Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
In anatomy, flexion is a position that is made possible by the joint angle decreasing. The skeletal (bones, cartilage, and ligaments) and muscular (muscles and tendons) systems work together to move the joint into a "flexed" position. For example the elbow is flexed when the hand is brought closer to the shoulder. The trunk may be flexed toward the legs or the neck to the chest.
The opposite term is extension, or straightening. Flexion decreases the angle between the bones of the limb at a joint, and extension increases it.
Note that specific flexion activities may occur only along the sagittal plane, i.e. from the forward to backward direction, and not side-to-side direction, which is further discussed in abduction.
Active range of motion exercises include movements such as flexion and extension. These exercises are used after an injury or surgery. They are done by a physical therapist or nurse initially, and may be continued by the patient.
In the healing process, active range of motion exercises, should avoid forcing the appendage into the extension or flexion position. The stress induced may re-injure the affected appendage (limb).
Muscles of flexion
- of forearm, at elbow
- of hand, at wrist
- of proximal phalanges, at metacarpophalangeal joint 
- of femur/thigh at hip (L1-L2)
- Tensor fasciae latae
- Rectus femoris
- (additional minor contributions from other hip flexors)
- of leg at knee (L5-S2)
- of toes
- of proximal phalanges at metatarsophalangeal joint
- ↑ http://www.exrx.net/Articulations/Shoulder.html#anchor102874
- ↑ http://www.exrx.net/Articulations/Elbow.html#anchor764071
- ↑ http://www.exrx.net/Articulations/Wrist.html#anchor260949
- ↑ http://www.exrx.net/Articulations/Fingers.html#anchor801484
- ↑ http://www.exrx.net/Articulations/Fingers.html#anchor783387
- ↑ http://www.exrx.net/Articulations/Thumb.html#anchor206196
- ↑ http://www.exrx.net/Articulations/Thumb.html#anchor212077
- ↑ http://www.exrx.net/Articulations/Hip.html#anchor845056
- ↑ http://www.exrx.net/Articulations/Knee.html#anchor22052
- ↑ http://www.exrx.net/Articulations/Toes.html#anchor229680
ca:Flexió (anatomia) de:Flexion (Medizin) nl:Flexie (anatomie) sv:Flexion