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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 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.
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.
To extend the leg straight at the knee you use the quadriceps. To bend the knee, you need the hamstring.Hamstring muscle groups.
The quadriceps extends the thigh and the hamstring muscles flex the knee