How Many Hamstring Muscles are in the Knee?

When someone says he or she "pulled a hamstring," you may get a vague idea of a muscle being stretched too far in the back of the leg. You may overlook the knee as part of the affected area. Unless you have been through medical courses or athletic training, or suffered a hamstring injury, it's hard to remember that there are actually three main muscles that constitute the hamstring and connect it to the back of the knee.

Hamstring Muscles

• The hamstring muscle set attaches at the back of the hipbone and runs from the back of the leg down to the sides of the knee area. Technically, the muscles are all connected and part of the functional group of the knee. The three muscles that make up the group are the semimembranosus, the biceps femoris and the semtendinosus muscles. All three work in tandem to provide the pull and damping effect necessary for working the knee.

  The actual connecting point to the hipbone is at the very lowest part of the pelvis structure where the hamstring group tendon attaches to what is called the ischial tuberosity. The muscles then actually overlap each other in an X and go past the knee to the lower part of the leg below the knee at the tibia. Tough fiber tissue (which takes on the strain of the leg's working in both walking and running) makes up the tendons that attach at both ends.

Primary Function

• The hamstring muscles provide power to propel the leg as directed. They constitute a fast-twitch muscle group, which means they work very quickly and powerfully with very low repetition to get started. The hamstring group owns responsibility for slowing or dampening the force of the leg as it returns downward to make another step.

Secondary Function

• Knee flexion by the hamstring muscles represents a secondary function to the leg motion--dampening effect. If this slowing effect did not occur, the leg power would fly forward and damage the knee joint.

  The secondary function of knee flexion occurs as the knee bends upward, expanding and stretching the front of the quadriceps in turn and readying it for retraction. The full force of the hamstring in motion pulls the leg backward at the hip to the point that the quadriceps is fully engaged in returning the leg forward.

Hamstring-Knee Injuries

• Knee injuries involving the hamstring group usually occur because another muscle gets pushed to being compromised. When the body has to adjust for one injured muscle or joint in the leg, it usually overcompensates with another muscle group. If the ankle or leg suffers from tenderness in a particular area, it's not surprising that the other muscles will experience added strain. Particularly during running or high-speed direction changes, the hamstring will be fully stretched as the leg returns forward and can easily tear. The resulting effect results in a sudden, clumsy deceleration of speed and control, which may cause twisting of the knee.

Injury Symptoms

• A primary hamstring injury won't manifest itself in the knee: You will usually feel it in the back of the leg as a sudden, large rubber-band pop. If the knee contorts as a result, the cause is a knee ligament's twisting or stretching wrong during deceleration. Even if the knee avoids injury, it will be subject to subsequent hampered flexion. The knee's ability to move forward becomes impossible as the attached-but-torn hamstring muscles trigger significant pain from stretching. The degree of pain will depend on how bad the muscle tear is.