quadricep
hamstrings
The Quadriceps.
It is the rectus femoris, which along with the action of knee extension, it also flexes the hip
The Gastrocnemius and the Soleus flexes the knee and the foot. The Plantaris is an unimportant muscle. Many people do not have it and doctors tend to use this muscle for tendon graphs.
hamstring group is the primary muscle group for flexing the knee
The flexor would be the hamstring, and gastrocnemius, which bend/flex the knee. The quadriceps, are extensors, which straighten/extend the knee.
Quadricep
The primary muscle responsible for extending the thigh is the quadriceps femoris, specifically the rectus femoris. To flex the knee, the primary muscles involved are the hamstrings, particularly the biceps femoris, semitendinosus, and semimembranosus. These muscle actions are important for movements like running and kicking.
Sartorius is one of the Quadricep muscles. It is the only muscle that I know of that has a double insertion point. It inserts on the distal part of the hip and serpentines to the medial part of the knee where it inserts.
Paralysis of the quadriceps femoris muscle would make an individual unable to flex the thigh. This muscle is responsible for extending the knee and flexing the thigh at the hip joint.
The plantaris is the smallest muscle in the human leg. It is used to flex the knee joint and is absent in about 7 percent of the human population.