The deep muscles that affect the knee are generally smaller that those that are directly involved in flexing the knee. Some are small in length, and others are thinner and less bulky than muscles that extend or flex the knee.

These muscles connect to the bones of the leg: the femur, the large bone of the thigh; the tibia, the large bone of the calf; and the fibula, the small bone of the calf.

The deep muscles that affect the knee’s movement include:

• Gracilis: This long, thin muscle extends from the pubic bone to the lower head of the femur at the knee on the inside of the thigh. Its primary function is to bring the hip inward, but it also helps flex the knee.
• Satorius: Like the gracilis, this long muscle aids hip movement and flexes the knee. It runs from the hip bone to the tibia on the inside of the thigh.
• Popliteus: This small, flat muscle begins at the back of the femur and wraps behind the knee. It rotates the knee as well as rotates the tibia inward, a small yet important movement in walking.
• Tensor fascia lata: This long, thin muscle stabilizes the hip and knee joints. It runs from the hip bone to the tibia on the outside of the thigh.
• Plantaris: Also long and thin, this muscle extends along the back of the leg from the bottom of the femur to the heel. It flexes the ankle and the knee.

Some of the muscles that begin near the knee do not affect it. Many are involved in actions of the foot and ankle. These include:

• Peroneus longus: This muscle begins at the head of the fibula and stretches down to the ankle. It flexes the ankle and supports the arch of the foot.
• Gastrocnemius: This large calf muscle connects to the head of the femur at the knee and extends to the foot to form the Achilles tendon. It is extremely important in flexing the ankle.
• Tibialis anterior: This muscle begins at the head of the tibia, crosses over the front of the leg, and connects to the back of the foot.