There are three main bones in each leg and arm of the human body. The four limbs are very similar in anatomical structure. The upper portions of the arms and legs have one large bone that articulates with the shoulder or hip respectively. The lower portion of the limbs each have two bones that are tightly bound together by an interosseous membrane and articulate with the upper portion of the limb and the wrist or ankle. The forearm has more motion than the lower leg, but the bones are very similar in structure and how they are joined. The leg has an additional bone in the knee called the patella or kneecap.
Upper Arm
The bone of the upper arm is called the humerus. The humerus has joints at the shoulder (the glenohumeral joint) and elbow. It has a rounded head, where it articulates with the glenoid fossa of the scapula (shoulder blade) and a wide, flattened end at the elbow that articulates with the radius and ulna bones of the forearm. There is a bump called the deltoid tuberosity, where the deltoid muscle connects about halfway down its length, and two large bumps called the medial and lateral epicondyles on either side of the elbow joint.
Forearm
The forearm is composed of two bones, the radius and ulna. The ulna is larger at the elbow, where it has a hinge-type joint with the humerus. The end of the ulna at the wrist has a small protrusion called the styloid process.
There is a ridge running down the length of both the ulna and radius where they connect through an interosseous membrane. This connection allows the radius and ulna to rotate about each other without sliding, which creates the rotation of the hand at the wrist.
Upper Leg
The upper leg or thigh has a single bone called the femur. This is the largest and longest bone in the human body. It has a rounded head where it articulates with the acetabulum pelvis, forming the hip joint. This portion is offset at an angle from the rest of the bone shaft and has a large bony bulge called the greater trochanter, where the angle is formed. There is also a lesser trochanter opposite the greater trochanter. These are the locations where the muscles of the hip, groin and buttocks attach to the femur.
At the knee the femur is enlarged, much like the humerus, were it articulates with the tibia, forming the knee joint. As with the humerus, the femur has two epicondyles on either side of this joint.
The leg also has a sesamoid-type bone that protects the front of the knee, called the patella. This kneecap is actually formed inside the tendon that connects the tibia and femur.
Lower Leg
The lower leg has two bones, the tibia and fibula. Like the bones of the forearm, these bones are joined by an interosseous membrane. Unlike the forearm, the bones of the lower leg are relatively fixed and do not rotate around each other. The tibia has a large flat area at the knee called the tibial plateau, where it articulates with the femur. There is a bump at the front of the tibia called the tibial tuberosity, where the tendon containing the kneecap connects.
The end of the tibia that forms the ankle joint articulates with the talus of the foot. Here both the tibia and fibula form a yoke on the talus and provide the stability of the ankle joint.
The fibula is much smaller than the tibia and is tightly bound to the tibia though the interosseous membrane. It is larger at the ankle and narrows to a small head near the knee, where it articulates with the underside of the tibial plateau.
References
- "Gray's Anatomy: The Anatomical Basis of Clinical Practice", Susan Standring (Editor), 2008
- "Trail Guide to the Body: How to Locate Muscles, Bones and More", Andrew Biel, 2005
