The deep muscles that affect leg movement 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 or foot.

Deep muscles connect to 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. Muscles are connected to bones via tendons, tough yet flexible bands of connective tissue.

The deep muscles of the upper leg include:

• Gracilis: This long, thin muscle extends from the pubic bone down 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 the hip and flexes the knee. It runs from the hipbone 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. It also 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 hipbone to the tibia on the outside of the thigh.
• Plantaris: Also long and thin, this muscle extends from the back of the femur to the heel. It flexes the ankle and the knee.

Some of the deep muscles of the rest of the leg 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 back of the femur and extends down to the rear of the foot to form the Achilles tendon. It is extremely important in flexing the ankle.
• Abductor hallucis: A muscle on the inner side of the foot that runs from the big toe to the heel, it pulls the big toe, or hallux, away from the body.
• Tibialis anterior: This muscle runs from the tibia to the big toe. This muscle helps make walking possible by flexing the foot upward and turning it inward.
• Extensor digitorum longus: This is one of three muscles that pull the foot upward. It also extends and lifts the toes as well as turns the foot outward.
• Flexor digitorum longus: This muscle in the calf and foot flexes the second through fifth toes and points those toes downward.
• Fibularis longus: This muscle helps in the sideways movement of the foot. It also helps flex the foot downward, like when you push down the gas pedal of a car.
• Fibularis tertius: This muscle helps move the foot from side to side at the ankle joint.

Fibularis brevis: This small muscle helps move the foot downward similar to the fibularis longus. It also helps flex the foot outward.