Difference Between Flexor and Extensor Muscles

The key difference between flexor and extensor muscles is that flexor muscles facilitate the process of flexion in the body, while extensor muscles facilitate the process of extension in the body.

Flexion is a bending movement where the angle between two body parts decreases. Contracting the biceps exhibits flexion as it brings the forearm closer to the upper arm, decreasing the angle between the two. Therefore, biceps are described as a flexor muscle (biceps flexing). On the other hand, extension is the extending movement where the angle between two body parts increases. When the triceps contract, the arm straightens and the angle between the forearm and upper arm increases. Therefore, the triceps are known as extensor muscles. These actions are intrinsic, which means they are an unchangeable property of the muscle.

CONTENTS

1. Overview and Key Difference
2. What are Flexor Muscles
3. What are Extensor Muscles
4. Similarities Between Flexor and Extensor Muscles
5. Side by Side Comparison – Flexor vs Extensor Muscles in Tabular Form
6. Summary

What are Flexor Muscles?

Flexion is typically instigated by muscle contraction of a flexor. Flexion refers to decreasing the angle between two body parts. Flexion at the elbow is decreasing the angle between the ulna (long bone found in the forearm) and humerus (long bone of upper limb). Flexor muscles further decrease between bones on two sides of a joint, as in bending the knee. When the knee flexes, the ankle moves closer to the buttock; therefore, the angle between the femur (long bone located within the human thigh) and tibia (the large bone in the lower leg) becomes smaller. Furthermore, when the head is flexed, the chin is against the chest. The trunk is flexed when a person leans forward. Flexion of hip or shoulder moves the arm or leg forward.

Hyperflexion refers to the movement of a flexor muscle beyond its normal limit. Hyperflexion may occur as a result of a fall or industrial or vehicular accident. In a situation of hyperflexion, the muscles, ligaments and other tissues surrounding the joints may be torn, dislocated or damaged. Cervical whiplash syndrome is an example of hyperflexion. It is an injury to the neck that occurs when the head is jerked forward and back very quickly.

What are Extensor Muscles?

Extension is the opposite of flexion. It describes a straightening movement that increases the angle between two body parts. For example, when standing, the knees are extended. The extension of the hip or shoulder moves the arm or leg backwards. Hence, extensor muscles are any of the muscles that increase the angle between members of a limb, as by straightening the elbow or knee or bending the wrist or spine backwards. In humans, certain muscles in the hand and foot regulate this function. Extensors in hand include extensor carpi radialis brevis, extensor carpi radialis longus, extensor carpi ulnaris, extensor digitorum, extensor indicis, extensor pollicis brevis, and extensor pollicis longus. Extensors in foot include extensor digitorum longus, extensor digitorum brevis, extensor hallucis brevis, and extensor hallucis longus.

Hyperextension is any extension that goes beyond 180 degrees and becomes reflexive. Hyperextension is also termed as an excessive joint movement in which the angle formed by bones of a particular joint opened beyond its normal healthy range. This increases the likelihood of dislocation and other potential injuries to the joint.

What are the Similarities Between Flexor and Extensor Muscles?

• Flexor and extensor are two muscles types.
• Both are regulating the movement of the human body.
• They are made up of a kind of elastic tissue.
• Both are commanded by a nerve.

What is the Difference Between Flexor and Extensor Muscles?

Flexor muscles facilitate the process of flexion in the body, while extensor muscles facilitate the process of extension in the body. So, this is the key difference between flexor and extensor muscles.  Flexion refers to a movement that decreases the angle between two body parts, while extension refers to a movement that increases the angle between two body parts. Therefore, the above terms refer to increasing and decreasing the angle between two body parts. Flexion and extension are extremely important for movement in the human body.

Below is a list of differences between flexor and extensor muscles in tabular form.

Summary – Flexor vs Extensor Muscles

Flexion and extension are movements that take place within the sagittal (anterior-posterior) plane and that involve anterior or posterior movements of the body. In flexion, the angle between two body parts decreases, while in extension, the angle between two body parts increases. Moreover, hyperflexion is excessive flexion at a joint. On the other hand, hyperextension is the abnormal extension at a joint. Thus, this is the summary of the difference between flexor and extensor muscles.

Reference:

1. “Types of Body Movements.” Lumen Learning, Available here.
2.“Flexor Muscle.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., Available here.

Image Courtesy:

1. “Flexor carpi ulnaris – Muscles of the Upper Extremity Visual Atlas, page 57” By Rob Swatski (CC BY-NC 2.0) via Flickr
2. “Extensor digitorum – Muscles of the Upper Extremity Visual Atlas, page 59” By Rob Swatski (CC BY-NC 2.0) via Flickr