Anatomically, the term "leg" refers just to the lower leg, below the knee. Flexing your leg is the same thing as bending your knee. Only one leg flexor, the gracilis, lies entirely in the region of the inner, or medial, thigh. However, the tendons of several other thigh muscles cross the inner thigh and knee, so they are included here as well.
The gracilis
The gracilis is one of a group of muscles in the inner thigh called the adductors. In general, their function is to adduct, or pull the thighs together. The gracilis is the only one of the group that also crosses the knee, so it is the only adductor that flexes the leg in addition to adducting the thigh. It originates on the pubis and inserts below the knee on the shin bone, or the tibia.
The sartorius
The sartorius is the longest muscle in the body. It originates on the pelvis at the anterior superior iliac spine and crosses over to the inner thigh to insert onto the inner upper tibia, just in front of the gracilis tendon. It is sometimes called the tailor's muscle, because its function is to fold the thighs and legs into the cross legged position in which tailors traditionally sat. To do that, it flexes, abducts and externally rotates the thigh while flexing the leg.
The semimembranosus
There are three hamstring muscles, located on the back, or posterior, of the thigh. All three flex the leg, but only two are located on the inner side of the posterior thigh. They are the semimembranosus and the semitendinosus. The semimembranosus originates on the ischial tuberosity, or sit bone, and inserts on the posterior side of the inner tibia. It extends the thigh at the hip, flexes the leg at the knee and medially rotates the leg.
The semitendinosis
The other medial hamstring is the semitendinosus. Like the semimembranosus, it originates at the sit bone. It inserts onto the inner tibia, just behind the gracilis tendon. The tendons of the sartorius, gracilis and semitendinosus make up what is called the pes anserinus, or goose foot, a distinctive bulge at the inner knee. Also like semimembranosus, the semitendinosus extends the thigh at the hip, flexes the leg at the knee and medially rotates the leg.
References
- "Human Anatomy & Physiology;" Elaine N. Marieb; 1998
- "Trail Guide to the Body"; Andrew Biel; 1997
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