The physiology of walking is a complex combination of muscles and bones that requires demanding coordination. The knees′ main actions are bending (flexion) and extension, and both are intrinsic to walking.
The hamstring muscles bend, or flex, the leg, and quadriceps muscles straighten, or extend, the leg at the knee. The hamstring muscles are three muscles at the back of the thigh–the semimembranosus, semitendinosus, and the biceps femoris–that make flexing and rotating the leg possible. The quadriceps, made up of four muscles of the front of the thigh, extend the leg for walking. The four quadriceps muscles are the rectus femoris, vastus lateralis, vastus intermedius, and the vastus medialis. These muscles attach to the top of the patella (knee cap) by the quadriceps tendon. The knee cap attaches to the upper front of the shin (tibia) by the patella tendon. The sartorius, gracilis, semimembranosus, and semitendinosus muscles are flexors of the knee. These four muscles also influence internal rotation of the tibia and protect the knee against rotary and valgus stress.
During normal walking the hamstrings are not at rest, but are working, bending the knee. The contraction of the muscles of the lower limbs propels the woman or man or child in walking or running. In between the tibia and fibula of the lower leg is the interosseous membrane that separates the muscles of the front and back of the calf. Plantarflexors at the back of the calf and the quadriceps at the front of the thigh contract in coordination to straighten the leg, bend the foot at the ankle, force the foot′s sole (plantar surface) against the ground, and propel the body forward.
This is a section from Dr. Jack E, Jensen’s book The One Stop Knee Shop. Read the next section The Jurassic Knee.