Here's why:

* Sagittal Plane: This plane divides the body into left and right halves. Movements in this plane involve flexion and extension. The push-up involves flexing (lowering) and extending (pushing up) the arms, which occur primarily in the sagittal plane.

While the push-up does involve some minor movement in other planes (like the transverse plane for shoulder rotation), the primary movement is within the sagittal plane.