Muscles and Tendons Knee Flexors
Knee Flexors
Seven muscles flex the knee: a) semi-tendinosis b) semi-membranosis c) biceps femoris d) sartorius e) gracilis f) popliteus g) gastrocnemius.
The semi-tendinosis, semi-membranosis and biceps femoris muscles are collectively known as the hamstrings and all originate from the ischial tuberosity of the pelvis. All the knee flexors except for the short head of biceps femoris and the popliteus are two joint muscles (ie crossing the hip and knee joint). Four of the flexors (popliteus, gracilis, semi-membranosis and semi-tendinosis) medially (or internally) rotate the tibia on the fixed femur, whereas the biceps femoris is a lateral rotator of the tibia. The semi-tendinosis, semi-membranous and biceps femoris muscles (hamstrings) flex the knee and extend the thigh. Because the muscles used reproduce a combined maximal excursion at one joint (eg extension of the thigh) they will limit movement at the other joint to less than maximal. Therefore they work most effectively at the knee joint if they are lengthened over a flexed hip.
Except for the plantaris muscle, the gastrocnemius is the only muscle at the knee that crosses both the ankle and knee joint. The gastrocnemius makes a relatively small contribution to knee flexion but is effective in preventing knee joint hyperextension thus the gastrocnemius appears to be more a dynamic stabiliser and less a mobility muscle at the knee joint.
The tendon of the gracilis muscle in addition to the sartorius and semi-tendinosis insert into the medial tibia to form part of the pes anserinus (goose's foot). These muscles collectively insert anterior and proximal to the insertion of the superficial medial collateral ligament (SMCL). The gracilis flexes the knee joint and produces slight medial rotation of the tibia. The three muscles of the pes anserinus appear to function effectively as a group to stabilise the medial aspect of the knee joint.
The popliteus muscle is a one joint knee flexor (in addition to the short head of biceps femoris). The popliteus flexes the knee and rotates it medially serving as a medial rotator of the tibia and the femur. The popliteus muscle may play a role in initiating unlocking of the knee because it reverses the direction of automatic external rotation that occurs in the final stages of knee extension (screw home mechanism).
