The concepts of extension and flexion apply only to the joints of the human body, not the muscles, which only contract or lengthen. A joint or articulation is an area of your body where two bones come together for the purpose of movement. The direction of this movement defines whether you are extending or flexing the joint.
It's All About the Angle
The main difference between joint extension and flexion is whether the angle between two body parts increases or decreases during the joint's movement. In general, when you extend or straighten a joint, the angle increases, and when you flex or bend a joint, the angle decreases.
Extension and Flexion Examples
Hip flexion takes place when you bend forward at your waist to touch your toes -- the angle between you upper and lower body decreases. When you return to an upright position, hip extension occurs -- the angle increases. To kick a soccer ball, for example, you flex your knee joint as you take your foot backward to prepare for the kick. The joint then extends as you move your lower leg forward to kick the ball.