The femur is the longest, heaviest bone in your body, and its primary job is bearing weight. It supports your body when you stand, walk, run, and jump, handling forces of up to 30 times your body weight during high-impact activities. But the femur does more than just hold you up. It acts as a lever for movement, produces blood cells, stores essential minerals, and forms half of two major joints.
Weight Bearing and Structural Support
The long middle section of the femur, called the shaft, is the main load-bearing structure of your thigh. When you’re standing still, it transfers the weight of your upper body down through your legs to the ground. When you’re moving, the forces multiply dramatically. Running, jumping, or landing from a height can send loads through the femur that far exceed your actual body weight.
The femur handles this because of its construction. The outer layer is dense, compact bone, while the interior uses a honeycomb-like pattern of spongy bone that distributes stress efficiently. This design gives the femur an exceptional strength-to-weight ratio, making it strong enough to resist bending and compression without being so heavy that movement becomes difficult.
How the Femur Powers Movement
Beyond passive support, the femur works as a mechanical lever that your muscles pull on to move your leg. The upper end of the bone has two bony bumps called the greater and lesser trochanters, which serve as anchor points for some of the body’s most powerful muscles. The three gluteal muscles (the muscles that form your buttocks) attach to the greater trochanter, giving them the leverage to extend, rotate, and stabilize your hip.
The femoral neck, the angled section connecting the ball of the bone to the shaft, is critical to this lever system. It meets the shaft at roughly 120 degrees, and this angle allows your leg to swing freely beneath your pelvis rather than jutting straight out to the side. The length of the femoral neck determines how much leverage your hip muscles have. A longer neck gives muscles a bigger mechanical advantage, which is one reason people with different bone proportions move differently.
At the knee end, the femur widens into two rounded surfaces called condyles that roll and glide against the top of the shinbone. This creates the hinge-like motion of the knee, letting you bend and straighten your leg with every step.
The Hip and Knee Joints
The femur forms a key part of both the hip joint and the knee joint. At the hip, the rounded femoral head fits into a cup-shaped socket in the pelvis called the acetabulum. This ball-and-socket design gives the hip its wide range of motion. You can flex your leg forward, extend it backward, swing it out to the side, and rotate it inward or outward, all because the femoral head can rotate and glide within that socket.
At the knee, the femur’s lower end meets the tibia (shinbone) and the patella (kneecap). While the hip allows movement in multiple directions, the knee is more constrained, primarily bending and straightening. The shape of the femur’s condyles guides this motion and, along with the surrounding ligaments, keeps the knee stable under load.
Blood Cell Production
Inside the femur, bone marrow produces the blood cells your body depends on. Stem cells in red bone marrow give rise to all the formed elements in blood: red blood cells that carry oxygen, white blood cells that fight infection, and platelets that help with clotting.
In children, red marrow fills much of the femur’s interior. As you age, most of the marrow in the shaft converts to yellow marrow, which is primarily fat. Red marrow remains concentrated in the spongy bone at the ends of the femur, particularly near the hip. This is why the hip region stays important for blood production throughout life.
Mineral Storage and Release
Your bones, including the femur, act as a mineral bank. About 85% of the phosphorus in your body is stored in bone tissue as part of a mineral compound called hydroxyapatite, along with a large share of your calcium. These minerals give bone its hardness.
When your blood calcium or phosphorus levels drop, specialized cells called osteoclasts break down small amounts of bone to release those minerals back into the bloodstream. When levels are sufficient, other cells called osteoblasts build bone back up. This constant cycle of breakdown and rebuilding keeps both your blood chemistry and your bones in balance, though the system can tip toward net bone loss with aging or poor nutrition.
Growth and Development
The femur begins forming remarkably early. A primary bone center appears in the shaft around day 43 of embryonic development. A secondary center near the knee appears late in pregnancy, between weeks 36 and 40, which is why its presence on an ultrasound can help confirm that a baby is near full term.
The centers near the hip develop over a wider window, appearing between ages 1 and 12. Growth plates at both ends of the femur are what allow the bone to lengthen throughout childhood and adolescence. The hip-end growth plate typically fuses between ages 11 and 19, while the knee-end plate fuses between 14 and 19. Once these plates close, the femur stops growing in length.
What Happens When the Femur Breaks
Because the femur is so central to mobility, a fracture can be life-altering. An estimated 350,000 hip fractures occur annually in the United States, most of them in older adults. The consequences are serious: research published in JAMA found that among patients hospitalized for hip fracture, 13.5% had died within six months. Among survivors, only 60% recovered their ability to walk at their previous level by the six-month mark, and nearly 13% still needed full assistance to move around.
By 12 months after a hip fracture, 24% of patients had died. Age and living in a nursing home before the fracture were both significant predictors of how well someone recovered their mobility. These numbers reflect the femur’s outsized role in everyday life. When it can no longer do its job, the impact extends well beyond the bone itself, affecting independence, cardiovascular health, and overall survival.