The transverse plane is a plane of movement used in anatomy to describe motions around a vertical axis. In the human body, that axis is most often the spine, and the transverse plane is where movements around the spine take place. An example of such a movement is trunk rotation, or twisting of the waist side to side. Another example but at a different vertical axis is the chest fly, a horizontal abduction and adduction in which both arms are held forward at shoulder height, moved horizontally out to either side, and then brought back together.
One of three planes through which the body moves, the transverse plane is typically depicted as a horizontal plane bisecting the body at the top of the hips, so that the body is partitioned into superior and inferior, or top and bottom, halves. Other planes of movement include the sagittal plane and the frontal plane. Movement in the sagittal plane, which separates the body into left and right halves, occurs front to back; examples are walking or performing biceps curls. In the frontal plane, movement occurs perpendicular to the sagittal plane in that it occurs side to side. The frontal plane separates the body into front and back halves, and examples of frontal plane movement include lateral shoulder raises and side lunges.
Relative to the sagittal and frontal planes, movement in the transverse plane is often more complex and at times occurs in combination with sagittal and/or frontal plane movement. As all movement in the transverse plane occurs around a vertical axis, it typically involves a rotation. The simplest of these movements is trunk rotation, in which the body is stationary and the abdominal and back muscles are used to turn the torso at the waist. Another example is rotation of the leg at the hip joint: when the leg is hanging vertically under the body and rotated internally and externally at the hip, that movement is taking place in the transverse plane.
A more complex example of transverse plane movement would be lunging at a 45-degree angle while chopping a medicine ball over the front knee. Stepping out at a 45-degree angle, rather than straight forward or out to the side, means that motion is occurring in both the frontal and sagittal planes as the body travels through space. Bringing the arms around in a circular motion and rotating the torso to chop the ball past the front knee is an action that is occurring in the transverse plane, even as the body moves through other planes.