Here's why:

* Sagittal plane: Divides the body into left and right halves. Movements in this plane are forward and backward, such as walking, running, and kicking.

* Horizontal extension: Refers to moving a limb away from the midline of the body in a horizontal direction. This movement is clearly in the sagittal plane.

Think of doing a horizontal extension with your arm. You're moving it away from the midline of your body, which is a movement within the sagittal plane.