Extension

Anatomically, extension is the verb used to indicate movement of a body part in relation to another so as to straighten their relationship. The usual direction is posterior. For example, extension of the hip involves movement of the femur in a posterior direction with rotation at the hip.

Exceptions include:

• knee extension, which entails movement in an anterior direction
• thumb extension, involving movement of the thumb across the plane of the palm
• extension of the foot, termed plantarflexion

An alternative definition for extension is an increase in the angle between bones or body areas.

Extension is the opposite of flexion.