Here's why:

* Sagittal Plane: This plane divides the body into left and right halves. Movements in this plane are forward and backward, like walking, running, or in this case, pulling yourself upwards.

* Frontal Plane: This plane divides the body into front and back halves. Movements in this plane are side-to-side, like jumping jacks or side lunges.

* Transverse Plane: This plane divides the body into upper and lower halves. Movements in this plane are rotational, like twisting your torso or swinging a golf club.

While the pull-up is primarily sagittal, there might be some slight movement in the transverse plane, especially if you are trying to engage your lats more by bringing your elbows down and back. However, the main movement of the pull-up is clearly in the sagittal plane.