Difference Between Flexion and Extension

Main Difference – Flexion vs Extension

Joints and muscles of the animal body are involved in motion. The straightening and contraction of muscles perform the motion of the joints. Flexion and extension are two anatomical terms used to describe angular motion. The direction of the movement or the change in the angle between the two bones determines the type of motion as a flexion or extension. The main difference between flexion and extension is that flexion is the action that brings the two bones together, decreasing the angle between the bones whereas extension is the action that increases the angle between the two bones.

Key Areas Covered

1. What is Flexion
      – Definition, Angle, Examples
2. What is Extension
      – Definition, Angle, Examples
3. What are the Similarities Between Flexion and Extension
      – Outline of Common Features
4. What is the Difference Between Flexion and Extension
      – Comparison of Key Differences

Key Terms: Angular Motion, Dorsiflexion Extension, Flexion, Hyperextension, Hyperflexion, Lateral Flexion, Plantar Flexion

What is Flexion

Flexion refers to a movement that decreases the angle between two body parts. The opposite movement of flexion is the extension. Flexion of the elbow, which decreases the angle between ulnar and the humerus, is a general example of flexion. Dorsiflexion, plantar flexion, and lateral flexion are special movements in flexion. Dorsiflexion is the backward bending. Bending of hand or foot are examples of dorsiflexion. Plantar flexion is the forward bending of hand or foot. Dorsiflexion and plantar flexion of the foot are shown in figure 1.

Figure 1: Dorsiflexion and Plantar Flexion

 Lateral flexion is bending to the side. The bending of the neck either to the left or right side is an example of lateral flexion.  

What is Extension

Extension refers to the process of straightening, which increases the angle between two body parts. Knees are extended when standing up. Most extensions lead to the movements in the posterior direction. For example, the extension of the shoulder or hip moves the arm or leg backward. The flexion and extension of shoulder, knee, and neck are shown in figure 2.

Figure 2: Flexion and Extension of Shoulder, Knee, and Neck

The abnormal motion of extension is called hyperextension. Hyperextension increases the stress on the ligaments of the joints, causing injuries.  

Similarities Between Flexion and Extension

• Flexion and extension are two types of anatomical movements of the body.
• Joints are involved in both flexion and extension.
• Both flexion and extension are defined based on the angle between the two bones.
• Both flexion and extension occur in the sagittal plane.
• Both flexion and extension can occur in a forward or backward direction.
• Both hyperflexion and hyperextension may lead to injuries.

Difference Between Flexion and Extension

Definition

Flexion: Flexion refers to a movement that decreases the angle between two body parts.

Extension: Extension refers to the process of straightening that increases the angle between two body parts.

Angle Between Two Bones

Flexion: The angle between the two joins decreases during the flexion.

Extension: The angle between the two joints increases during the extension.

Action

Flexion: Flexion leads to the bending of the bones, bringing the two bones together.

Extension: Extension leads to the straightening of the bones, dispersing the two bones.

Examples

Flexion: Upward phase of a bicep curl and bending of the knee are examples of flexion.

Extension: Downward phase of a bicep curl and standing up are examples of the extension.

Abnormal Movement

Flexion: The abnormal movement of flexion is hyperflexion.

Extension: The abnormal movement of extension is hyperextension.

Conclusion

Flexion and extension are two types of angular moments of the body in which both joints and muscles are involved in the motion. Flexion is the bending, which decreases the angle between two anatomical parts of the body. The extension is the opposite of the flexion in which the angle between the two anatomical parts is increased. The main difference between flexion and extension is the change in the angle between the anatomical parts of the body during each type of the motion.

Reference:

1.“Flexion (Joint Movement).” Flexion : Muscle Actions : Joint Movements, Available here.
2.“Types of Body Movements.” Types of Body Movements | Anatomy and Physiology I, Available here.

Image Courtesy:

1. “Dorsiplantar” By Connexions – (CC BY 3.0) via Commons Wikimedia
2. “FlexsionExtension2017” By Tonye Ogele CNX –  (CC BY-SA 3.0) via Commons Wikimedia

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