Knee Muscles

Muscles around the knee

There are seven muscles (maybe 8) that flex the knee. These are the semimembranous, semitendinosus, biceps femoris (long and short), saratorius, gracillis, politeus and gastrocnemius. The plantaris is sometimes considered an eighth knee flexor.
Other than the short head of biceps femoris and politeus the knee flexors are two joint muscles (they cross two joints either hip and knee or knee and ankle). Therefore the ability for these muscles to produce effective force is influenced by the other joints relative position.
Five of these flexors are able to medially rotate the tibia on the femur, these are the popliteus, gracilis, sartorius, semimembranosus and semitendinosus. The biceps femoris is able to laterally rotate the tibia.

The semitendenous, semimembranosus and long and short heads of biceps femoris are collectively known as the hamstrings. These muscles attatch to the iscial tuberosity (other than the short head that attatches to the posterior femur). The semitendinosus attatches with the sartorius and gracillis into the anteromedial aspect of the tibia into a common tendon called the pes anserinus or gooses foot.
Both heads of the biceps femoris muscle attach distally to the head of fibula with a slip to the lateral tibia.
The short head of biceps femoris is unique that it only acts on the knee but the rest of the hamstring act as hip extensors and knee flexors. This means that greater hamstrong force is produced when the hip is in flexion as the hamstring is lengthened over that joint. When the hamstrings are required to contract with the hip extended and knee flexed to 90 degrees or more the hamstring has to shorten at both the knee and hip. This is a very weak position for the hamstring as its at its shortest combined with the rectus femoris at a full passive stretch.
The hamstring muscles can produce a posterior sheering force on the tibia which peaks between 75 and 90 degrees of flexion (non weight bearing) this translational force can reduce strain on ACL.

The gastrocnemius muscle does have the ability to flex the knee but the ability for it to produce force once the knee is flexed is severely diminished. therefore it is thought to act as a dynamic stabilizer in agait giving stiffness to the knee joint.


The four extensors of the knee are known collectively as the quadriceps femoris muscle. Only one portion of this muscle is a 2 joint muscle (rectus femoris) whereas the vastus intermedius, lateralis and medialis orgiginate from the femur and merge with the rectus femoris into the common tendon.
 The quadriceps tendon inserts proximally into the patella and then continues where it becomes the paterllar ligament which runs into the tibial tuberosity.

The vastus medialis muscle has a different degree of pull dependent on the part of muscle assessed. Theupper fibres angle 15-18 degrees medially to the femoarl shaft but the distal 50-55 degrees medial. this brought about the seperation of this muscle into the vastus medialis longus (VML) and vastus medialis oblique VMO. This is why VMO recruitment is thought to be important in moving the patella medially in PFJ pain.

The patella and quadriceps muscle influence each others function. The patella increases mechanical advantage of the quadriceps by deflecting the action of the muscle away from the joint center increasing torque production. When the knee is flexed the patella dictates the angle of pull, in full extension though it acts as a normal pully when fixed in the intercondylar notch. Due to the mechanical advantage produced by the patella peak torques are typically seen between 45 and 60 degrees.

This increase in mechnical advantage does come at a price. It also increases the anterior sheer of tibia on femur. The ACl acts as a passive restraint to this force. As the knee reaches full extension the strain on the acl also increases. In the abscence of the ACL a near full quad contraction can generate tibial translation which may feel like giving way.

The quads function differently dependent on activity. When not weight bearing the mechanical advantage is greater at 90 degrees and decreases as the leg extends. However when weight bearing the mechanical advantage is at its greatest in full extension. Therefore the greater the knee flexion when weight bearing e.g squat the higher the quad effort needed.