Here's why:

* Sagittal Plane: This plane divides the body into left and right halves. Movements in this plane involve forward and backward motion. A rear lunge involves stepping backward with one leg while keeping the other leg stationary, fitting the definition of movement in the sagittal plane.

While the rear lunge primarily occurs in the sagittal plane, there is also some movement in the frontal plane due to the slight lateral shift of the body during the lunge. However, the primary motion remains in the sagittal plane.