Extension (kinesiology)
Extension is a movement of a joint that results in increased angle between two bones or body surfaces at a joint. Extension usually results in straightening of the bones or body surfaces involved. For example, extension is produced by extending the flexed (bent) elbow.  Straightening of the arm would require extension at the elbow joint. If the head is tilted all the way back, it is said to be extended. Note: while, extension is a movement, the terms "extend" versus "flexed refer to the final position of a body part with reference to the anatomical position of the body. For, example, if an arm fully bent at the elbow is slightly extended, it will still be called "flexed" or "semiflexed", and will be called "extended" only upon complete straightening of the arm.
The movement in the opposite directions is called flexion. Flexion decreases the angle between the bones of the limb at a joint, while extension increases it.
With the foot, extension (elevation) is usually called dorsiflexion.
If a part of the body is overstretched because of exaggerated extension motion, then one speaks of a hyperextension (as with the knee). This puts a lot of stress on the ligaments of the joint, and need not always be a voluntary movement, but may occur as part of accidents, falls, or other causes of trauma.
In some anatomical muscle names, particularly with muscles of the forearm and the Cnemis, the term occurs explicitly as the second word in the Latin spelling of the name (for example, Musculus extensor carpi ulnaris).

# Muscles of extension

## Upper limb
- of forearm at elbow
Posterior compartment of the arm
Triceps
Anconeus
- Posterior compartment of the arm
Triceps
Anconeus
- Triceps
- Anconeus
- of hand at wrist
Posterior compartment of the forearm
Extensor carpi radialis longus
Extensor carpi radialis brevis
Extensor digitorum
- Posterior compartment of the forearm
Extensor carpi radialis longus
Extensor carpi radialis brevis
Extensor digitorum
- Extensor carpi radialis longus
- Extensor carpi radialis brevis
- Extensor digitorum
- of phalanges, at all joints
Posterior compartment of the forearm
Extensor digitorum
Extensor carpi ulnaris
Extensor digiti minimi (little finger only)
Extensor indicis (index finger only)
- Posterior compartment of the forearm
Extensor digitorum
Extensor carpi ulnaris
Extensor digiti minimi (little finger only)
Extensor indicis (index finger only)
- Extensor digitorum
- Extensor carpi ulnaris
- Extensor digiti minimi (little finger only)
- Extensor indicis (index finger only)
- of phalanges, at interphalangeal joints
Lumbricals of the hand
Dorsal interossei of the hand
Palmar interossei
- Lumbricals of the hand
- Dorsal interossei of the hand
- Palmar interossei
- of thumb
Extensor pollicis brevis (proximal phalange)
Extensor pollicis longus (distal phalange)
- Extensor pollicis brevis (proximal phalange)
- Extensor pollicis longus (distal phalange)

## Lower limb
- of thigh/femur at hip
Gluteus maximus
Posterior compartment of thigh
Biceps femoris
Semitendinosus
Semimembranosus
- Gluteus maximus
- Posterior compartment of thigh
Biceps femoris
Semitendinosus
Semimembranosus
- Biceps femoris
- Semitendinosus
- Semimembranosus
- of leg at knee (L3-L4)
Quadriceps
Rectus femoris muscle
Vastus medialis
Vastus lateralis
Vastus intermedius
- Quadriceps
Rectus femoris muscle
Vastus medialis
Vastus lateralis
Vastus intermedius
- Rectus femoris muscle
- Vastus medialis
- Vastus lateralis
- Vastus intermedius
- of toes
Extensor hallucis longus
Extensor digitorum longus
Extensor digitorum brevis
Extensor hallucis brevis
- Extensor hallucis longus
- Extensor digitorum longus
- Extensor digitorum brevis
- Extensor hallucis brevis