Your knee joint is a complex structure of bone, cartilage, ligaments and tendons connecting to muscles. Knee flexion uses several muscles on the back of your thigh, as well as some supporting muscles in your inner front thighs. Flexion involves bending your knee so the angle between your lower leg and upper leg decreases. A soccer player flexes her knee before extending it to kick the ball, for example.
Flexing your knee involves your hamstrings, a group of muscles including the semitendinous, biceps femoris and semimembranous. The biceps femoris are closest to the outer, or lateral, side of your thigh and have a "long head" and "short head." The heads are two separate branches of the muscle that attach to the femur, or large bone of your upper leg. Your semitendinous is next to your biceps femoris toward the center of your leg and your semimembranous lies just underneath your semitendinous.
Your sartorius is a strap-like muscle that runs across the front of your thigh. The sartorious attaches on the upper spine of your ilium, a bone of your pelvis, and crosses down to attach on your inner, or medial, femur. In addition to producing flexion of the knee, your sartorius helps produce the cross-legged position and assists thigh rotation and abduction, or bringing your leg in toward the midline of your body.
The gracilis is a long, thin, superficial muscle running along your interior thighs. It connects to the lower, interior spine of your ilium straight down to your inner tibia and helps you flex your knee as well as rotate your thigh inward, especially while you're walking.
Lower Leg Muscles
You use your popliteus and gastrocnemius to support the major muslces of knee flexion on your upper leg. Your popliteus is a thin, triangular muscle that crosses behind your knee and assists with knee flexion. The popliteus connects from your femur to your tibia. The grastrocnemius is a large, superficial pair of calf muscles that allow you to flex your knee when your foot is also flexed. Beginning at your femur, the two heads of gastrocnemius connect down to the calcaneous bones on each heel.