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Knee flexion means bending your leg at the knee joint, reducing the angle between your lower leg and upper leg.
Hamstrings. The hamstrings are three muscles that run down the back of the leg to cross the knee joint. They work together to bend or flex the knee. The tendons of the semimembranosus and the semitendinosus insert on the medial side of the knee.These two muscles aid in the kneeâs slight internal rotation.ther Important Knee Muscles. While the hamstrings and quadriceps are the muscles that cross the knee, they donât act alone. The sartorius aids in flexion and internal rotation of the knee. The gracilis, mainly known for hip adduction, also contributes to flexion. The popliteus serves an especially important function.