Here's why:

* Sagittal Plane: This plane divides the body into left and right halves. Movements in this plane involve forward and backward motion. Lunges primarily involve the forward and backward motion of the legs and torso.

* Frontal Plane: This plane divides the body into front and back halves. Movements in this plane involve side-to-side motion. Lunges don't involve significant side-to-side motion.

* Transverse Plane: This plane divides the body into upper and lower halves. Movements in this plane involve rotational motion. Lunges don't involve significant rotational motion.

Important Note: While the primary plane of motion for a lunge is sagittal, there might be some minor movement in other planes depending on the variation of the lunge and individual technique. For example, a walking lunge might involve some slight rotation in the transverse plane as you step forward.