The deep muscles that affect the knee are generally smaller than those that are directly involved in flexing the knee. The muscles tend to vary in terms of size, length, mass, and thickness.
These muscles connect to the three 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 movements include:
- Gracilis: This long, thin, muscle extends from the pubic bone to the lower portion 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.
- Sartorius: Like the gracilis, this long muscle aids in 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, attaching to the lateral meniscus. It rotates the knee as well as the tibia inward, a small but 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 both 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 plays a vital role in flexion of 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.