A leg is the entire lower limb of the human body, stretching from the hip down to the foot. It contains the longest and strongest bones in your body, more than a dozen major muscle groups, three large joints, and a network of blood vessels and nerves that keep everything working together. In everyday language, “leg” means the whole limb, though in medical anatomy the term technically refers only to the section between the knee and ankle.
Bones That Make Up the Leg
Your leg is built around four major bones. The femur, or thighbone, is the largest bone in your body. It connects your hip to your knee and bears most of your weight when you stand. Below the knee, two bones run side by side: the tibia, the larger and stronger of the pair, and the fibula, a thinner bone along the outside of your lower leg. The fourth bone is the patella, your kneecap, which sits in front of the knee joint and acts as a shield while giving your thigh muscles extra leverage when you straighten your leg.
These bones aren’t fully solid until late adolescence. Growth plates near the ends of the femur and tibia are made of cartilage during childhood and gradually harden into bone as puberty ends. In girls, growth plates typically close between ages 13 and 15. In boys, closure happens between 15 and 17, which is why boys often keep gaining height a couple of years longer.
Major Muscle Groups
Your legs contain some of the largest and most powerful muscles in your body, organized into groups that pull bones in different directions.
Upper Leg
The front of your thigh is dominated by the quadriceps, a set of four muscles that run from your hip and upper thigh down to your knee. They straighten your knee and help flex your thigh at the hip. On the back of the thigh, three muscles form the hamstrings, which bend the knee and pull the leg backward. Along the inner thigh, five smaller muscles called the adductors pull your leg toward your body’s midline and help rotate the thigh.
Lower Leg
The front of your lower leg contains four muscles responsible for lifting your foot and extending your toes. You use these every time you take a step and need to clear your foot off the ground. The back of the lower leg is home to the calf muscles, primarily the gastrocnemius and soleus, which point your foot downward and generate the push-off force for walking, running, and jumping. Two muscles along the outer edge of your shin stabilize your foot during movement and let you tilt it side to side.
Three Major Joints
The leg has three joints, each with a different range of motion suited to its role.
The hip joint is a ball-and-socket design that allows movement in nearly every direction. A healthy adult hip can flex forward roughly 130 to 134 degrees and extend backward about 17 to 18 degrees, with additional range for rotation and side-to-side movement. The knee is a hinge joint that primarily bends and straightens. Adults can typically flex the knee about 138 to 142 degrees, enough to pull the heel close to the back of the thigh. The ankle joint lets you point your foot downward (about 55 to 62 degrees) and pull it upward (about 13 to 14 degrees). Women tend to have slightly more range of motion at each of these joints than men.
Nerves and Blood Supply
Several major nerves travel through the leg. The femoral nerve runs down the front of the thigh and controls the quadriceps and sensation in that area. The obturator nerve serves the inner thigh. The largest nerve in the body, the sciatic nerve, runs from the lower back down through the buttock and along the back of the thigh, where it splits into two branches: the tibial nerve, which continues down the back of the calf, and the common fibular nerve, which wraps around the outside of the knee toward the shin and foot. Compression or irritation of the sciatic nerve is what causes the radiating pain known as sciatica.
Blood reaches the leg through the femoral artery, which branches into smaller vessels as it descends toward the foot. Veins return blood upward against gravity, relying on one-way valves and the squeezing action of leg muscles to push blood back toward the heart. This is why prolonged sitting or standing can cause blood to pool in the lower legs, leading to swelling or discomfort.
How Your Legs Power Walking
Walking looks simple, but each step involves precise coordination among dozens of muscles firing in sequence. One full gait cycle, measured from the moment one heel hits the ground to the next time that same heel hits again, splits into two phases. The stance phase, when your foot is on the ground, accounts for about 60% of the cycle. The swing phase, when that foot lifts and moves forward, takes up the remaining 40%.
When your heel first strikes the ground, your hip extensors and quadriceps contract to absorb the impact and keep your knee from buckling. As your body rolls forward over that foot, your calf muscles and the muscles along the sole of your foot contract to stabilize the arch and prepare for push-off. Then, as your weight shifts to the other leg, hip flexors pull the trailing leg forward into the swing phase while muscles in the front of your shin lift the foot so your toes don’t drag on the ground. In the final instant before your foot lands again, the hamstrings fire to slow the forward momentum of your thigh and set up for another controlled landing.
Why Human Legs Look the Way They Do
Human legs are dramatically different from those of other primates, and nearly every difference ties back to walking upright. Chimpanzees have a short connection between the top of the thighbone and the hip, which limits how effectively their hip muscles can support the body during upright walking. In humans, that connection is longer and its base much thicker, creating a strong bridge that can handle the repeated stress of bipedal movement.
The human knee joint is also far more heavily built than a chimpanzee’s. Every step briefly puts your full body weight on a single leg, and the broad area of bone just below the knee evolved to absorb that stress. Chimpanzees, with their lighter knee joints, cannot comfortably walk on two legs for extended periods. Human legs are also proportionally longer than those of earlier human ancestors, a change visible in fossils of Homo erectus. Longer thighbones meant longer strides, which allowed early humans to cover more ground with less energy. The broad, basin-shaped human pelvis further supports upright posture by anchoring large gluteal muscles that keep you balanced over one leg with each step.
These structural changes didn’t happen overnight. Fossils dating back roughly six million years show an early ancestor, Orrorin tugenensis, with a thighbone already angled in a way that resembles modern humans more than apes, suggesting that the shift toward upright walking began very early in the human lineage.