The femur is the longest, strongest, and heaviest bone in the human body, making it a difficult one to break. It’s also protected by various muscles and helps you maintain your posture and balance.

Femur bone function

Your femur connects many important muscles, tendons, and ligaments in your hips and knees to the other parts of your body, including parts of the circulatory system. More than that, it helps you carry your body weight when you stand and move. Your femur is critical in helping you maintain stability so you don’t fall over easily.

The femur also contains bone marrow, which is a soft, fatty tissue made up of stem cells. Stem cells have two unique abilities that make them essential to survival — they can make more stem cells, and they can “morph” or develop into other types of cells (this process is called “differentiation”).

The femur is the longest, strongest, and heaviest bone in your body. (Photo Credit: iStock/Getty Images)

These stem cells form two types of bone marrow: red and yellow. Each has an important job. Red bone marrow cells produce all the components of your blood (red and white blood cells and blood platelets). Yellow bone marrow cells store fat, which is needed for energy and to produce bone, cartilage, and muscles. From birth to around age 7, your bones contain only red marrow. From then on, yellow bone marrow gradually replaces red.

Some basic knowledge about the importance of the femur and its role in your overall health can help you avoid injury and protect your bone health.

Where is the femur located?

The femur is more commonly referred to as the thigh bone. It is the only bone in the upper portion of your leg and is surrounded by your thigh muscles, including your hamstrings and quadriceps. The adult femur is typically around 18 inches long.

Parts of the femur

The long middle part of your femur is called the femoral shaft. The femoral shaft supports your body weight and forms your thigh structure. This hollow portion of the bone is about 1 1/2 inches thick and has a rounded shape on either end. It starts below the hip and ends at the knee where the bone begins to widen.

On the femur’s upper end (proximal aspect) are the femoral head and neck. The hip joint connects the head of the femur and the pelvis like a ball and socket. Stress fractures, which can affect long-distance runners, can occur in the femoral neck. Fractures of the femoral head are rare but may result from a high-impact injury.

Below this is the intertrochanteric area (below the neck but above the longer shaft of the femur), which has two bony spots: the greater trochanter and the lesser trochanter. Directly below this is the subtrochanteric area. Most hip fractures occur in the femoral neck or intertrochanteric area. 

The bone widens at the lower end (distal aspect), which forms the top of the knee joint and connects to your shin bone (tibia) and patella (kneecap).

Femoral muscles

Your femur is covered on all sides by your thigh muscles, which are divided into three compartments — anterior, medial, and posterior. The femur is located in the anterior compartment, which is made of the muscles you use to move your hip and extend your knee.

Within the three compartments are four important muscles, which include:

The most common problems that affect the femur are fractures, osteoporosis, and patellofemoral pain syndrome.

Femur fractures

As the strongest bone in your body, your femur can support as much as 30 times your body weight. Because it’s so strong, it usually takes a lot of force to break or fracture it.

Common types of femur fractures include:

Despite how strong your femur is, it can break. Some causes include car accidents and high-impact falls. If your femur bone is fractured, you'll feel a lot of pain, but other common symptoms include:

Swelling

Tenderness

Inability to move your leg normally

An abnormal bump on your leg

Bruising or discoloration

After taking your medical history and conducting a physical exam, your provider may perform the following tests to diagnose your fracture:  

X-ray: Invisible, electromagnetic beams are used to create an image of organs and bones. 

MRI: A combination of large magnets, radio waves, and a computer creates detailed images of the inside of your body.

CT scan: Both X-rays and computers are used to produce detailed images of your bones, muscles, fat, and organs to provide a more detailed image than standard X-rays.

If you do experience a fracture, you’ll likely need surgery to repair your bone and physical therapy to help you regain your strength and ability to move. To properly fix your femur, you would need to have an operation. Depending on the severity of the injury, your doctor may use strong metal screws and plates, or even long titanium rods to piece the femur bone back together. These materials are commonly used in surgery today to help heal broken bones in various parts of the body.

Osteoporosis

Your femur, like all bones, can be affected by osteoporosis — a condition that causes your bones to gradually become weak and brittle. The older you get, the more likely you are to be affected by osteoporosis. The weakening of your bones can make you much more prone to getting fractures in your femur and other bones in your body.

To find out if your femur is weakening due to osteoporosis, your doctor may order a bone densitometry test, also known as a dual-energy X-ray absorptiometry (DEXA or DXA) test. This uses a small amount of radiation to generate pictures of your lower spine and hips (including the femur). This quick and easy procedure is the standard method of diagnosing osteoporosis.

Patellofemoral pain syndrome

This is a condition that causes pain in the front of your knee near the patella (kneecap). It’s sometimes called “runner’s knee” because it usually affects people who play sports that involve running or jumping. The patella sits on a tiny groove at the end of the femur, which is why this syndrome affects both of these structures and the surrounding area.

Patellofemoral pain syndrome usually causes a dull, aching pain in the front of your knee, which can worsen when you:

Walk up or down stairs

Kneel or squat

Sit with a bent knee for long periods

If there is something wrong with your femur or the surrounding area, you might experience: 

Difficulty moving the leg

Inability to stand or walk

Pain or swelling in the thigh, possibly with bruising

Deformity (abnormal shape) of the thigh

Bone pushing out through the skin, which indicates a severe fracture

Your femur is a big part of your bone structure, and just like other bones in your body, it's constantly changing. Building strong and healthy bones in your childhood and adolescence ensures they sustain you through adulthood. But as you age, your bones start to lose slightly more mass than they gain.

Luckily, there are ways to prevent or slow this process, some of which include:

Be sure to talk to your doctor about any potential concerns about bone health issues such as osteoporosis so they can guide you.

The femur is the longest, strongest, and heaviest bone in the human body. The most common problems affecting this bone are fractures, osteoporosis, and patellofemoral pain syndrome. To diagnose a fractured femur, your health care provider may order tests such as an X-ray, MRI, or CT scan.

Femur Bone FAQs

How painful is a broken femur?

A femoral shaft fracture usually causes immediate, severe pain. You won’t be able to put weight on the leg. The injured leg may look shorter than the other leg and no longer straight.

Can you walk after breaking your femur?

No, because you won’t be able to put weight on the injured leg.

Why is the femur called a long bone?

Long bones are hard and dense. It has a shaft and two ends, and it provides strength, structure, and mobility. Long bones contain both yellow and red bone marrow, which make blood cells.