Extension Is Bending of a Joint so the Bones Are Moved Farther Apart

Learn About Extension

Extension of the elbow joint.
Extension of the elbow joint. Halfdark/Getty Images

Extension is a physical position that increases the angle between the bones of the limb at a joint. It occurs when muscles contract and bones move the joint from a bent position to a straight position. It is a posterior movement for joints that move backward or forward, such as the neck. It is the opposite of flexion.

Typically, extension of a joint is limited to 180 degrees or less. In other words, that joint can basically be opened until it is straight. Think of your arm or leg as an example, as these can be unbent until they are virtually straight, but not beyond that point.

Hyperextension

Just as it sounds, hyperextension is an extreme version of an extension. Hyperextension is an excessive joint movement in which the angle formed by the bones of a particular joint is opened, or straightened, beyond its normal, healthy, range of motion. Such a movement may potentially make that particular joint unstable, and, in turn, increase the risk and likelihood for dislocation or other potential injuries of the joint.

The Joints

A joint is a physical point of connection between two separate bones. For example, the knee joint refers to the point of connection between the femur, or thigh bone, and the tibia, or shin bone. Joints also contain a variety of fibrous connective tissue such as ligaments, which connect the bones together, and tendons which connect muscle to bone and cartilage. Cartilage covers the ends of bones and provides some cushioning in joints.

Range of Motion of a Joint

A majority of the human body’s joints allow for movement, other than the skull joints. Joints such as the knee and elbow have a predetermined range of motion that limits how far an individual joint can bend comfortably. Each joint has a separate range of motion that is usually measured in degrees. The range of motion can be reduced due to injury or surgery and exercises can be done to improve or restore the range of motion.

Extension and Extensor Muscles

Extension is seen in these movements:

  • Straightening the knee
  • Unbending your elbow
  • Moving the upper leg backward from the hip
  • Moving your arm backward from the shoulder
  • Moving your head backward from the neck
  • Raising your chin
  • Leaning backward from the waist
  • Straightening your finger
  • Extension at the wrist moves the hand back towards the back of the forearm and is called dorsiflexion.

Muscles that contract to produce extension are called extensors. In the upper limb, these include the latissiumus dorsi, teres major, triceps brachii, aconeus, and extensors in the hand and fingers. In the lower limb, the extensors include the gluteus maximus, biceps femoris, quadriceps, and toe extensors.

Flexion

The opposite of extension is flexion. Flexion is defined as the bending of a particular joint so that the bones that form that joint are pulled closer together. Flexion is a physical position that decreases the angle between the bones of the limb at a joint. For the neck and torso, it is movements that bend forward such as leaning forward at the waist or nodding the head. It occurs when muscles contract and bones move the joint into a bent position.