Extension is a movement of a joint that results in increase angle between two bones ofr 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.