also flexes the foot.
