Which of the following activities primarily involves the sagittal plane?

The activity that primarily involves the sagittal plane is bending forward and backward. The sagittal plane divides the body into left and right halves and encompasses movements that occur in a forward and backward direction. Bending forward or backward directly involves flexion and extension at the joints, particularly in the spine and hips, which are key movements occurring within this plane.

Jumping, while it does involve vertical motion (which may seem related), primarily engages the frontal and transverse planes during the take-off and landing phases. Twisting primarily occurs in the transverse plane, which involves rotation around the body's vertical axis. Side lunging predominantly takes place in the frontal plane, focusing on movements that go laterally (side to side) rather than forwards and backwards. Therefore, bending forward and backward is the best representation of motion that strictly occurs in the sagittal plane.