Your knee joint is a complex structure of bone, cartilage, ligaments and tendons connecting to muscles. Knee flexion — that is, bending your knee — requires the use of several muscles on the back of your thigh, as well as some supporting muscles in your inner front thighs. If you have pain or an injury in any of these muscles, you might feel it when you bend your knee.
Flexing your knee involves your hamstrings, a group of muscles including the semitendinosus, biceps femoris and semimembranosus. 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 semitendinosus is next to your biceps femoris toward the center of your leg and your semimembranosus lies just underneath your semitendinosus.
Your quadriceps, or the front of your thighs, have four parts — the rectus femoris, vastus lateralis, vastus medialist and vastus intermedius — and they're all powerful extensors of the knee joint that are vital to walking, running, squatting and jumping. These muscles extend the lower leg at the knee joint, while also stabilizing the patella.
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 muscles 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 gastrocnemius 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 calcaneus bones on each heel.